.

ExamensarbeteMMK 2017: 165 MDA 605

Uppvärmning av litiumjon batterier med växelström vidtemperaturer under noll grader

Dieter Henz

Godkänt Examinator Handledare2017-08-30 Prof. Lei Feng Mikael Hellgren

Uppdragsgivare KontaktpersonRúdi Soares Rúdi SoaresAlexander BessmanProf. Oskar Wallmark

.

Sammanfattning.Att göra trafiken eldriven är ett tydligt mål hos många myndigheter i hela världen. Lokaltsett är elektriska fordon utsläppslösa och kan därför bidra till att förbättra luftkvaliteteni snabbt växande städer. Trots att det finns många anledningar till att stödja elektriskafordon accepterar konsumenterna dem inte på en önskad nivå. En grund till den lågapopulariteten är det dåliga förhållandet mellan körsträcka och inköpskostnad. En möj-lighet till att förbättra förhållandet är att minsta antalet komponenter i batteripaketet,eftersom kostnaderna då sjunker samtidigt som körsträckan ökar.

Moderna batterier i elektriska fordon bygger på litiumjon teknologin. Denna typav batterier har ett begränsd brukstemperaturområde och behöver därför värmas uppvid kalla förhållanden. I många elektriska fordon blir batteriet uppvärmt via ett egetvärmeelement som byggs in i batteripaketet. Detta värmeelement orsakar mer komplex-itet och behöver energi för att fördela värmen över batteriet. I den här studien undersöksmöjligheten att använda en ny metod som kallas växelströmvärmning. Denna metodutnyttjar värmeutvecklingen i batteriets inre impedansen för att värma upp batteriet in-ifrån.

Arbetet innehåller både en praktiskt del och en del med simulationer. En litteraturstudieöver moderna uppvärmningsmetoder är presenterad i början av rapporten. Efter detfinns en impedansmätning av två battericeller. Därefter testas olika driftpunkter för attbestämma hur växelströms frekvens och amplitud påverkar uppvärmningseffekt i en cell.Resultatet av impedansmätningen används för att bestämma driftpunkter för ett följandeexperiment och härleda ett ekvivalentskrets-schema för en battericell.

Studiens slutsats är att det i allmänhet är möjligt att värma litiumjonbatterier med väx-elström. Låga frekvenser har bättre uppvärmningseffekt på samma strömamplitud ochhöga strömamplituder har bättre effekt än låga strömamplituder. På grund av detta visarstudien också att det behöver genomföras flera undersökningar som fokuserar på hurfrekvensen är kopplad med åldringen av batterikapaciteten vid låga temperaturer. Avs-lutningsvis presenteras en idé som visar hur det är möjligt att höja strömmen i batterietför att förbättra uppvärmningseffekten.

ii

iii

.

Master of Science ThesisMMK 2017: 165 MDA 605

Heating Effect of Alternating Current on Lithium-IonBatteries at Subzero Temperatures

Dieter Henz

Approved Examiner Supervisor2017-08-30 Prof. Lei Feng Mikael Hellgren

Commissioner Contact personRúdi Soares Rúdi SoaresAlexander BessmanProf. Oskar Wallmark

.

Abstract.Electrification of transport is a distinct goal of many authorities. Electric vehicles arelocally emission-free and can help to improve the air quality in cities. Despite severalstimuli to promote electric vehicles, customers do not accept them at the desired level.One reason is the poor ratio between driving range and purchase cost. One possibility totackle this poor ratio is to reduce complexity and weight of a battery pack, which helpsto reduce its costs while increasing the driving range at the same time.

Recent batteries in electric vehicles use lithium-ion technology. This type of batterieshave a limited operating temperature, which requires pre-heating in cold ambient. Inmany electric vehicles pre-heating is realized with a heating element in the battery pack.This heating element does not only bring along additional weight but requires extra en-ergy for distributing the heat. The idea of this study is to propose a more efficient heatingmethod which is called alternating current (AC) heating. This method uses the internalimpedance of a battery to generate dissipative heat, heating it up from inside.

This work consists of practical measurements and experiments on a cell level as wellas simulations. First, a literature research is conducted to present the latest heating tech-nologies. Then, an impedance measurement is performed. The result of the impedancemeasurement is used to define the operating points for the following experiment. Theexperiment shows how AC frequency and amplitude influence the heating effect. The re-sults from the impedance measurement and the experiment are further used to constructa cell simulation model that connects the thermal properties with the electrical properties.After verification of the model, the simulation is expanded to a battery pack level.

The conclusion of the study is that it is generally possible to heat lithium-ion batter-ies with alternating current. Lower frequencies provide better heating effect at the samecurrent amplitude and a higher current amplitude gives better heating effect than loweramplitudes. Therefore, the study also shows that further research is necessary to deter-mine how the AC frequency affects aging of lithium-ion batteries at low temperatures.Finally, a method for increasing the current amplitude is proposed.

iv

v

Dedication

I dedicate this work to my mother and my father who passed away in 2016. So manybeautiful pieces of my life which I can not count and not describe are thanks to them.

Acknowledgement

This work was partially sponsored by Scania AB. Therefore I thank the company andespecially Pontus Svens as representative. I thank Rúdi Soares, Alexander Bessman andOskar Wallmark for this interesting thesis project, for introducing me to the topic as wellas for their positive support throughout the whole thesis. Furthermore, I thank NiclasJohannesson and Jesper Freiberg for their practical support during the laboratory work.Last but not least I thank my family, girlfriend and other loved ones who have supportedme from far away during my whole master studies.

vi

vii

List of Figures

1.1 Comparison of car models with purely electric and internal combustionengine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1 Equivalent circtuits of a battery . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.1 Battery with connections and part of insulation . . . . . . . . . . . . . . . . . 173.2 Schematic overview of the experimental setup . . . . . . . . . . . . . . . . . 193.3 Insulated batteries inside climate chamber . . . . . . . . . . . . . . . . . . . . 223.4 Applied equivalent circuit model . . . . . . . . . . . . . . . . . . . . . . . . . 223.5 Model of battery pack with controller . . . . . . . . . . . . . . . . . . . . . . 25

4.1 Bode graph of battery impedance . . . . . . . . . . . . . . . . . . . . . . . . . 274.2 Nyquist graph of battery impedance for cold temperatures . . . . . . . . . . 274.3 Nyquist graph of battery impedance for higher temperatures . . . . . . . . . 284.4 Surf graph of real part of battery impedance . . . . . . . . . . . . . . . . . . 294.5 Surf graph of imaginary parts of battery impedance . . . . . . . . . . . . . . 304.6 SS18 warmup during experiment at 0.5 C . . . . . . . . . . . . . . . . . . . . 314.7 SS20 warmup during experiment ay 0.5 C . . . . . . . . . . . . . . . . . . . . 324.8 SS18 warmup during experiment at 1 C . . . . . . . . . . . . . . . . . . . . . 324.9 SS20 warmup during experiment at 1 C . . . . . . . . . . . . . . . . . . . . . 334.10 SS18 warmup during experiment at 1.25 C . . . . . . . . . . . . . . . . . . . 344.11 SS20 warmup during experiment at 1.25 C . . . . . . . . . . . . . . . . . . . 344.12 Series resistance and inductance . . . . . . . . . . . . . . . . . . . . . . . . . 354.13 First parallel R-C branch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364.14 Second parallel R-C branch . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364.15 Nyquist graph with both measured data and equivalent circuit I . . . . . . . 374.16 Nyquist graph with both measured data and equivalent circuit II . . . . . . 384.17 Model comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394.18 Model comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404.19 Model comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414.20 Model comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424.21 Model comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434.22 Model comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444.23 Temperature increase with controller in either battery pack or single battery 454.24 Controlled current for temperature increase in either battery pack or single

battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

viii

LIST OF FIGURES

6.1 Circuit with power supply, resonance capacitor and battery equivalent circuit 546.2 Current from power supply and current in resonance circuit . . . . . . . . . 54

ix

List of Tables

2.1 Resulting temperatures using different heating methods with different en-ergy inputs with data from [32] . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.1 Specifications of the lithium-ion battery used in this study . . . . . . . . . . 17

4.1 Qualitative overview of the temperature behavior of the real (R), imagi-nary capacitive (XC) and imaginary inductive (XL) part of the measuredimpedance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.2 Comparison of the four electro-thermal battery simulations . . . . . . . . . . 454.3 Time and power consumption to heat a battery pack . . . . . . . . . . . . . . 46

5.1 Overview of battery heating methods . . . . . . . . . . . . . . . . . . . . . . 50

6.1 Parameters of the equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . 55

x

Nomenclature

Abbreviations

AC Alternating current

BMS Battery managment system

C − rate Current that charges or discharges a battery within one hour

CAN Controller area network

CRG Current ripple generator

CVM Constant voltage method

DC Direct current

EIS Electrochemical impedance spctroscopy

EV Electric vehicle

LIB Lithium-ion battery

PCM Pulsed current method

PCS Phase change slurry

SEI Solid electrolyte interface

SOC State of charge

SOH State of health

Equation Variables

∆T Temperature difference [K]

q Heat generation rate [J]

ω Angular frequency [1s]

C Capacitor [F]

Cth Heat capacity [ JK

]

F (s) Controller transfer function

xi

LIST OF TABLES

G(s) System transfer function

ki Integral gain

kp Proportional gain

L Inductor [H]

P Power [W]

R Imaginary reactive impedance [Ω]

Rth Thermal resistance [ KW

]

X Real active impedance [Ω]

Z Impedance [Ω]

xii

Contents

1 Introduction 11.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Problem Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Research Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.4 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.5 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.6 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Frame of Reference 52.1 Battery Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1.1 Electrochemical Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1.2 Lithium-Ion Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1.3 Causes of Aging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.1.4 Temperature Dependent Aging . . . . . . . . . . . . . . . . . . . . . . 72.1.5 Causes of Heat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 Battery Heating Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2.1 External Battery Heating . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2.2 Internal Battery Heating . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.3 Battery Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3 Implementation 163.1 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163.2 Battery Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163.3 Electrochemical Impedance Spectroscopy . . . . . . . . . . . . . . . . . . . . 183.4 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.5 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.5.1 Electrical Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223.5.2 Thermal Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.5.3 Temperature Controller . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4 Results 264.1 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.1.1 Electrochemical Impedance Spectroscopy . . . . . . . . . . . . . . . . 264.1.2 Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304.1.3 Simulation Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354.1.4 Heat Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

xiii

CONTENTS

4.1.5 Temperature Controller . . . . . . . . . . . . . . . . . . . . . . . . . . 454.2 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4.2.1 Electrochemical Impedance Spectroscopy . . . . . . . . . . . . . . . . 474.2.2 Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474.2.3 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484.2.4 Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

5 Conclusion 495.1 Answers to Research Questions . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5.1.1 What are the existing methods to heat LIBs? . . . . . . . . . . . . . . 495.1.2 To what extent is it possible to heat LIBs with current? . . . . . . . . 505.1.3 What are the limitations of the AC heating method . . . . . . . . . . 51

5.2 Ethical Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

6 Outlook 536.1 Usability in Real Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . 536.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Bibliography 56

A Battery pack with temperature- and current controller 59

xiv

Chapter 1

Introduction

1.1 Background

Electric vehicles offer many advantages. High efficiency, little noise disturbance and localzero-emission are only a few of them. Even so, electric vehicles are not nearly as pop-ular as their conventional equivalent. Statistical data from the year 2016, provided bythe German Federal Motor Transport Authority Kraftfahrt-Bundesamt [1], supports thatstatement. Less than 2% of all newly registered vehicles in Germany were equipped withalternative propulsion, which also counts in hybrid vehicles. Of all cars with alternativepropulsion only 23% were purely electric, which results in a market share of 0.3% purelyelectric vehicles. This can have many causes like missing charging infrastructure, too longcharging time and little driving range combined with higher purchase cost to name onlya few of them. This disadvantage becomes more visible when comparing car models thatare available with both purely electric and combustion propulsion like VW e-Golf [2] andGolf 1.0 TSI [3], Ford Focus Electric [4] and Focus 1.6 Ti-VCT [5] as well as Smart FortwoCoupé Electric Drive [6] and Fortwo Coupé 1.0 [7], all in their most basic version. Figure1.1 shows the ratio: The electric version of a car costs double when bought newly, whileoffering only about a quarter of the driving range.

1

1.2. PROBLEM DESCRIPTION

VW Golf

Ford Focus

smart fortw

o0

5 000

10 000

15 000

20 000

25 000

30 000

35 000

40 000electric

combustion

VW Golf

Ford Focus

smart fortw

o0

200

400

600

800

1000

1200

Ra

ng

e in

km

electric

combustion

Figure 1.1: Comparison of car models with purely electric and internal combustion engine

In order to increase the popularity of electric vehicles the driving range has to beincreased, while the purchasing cost has to be reduced at the same time. Both factors aredirectly linked to the battery. Storing electric energy in batteries is expensive and one ofthe main reasons why so little capacity is installed. Currently, the main battery technologyused is lithium-ion.

1.2 Problem Description

Lithium-Ion Batteries (LIBs) lose capacity much quicker when used at low temperaturecompared to usage at room temperature. A rule of thumb says, that LIBs are at the endof their lifetime when they have decreased to 80% of their original capacity. Therefore, itwould be desirable to avoid operation at cold temperatures. In cold ambients, this wouldrequire preconditioning before usage. This could lead to a longer lifetime of the batteriesand therefore reduced costs as well as the environmental impact. If in addition a properoperating temperature could be reached without requiring additional heating elementsin the battery pack, costs could be reduced at the same time as the driving range wouldincrease due to a lighter battery pack.

Since every battery carries an inherent inner ohmic resistance, LIBs generate heatwhen electric current is flowing into or out of the battery. As under normal operationthis heat generation is considered as losses. At cold temperatures this effect can be har-vested to heat up the battery from the inside. By applying alternating current (AC), cur-rent would flow over the inner resistance, causing ohmic heat but it would not changethe state of charge (SOC). If on top of that, the frequency of the alternating current is toohigh for the chemical reactions in the cell to start, this procedure would not cause aging.

2

CHAPTER 1. INTRODUCTION

1.3 Research Questions

In this work, answers are sought for the following questions.

• What are the existing methods to heat LIBs?

• To what extent is it possible to heat LIBs with current?

• What are the limitations of the AC heating method?

1.4 Outline

Before the practical activities, a literature study will be presented. The first part of the lit-erature review explains the fundamental principles of LIBs. The second part is dedicatedto state of the art research about heating procedures for LIBs. Battery principles will bestudied in standard literature books. Heating procedures and current achievements inthis discipline will be summarized from various research articles. To achieve a broad andobjective overview, studies from multiple research groups with different approaches willbe looked into.

The practical part is divided in three parts. Firstly, an electrochemical impedancespectroscopy (EIS) is carried out at different temperatures to see how the cell impedanceevolves over temperature. The outcome of the EIS will be used to identify the parametersfor the following experiment. Secondly, an experiment is conducted. This is to identifythe ability to raise a cell’s temperature with current. Therefore, the battery cells are in-sulated to reach a more realistic behaviour. For this experiment, a test bench is set upto test the impact of both AC frequency and amplitude on the heating effect. Finally, asimulation is performed. The goal of this simulation is not only to predict the cell heatingunder different circumstances but also to draw conclusions about a realistic battery packfor real applications. Therefore, an electric battery model is connected with a thermal bat-tery model. For the electric model, data from the EIS is used and for the thermal model,data from the experiment is used.

1.5 Limitations

This project will be conducted on two single lithium-ion cells. A whole battery pack, as itis used in vehicles, is normally composed of many cells and a battery management system(BMS), mounted into a battery box. This changes the temperature behaviour comparedto a single cell. Due to practical aspects, such as current output of the supply, this studystarts at the cell level. After simulation models for the same case are verified, they can beexpanded to a more realistic case by extrapolation of the gathered data.

1.6 Methodology

The research fashion of this work is qualitative and will be conducted as critical casestudy. Generally, in a qualitative study, a phenomenon is investigated to gain more de-tailed knowledge about it. The idea of a critical case study is to consider unfavorable

3

1.6. METHODOLOGY

conditions and proof a certain capability under these unfavorable conditions. In this waya general conclusion can be drawn if the study is successful despite the limited numberof samples.

The qualitative fashion in this work is a result of the number of samples. There are twocells of the same kind in this study. To ensure data consistency the result of one sampleverifies the result of the other sample if equal. The phenomenon in this work is heatgeneration from internal losses. Naturally, losses are tried to be kept as small as possible.In the batteries’ case this is done by minimizing the inner impedance. Therefore, thesecells are ill suited for being heated by internal losses. Furthermore, the used cells havehigh thermal capacity, resulting in longer warm up time and large surface area, whichleads to more heat loss towards the cold ambient. In this way, it is possible to proof thegeneral feasibility of the AC heating method if it can be proven to work with the describedcells.

4

Chapter 2

Frame of Reference

2.1 Battery Principles

2.1.1 Electrochemical Cell

In general an electrochemical cell can act either as a galvanic or as an electrolytic cell.Which type it is acting as, depends on the mode of operation.

A galvanic cell works by equalizing a thermodynamic imbalance between the twoelectrodes. A galvanic process is therefore spontaneous and electrons are moving fromthe negative to the positive electrode. Electrons transport the electric charge outside of thecell. Ions transport charge inside the cell and can move from the positive to the negativeelectrode or in the opposite direction, depending on their charge. Even both movementscan happen at the same time, if ions of both negative and positive charge are present inthe cell. During the galvanic process net electric energy can be extracted, as it is done ina fuel cell or while discharging a battery. The opposite is true for an electrolytic cell; itdoes nothing on its own, but reactions can be forced by inserting electric energy. In a fuelcell this process is described as extracting hydrogen as well as oxygen from water and ina battery as charging. [8] In theory, this process could be repeated unlimitedly.

2.1.2 Lithium-Ion Batteries

As Palacín [9] states, the term battery is, even in academia, equivalently used as the termcell. This is common practice, despite battery originally describes a stack of cells. Bat-teries are divided into primary and secondary batteries, where primary types are non-rechargeable and secondary types are rechargeable [10] [9].

Every LIB consists out of two electrodes, electrolyte and a separator. An electrode itselfconsists of a chemical structure and a current collector. The chemical structure is metaloxide at the positive electrode and graphite at the negative electrode. These materials arethe reaction partner for the lithium ions. The electrodes act as a host for the lithium, whichmoves in between the two electrodes when the battery is charged or discharged. Theelectrode materials are electrically conductive, enabling the electrons to move throughthem to reach the current collectors. The current collectors are of highly conductive metalsuch as aluminum or copper, which collect the electrons from the chemical structure (orreleases them into it) and connects the cell to the externally connected electrical load. As

5

2.1. BATTERY PRINCIPLES

electric current is flowing outside of the cell, the same charge has to be transported insidethe cell, to keep the balance. Inside the cell this electric charge is transported with Li+ions, which separate from one electrode, move through the electrolyte and join the otherelectrode. For this process, the electrolyte acts as an ion conductor, similar as metal actsas an electric conductor for electrons, and is non-conductive for electrons. To prevent thetwo electrically conductive electrodes from touching, a separator is inserted in betweenthem. This separator lets the lithium ions pass but poses a mechanical, electrically non-conductive barrier to prevent a short-circuit inside the cell. [11] [12]

Charging or discharging the battery means to transport charge between the electrodes.In the case of LIBs, lithium is bound at the negative electrode, in the layered graphitewhen it is charged. When the cell is discharged, the lithium is bound in the tunneledstructure of the metal oxide at the positive electrode.

When the battery is used as an electric source, lithium at the negative electrode losesone electron and dissolves into the electrolyte. The electron moves through the graphite,into the current collector and goes through the externally connected electrical load. Insidethe cell, the lithium ion moves through the electrolyte, passes the separator and interca-lates into the tunneled structure of the metal oxide at the positive electrode, where it joinswith the electron again. This process can continue as long as there is still lithium bound atthe negative electrode. Once all lithium has moved to the positive electrode, the batteryis discharged and cannot provide any more electrical current. [12]

In order to charge a battery, an external current source is applied to the electrodes.This external electrical source reverses the charging process. All lithium is bound at thepositive electrode when the battery is discharged. The lithium atoms lose one electron,become a Li+ ion and dissolve into the electrolyte. The electron moves through the metaloxide and the current collector towards the external source and then to the negative elec-trode. Inside the cell, the lithium ion again moves through the electrolyte, passes theseparator and intercalates into the layered structure of the graphite at the negative elec-trode, where it obtains the missing electron again. [12]

There is several types of LIBs, all with different advantages and disadvantages. De-tailed information about the type used in this study can be found in table 3.1 in section 3.1.

2.1.3 Causes of Aging

Aging is a general description for the decrease of a battery’s performance. The per-formance can be diminished by either capacity fade or impedance raise. Capacity lossmostly originates in either loss of cyclable lithium or dissolution of electrode materialinto the electrolyte. [13] [14]. Loss of cyclable lithium is mainly caused by the forma-tion of a solid electrolyte interface (SEI) at the electrode|electrolyte interface [15], whichhappens mostly at the first cycle of a battery. The formation of a SEI layer around the elec-trodes is actually desired, since it slows down the process of the other two causes of ag-ing (electrode-material dissolution into the electrolyte and impedance increase). [16] [17]This SEI layer can receive cracks and will form a new layer at this crack. This formationscause the SEI layer to grow, which uses up more cyclable lithium as well as increasing thecell impedance. In order to increase the electrical conductivity in the positive electrode,graphite is doted into the metal oxide. At high currents these graphite particles can crackand lose connection with the surrounding metal oxide, causing higher impedance, too.

6

CHAPTER 2. FRAME OF REFERENCE

[12] [18]

2.1.4 Temperature Dependent Aging

Another process, that causes loss of cyclable lithium is lithium plating. It happens at thenegative electrode while charging, when the diffusion rate of the lithium ions is limited,as it is at low temperatures [19] [20] and especially below 0C [21]. As consequence ofthe low temperature, the potential of the graphite is decreased. After the lithium ionshave passed the SEI layer, they should react with the graphite but cannot due to thedecreased potential of the graphite electrode. Anyhow, the lithium ions still receive anelectron but since they are not bound to the graphite structure, they depose on top ofthe graphite as lithium metal. Those lithium ions are now lost for storing electric energy[17]. Deposited lithium ions can even grow a dendrite which can be compared to a spike,which grows from the negative electrode towards the positive electrode. This dendritecan even perforate the separator and cause an electric short circuit inside the battery [12].

2.1.5 Causes of Heat

As already mentioned, LIBs generate heat during any operation. This heat has threesources: charge transfer overpotential, mass transfer overpotential and ohmic losses.Charge transfer overpotential results in reaction heat. It occurs when the Li+ ions performa chemical reaction at the electrodes. Heat from mass transfer overpotential is caused byfriction when the Li+ ions move through the electrolyte. Ohmic heat is generated whenelectrons move through the electrode structure and the current collectors. [12] As men-tioned in section 2.1.4, performing charging actions at low temperatures will cause capac-ity loss. Therefore the AC frequency must be chosen high enough to prevent the chemicalreactions from happening.

2.2 Battery Heating Procedures

As described in numerous articles, low temperatures not only affect a LIB’s aging andsafety during operation but also decreases the efficiency and limits the power outputdue to an increased inner resistance. Therefore, various research has been conducted onbattery warming. Generally spoken, the methods for battery heating can be divided intointernal and external heating. In the following section an overview of current heatingmethods is given.

2.2.1 External Battery Heating

Heating with the Air Conditioning System

Zou et al. [22] are investigating how a vehicle’s air conditioning system can be used tocontrol both the passenger cabin temperature and the battery pack temperature at thesame time. A vehicle’s air conditioning system is well known from current vehicles. Inthis case, however, the refrigerant cools/heats both heat exchange media, the cabin’s air

7

2.2. BATTERY HEATING PROCEDURES

and the water which runs through the battery pack. The system was tested experimen-tally with a battery pack of lithium-ion cells with a mass of 16kg. It was found that theregular air conditioning system was not capable of heating the battery pack at −20C onits own. It is explained that the decrease of the refrigerants ability to transport heat andthe decrease of the scroll compressor at low temperatures make it necessary to includea positive temperature coefficient (PTC) heating element. This is a simple ohmic heater,which helps the refrigerant to heat up to a suitable temperature under which the knownair conditioning system works properly. With this additional PTC element, it was possi-ble for the authors to heat the battery within approx. 800s with a power input of 1.5 kW.It makes sense that it was necessary to add an ohmic heater in the heat exchange mediumof the battery. Water freezes below 0C and cannot be pumped through pipes in that case.This would mean that the air conditioning system first would have to defrost the waterbefore it could be used for heating.

Heating using Heat Pipes

Wang et al. [23] use heat pipes to transport heat into batteries. In their setup they donot use actual batteries but a metal plate, with a specific thermal capacity that is similarto the one of lithium-ion batteries. Heating block and "batteries" are both placed in afreezer. The heat pipes reach from inside of the heating block into the cell. It is notmentioned what kind of heat transport medium is inside the heat pipes but it freezes ata certain temperature. The authors show that it is possible to heat the batteries with thisprocedure under different circumstances. The circumstances are different temperaturesin the heating block (20C and 40C). These circumstances are compared to each otherby the time it takes to rise the cell temperature from −15C or −20C to 0C. The risetime to 0C differs from 300 s (heater at 40C, start temperature −15C) to 1500 s (heaterat 20C, start temperature −20C). The authors state that it was possible to start thesystem even when the heat transport medium was frozen initially. They further state thatheat pipes are a very efficient method to transport heat. They however do not state howmuch energy is used to maintain a temperature of 20C or 40C in the heating block in anenvironment that has −15C or −20C.

Thermodynamic Assessment of Different Heating Methods

In the study of Zhang et al. [24] three heating methods are compared and evaluated basedon the amount of energy they require to heat up a battery pack. The authors call themethods "direct cabin air blow", "PCS cycle" and "refrigerant circulation". PCS standsfor phase change slurry and is a mixture of water and micro-encapsulated PCM (phasechange material). In all three methods the heat exchange medium inside the battery packis air, which is circulated with a fan. The battery pack itself is insulated from the ambientair with a box. The difference in the methods is how the air in the battery pack gets heatedup.

• With direct cabin air blow, a channel between the cabin and the battery pack isengaged. Warm air from the cabin is directed into the battery pack and heats thebattery.

8

CHAPTER 2. FRAME OF REFERENCE

• PCS cycle means that there is a pipe system installed, in which the slurry circulatesbetween the passenger cabin and battery pack. In the warm cabin the PCM insidethe slurry changes from solid to liquid phase, absorbing heat. In the battery packthe slurry changes phase from liquid to solid releasing heat.

• The refrigerant method makes use of the air conditioning system. Heat is absorbedfrom ambient air and released in the condenser. In this case two condensers areincluded in the closed circuit. One in the passenger cabin and one in the batterypack.

In the study a 27.7 kWh battery, consisting of 2400 lithium-ion 18650-type cells, is lookedinto. The authors conduct a thermodynamic assessment with both the 1st and the 2ndlaw of thermodynamics to evaluate which method is the most efficient in heating thisbattery. The authors find that the refrigerant method and the PCS cycle cause the sameextra power load at any operating point. For mild heating, blowing air from the cabincauses the least power to transport heat to the battery. However, if the ambient condi-tions become more extreme and the need for heating increases, the required power for aircirculation increases much faster than the required power for refrigerant or PCS circula-tion. These results are intuitive as it is generally known that heat can be transported moreefficiently in liquids as in air, due to higher relative thermal capacity. However, the au-thors do not state how much energy is required to produce the heat. This is an importantfactor as the passenger cabin cannot be assumed as an infinite source of heat.

2.2.2 Internal Battery Heating

Heating Element Inside a Battery

Wang et al. [25] state correctly in their paper that there is no external heat transportmedium required if the battery is self-heating. In their study the authors have built aprismatic cell with an additional nickel foil of 20µm thickness inside. The foil is placedbehind the negative electrode from the positives electrode perspective and electricallyconnected to the negative electrode inside the cell. On top of the cell the foil has its ownterminal, which the authors call activation terminal. There is also a switch between acti-vation and negative terminal, which can be used to short-circuit those two terminals. Theload is always applied between positive and activation terminal. If the cell needs to beheated, the switch is opened and the current flows from the positive terminal through theload to the activation terminal through the foil and then reaches the negative electrode.In this way the foil acts as an ohmic resistance and dissipates heat into the electrolyte.Once the cell has reached a proper temperature, the switch can be closed and the foil isbypassed. The authors say it was possible to heat the cell from −20C to 0C within 20 sand from−30C to 0C within 30 s while reducing the SOC 3.8% and 5.5% respectively at1C DC discharge.

Heating with Vehicle’s Flattening Capacitor, Interter and Electrical Machine

Baba and Kawasaki [26] are studying how existing hardware in an electric or hybrid ve-hicle could be utilized to heat up the battery. The system the authors look into is generalmotor control circuit. It is composed of the battery as power source in parallel with a

9

2.2. BATTERY HEATING PROCEDURES

smoothing capacitor, the three phase inverter with free wheel diodes and a three phaseelectrical machine in Y-connection. The three phase inverter is connected to the batteryat two points, each individually controllable by a relay. The first relay connects the threephases with the positive terminal of the battery. The second relay is special and connectsonly one phase between the two transistors with the positive terminal of the battery. Bydisconnecting the first relay and connecting the second relay the inverter can be used toform a buck-boost converter by utilizing the parallel capacitor and the coils of the electricmachine. If the transistors in the three phases are called U+ V+ W+ and U- V- W-, thenthe second relay connects the positive terminal of the battery with the part between U+and U- of the inverter. The buck-boost procedure then happens in four sequences:

Transistors V- and W- are closed, the remaining are open. The ’inductor’ is charged.Here, the inductor is comprised as follows: 2 phases are in parallel, the parallelbranch is in series with the third phase of the electrical machine.

All transistors are open, the inductor builds up a higher voltage in order to maintainthe current flow. The voltage in the inductor rises above the capacitor’s voltage andcurrent flows over the freewheeling diodes from the inductor into the capacitor. Thecapacitor has now a higher voltage than the battery.

Transistors V+ and W+ are closed, all others are open. The capacitor now acts as a source;battery and inductor are in series, the capacitors voltage discharges into the battery.

Transistor V+ and W+ are opened, V- and W- are closed. The voltage in the inductor risesabove the battery voltage in order to maintain the current flow. Current continuesto flow into the battery until the inductor is discharged.

The described charge/discharge currents cause ohmic losses in the battery impedance butthe battery is not actually discharged, since the frequency of these currents is too high tooxidize the lithium. Another benefit is that not only the battery heats up, also the motorheats up due to the unavoidable ohmic resistance in the windings. This also causes atemperature rise in the surrounding oil, which is especially beneficial for hybrid vehicles.

The authors find that with this method it was possible to heat the system within fiveminutes from −20C to 0C. The procedure was tested on a real setup. In this specificstudy, the authors used a Toyota Prius to prove their concept. It is worth mentioningthat this method presents a very efficient heating method since it only alternating energybetween several electric reservoirs, while the occuring losses are acutally intended andtherefore of nearly 100% efficiency. Furthermore, the described method can be applied inboth cases, with and without a connected charger and as well over a wide battery SOClevel.

Pulsed Current and Constant Voltage Heating

In their study, Mohan, Kim, and Stefanopoulou [27] connect an electrical model with athermal model of a battery and compare two different heating methods. The authorscall them constant voltage method (CVM) and pulse current method (PCM). CVM is DCdischarge only with varying current at a constant voltage. PCM cycles energy betweenthe battery and an external energy storage, resulting in alternating current. In the study

10

CHAPTER 2. FRAME OF REFERENCE

the authors formulate an optimal control problem with different penalty on energy lossfor PCM. The optimal control problem penalizes large energy transport in PCM, since itcauses higher losses and requires a larger and therefore more expensive external energystorage. The frequency applied is 10 Hz. The authors find that CVM is faster in any case,even without any penalty in PCM.

In other words, this means that PCM can not, even with very high pulse amplitudes,heat a battery as fast as CVM. This can be explained with the lower charge transfer in thebattery. With CVM not only dissipative energy from the battery causes heat but also thechemical reactions contribute to the temperature rise, while with PCM almost only ohmiclosses cause the temperature rise. It is worth mentioning, that the goal of the authorswas to increase the battery’s power output. This is achieved by rising the temperature.However, they did not focus on capacity loss and how it would decrease the battery life.

Heating with Alternating Current at High Frequency

Stuart and Handeb [28] only take AC-heating into consideration and look into the in-fluence of different C-rates, SOC-levels and ambient temperatures. The authors madeexperiments with lead-acid and 16 series connected 6.6 V nickel metal hybrid (NiMH)batteries. The lead-acid battery was used in a simple setup to prove that the concept ofAC heating is actually functional. It was connected to 60 Hz as it is the common gridfrequency in American countries and therefore easy to implement. After that the au-thors built a setup, which could deliver 10-20 kHz and connected the NiMH battery andtested the heating effect at different amplitudes, SOC levels and ambient temperatures.In contrast to the previous study, the authors state that it was not possible to achieve atemperature rise with only DC. They also mention that charging at cold temperaturescauses permanent capacity loss and even gassing. Gassing bloats the battery, thereforecharging a cold battery should be avoided. Even 1

60s was time enough to start oxidizing

the lead, causing the same effect as pure charging, just at a slower rate. Therefore agingwas possibly caused, too. For the NiMH battery, 10-20 kHz was applied with current inthe range of 60 − 80Arms (corresponding to 9.23-12.3C). In a first test the authors founda higher current to be more effective. It took 3 min to raise the temperature from −20C

to +20C with 80Arms and 6 min with 60Arms. This result was confirmed in another testwhere the ambient temperature was chosen to be −30C. In this case it also took 3 min toraise the temperature from −30C to +20C with 80Arms (12.3C) but 7 min with 60Arms

(9.23C). Both tests were done at a SOC-level of 55%. In another test the authors compareddifferent SOC-levels at 60Arms (9.23C). They found that a higher SOC-level is beneficialfor heating purposes. While it took 8 min at 25% SOC for the temperature to raise from−30C to +20C it took only 6 min at 75% SOC. During the tests the authors as wellmeasured the battery impedance. They found that the impedance in the NiMH batteriesdecreased from 1.3Ω to 0.4Ω over the mentioned temperature range.

Heating with Alternating Current at Mid-Frequency

Zhu et al. [29] perform simulations and real measurements with and 18650 2.3 Ah lithium-ion battery. Different AC amplitudes and frequencies as well as different AC waveforms(sinusoidal and rectangular) were used. The equivalent circuit is assumed to an ohmicresistance in series with two parallel RC (ohmic resistance in parallel with capacitor)-

11

2.2. BATTERY HEATING PROCEDURES

branches. The values for the components were chosen in such way that the equivalentcircuit had the same electrical behaviour as the cell. The authors did AC impedance mea-surements and fitted the data into their equivalent circuit. The figures of the measurementreveal the impedance change over temperature. The real resistance varies between 0.35Ω

and 0.02Ω between −25C and +40C. In a first step the authors simulated 300 Hz in arange of 1.5 A to 10 A with both sinusoidal and rectangular waveform. In a second stepthey simulated 8 A and 10 A in a frequency range of 1 Hz to 600 Hz with both waveforms.

Result of the first experiment was that, as expected, higher current amplitudes hadbetter heating effect than lower amplitudes and rectangular waveforms also deliver fastertemperature rise. In detail

• Sinusoidal: −24C to +3C with 10 A amplitude (3.0C), −24C to −5C with 6 Aamplitude (1.8C) and no temperature rise with 1.5 A amplitude (0.5C)

• Rectangular: −24C to +23C with 10 A amplitude (4.3C), −24C to +10C with 6A amplitude (2.6C) and no temperature rise with 1.5 A amplitude (0.6C))

This is as expected since power dissipation in an ohmic resistance follows Pdiss =

i2rmsR and rectangular waveforms have a higher RMS value at the same peak amplitude.The result of the second experiment was that 30 Hz delivered the highest temperature

rise with both waveforms, sinusoidal and rectangular. Continuing from that, neighbour-ing frequencies (10 Hz and 80 Hz) delivered similar results as 30 Hz while 1 Hz, 300 Hzand 600 Hz gave worse results. Further, 10 A (3.0C for sinusoidal, 4.3C for rectangular)resulted in a higher temperature rise as 8 A (2.5C for sinusoidal, 3.5C for rectangular) andrectangular in a higher temperature than sinusoidal. The best result was achieved with30 Hz, 10 A rectangular waveform, where the temperature rose from −24C to +25C

within 600 s. The used equivalent circuit model was able to predict the resulting tem-perature very well with less than −1C error. Further findings were that at 1 Hz/10A/rectangular waveform the capacity faded by 3.7% and by 6.6% after 20 and 40 heatingcycles respectively compared to the initial capacity. No capacity fade was observed athigher frequencies.

Heating with Alternating Current at Low Frequency

Zhang et al. [30] have done a similar study as explained in the preceding study. A modelof a 18650 lithium-ion battery was developed according to data gathered from an elec-trochemical impedance spectroscopy (EIS). The assumed circuit was a series connectionof an inductance, an ohmic resistance, a R/Q (ohmic resistance in parallel with a capaci-tor but with compressed semi-circle in the nyquist diagram) and a Q/R-W (compressedsemi circle capacitor in parallel with series connection of ohmic resistance and warborgimpedance) branch. The study was carried out with sinusoidal waveform and 3 A to 7 A(1C to 2.25C) at 0.1Hz / 1Hz / 10Hz. The ambient temperature was always assumed tobe −20C and two different heat insulations (Rth1 = 21.6 W

m2Kand Rth1 = 15.9 W

m2K) were

applied. As in previous studies, the authors found that higher currents work better forheating purposes compared to lower currents. They further state that 0.1 Hz and 1 Hzachieve similar results, while 10 Hz works less well. As expected, a higher thermal re-sistance allows a better heating than the insulation with better heat conductivity. Duringheating the real part of the battery impedance decreased from 0.55Ω at −20C to 0.15Ω at

12

CHAPTER 2. FRAME OF REFERENCE

+5C The authors also mention that the heat distribution in the cell is very uniform andstate that if the battery is discharged, the risk of lithium plating is low.

Comparison of Heating Methods

Ji and Wang [31] compare three internal and one external heating method. The authorscall the internal methods self-internal-, mutual pulse- and AC-heating. The externalmethod is called convective heating. All data is gained from simulations. The authorscompared the time that was necessary for heating the battery from −20C to +20C.

Self-internal heating can be compared to DC discharge, where the battery simply powersan electrical load and heats up from the dissipated power in the battery impedance.

For mutual pulse heating the battery is divided into two parts, interconnected with abuck-boost converter. Energy is cycled back and forth between the two parts andheat is dissipated in the internal impedance.

For AC-heating an external power source is connected to the battery and injects AC. Thebattery heats up from the dissipated power of the impedance.

During convective heating the battery powers an external ohmic resistance and a fan.In this case the battery heats from in- and outside due to internal losses and heatedambient.

For self-internal heating the authors simulate different discharge rates and find that thehigher the current, the faster the temperature rise. More precisely they found the rise timefrom −20C to +20C to be 132 s / 228 s / >420 s for 2C / 3C / 4C-rate respectively.

In the case of mutual pulse heating the authors only give voltage levels for the dis-charge pulse and no C-rates. However, the pulses are of 1 s length and it is stated, thatthe more the discharge voltage level differs from the cell voltage, the higher the currentand the faster the temperature increase. For 2.2V / 2.4V / 2.8V discharge voltage it took80s / 120s / 204s respectively to heat from −20C to +20C

Considering AC heating the authors assume an AC-voltage source with a 1 V sinu-soidal amplitude around the cell voltage. Different frequencies are investigated and, incontrast to other studies, it is found that higher frequency heats the battery faster. It isstated that the dissipated power follows Pdiss =

U2RMS

Rand with constant voltage and in-

creasing frequency the real part of the battery impedance decreases, which maximizesthe power. This is in fact not applicable, since in reality the limiting factor usually is themaximum available current. It then follows Pdiss = i2RMSR and with constant current it isobvious that the dissipated power decreases with increasing frequency. With AC-heatingthe temperature rise time found was 80s / 175s / 270s / 276s / 340s for 1kHz / 60Hz /1Hz / 0.1Hz / 0.01Hz. As in [30] 1 Hz and 0.1 Hz show barely any difference.

Convective heating was carried out at different power levels. The total power wasregulated with the ohmic resistor, which produces ohmic heat, while the fan to convectthe air was assumed to take 3 W constantly. The different power levels added up to 3.9C/ 2.9C / 2.3C, which resulted in a heating time of 80s / 140s / 200s respectively.

In a comparison, the required energy for heating is compared and how much thebattery-self powered methods have decreased the SOC. Obviously for the AC-heating

13

2.2. BATTERY HEATING PROCEDURES

method no discharge rate can be found, since the power comes from an external source.The authors found the following decrease of the SOC level for the three methods:

• self-internal heating: 14% - 22.5%, depending on the discharge rate (4C - 2C)

• mutual-pulse heating: 5%

• convective heating: 7.5%

The authors find the mutual-pulse heating to be most suited as a battery self-poweredheating method. That is not only due to the fewest power consumption but also due tothe more uniform heat distribution. For the AC-heating method the authors suggest 60Hz frequency. Even though it did not deliver the fastest rise time, they state that it wouldbe most easy to implement, since the grid has the same frequency and therefore only thevoltage level would have to be adjusted.

Vlahinos and Pesaran [32] compare six different heating methods, which are listedbelow. The authors created several simulations and then compared the different methodsregarding their heat uniformity, energy demand and rise time. The battery pack wasconstructed of six modules. Each module was encapsulated in a plastic module. Allmodules were held together by another plastic case.

• Internal core heating: Heating the battery core with dissipative electric power fromconstant discharge

• External jacket heating: Using an ohmic heater, that surrounds each module

• Internal jacket heating: same as external, but ohmic with an ohmic heater aroundeach cell

• internal fluid heating: heating with a warmed fluid, that flows around the cells

• AC-heating: same as internal core heating, but with alternating current (only brieflydiscussed)

As shown in table 2.1 the authors simulated all heating methods with the same mag-nitude of energy input and compared the temperature rise after 600 s. It can be seen thatsome methods deliver a more uniform heat distribution among the cells compared withothers.

The authors state that the core and internal heating methods result in the most uni-form heat distribution. The internal core heating showed the fastest temperature rise andwas also among the most efficient in terms of energy usage, together with internal jacketheating and internal fluid heating. The authors also found that it is possible to heat withan external AC source.

14

CHAPTER 2. FRAME OF REFERENCE

Method 1.09 Wh 2.90 Wh 4.71 Wh 6.53 WhInternalcore h.

−35C −26C −17C −8C

Externaljacket h.

-

min:−37.8C

max:−37.3C

-

min:−32C

max:−24C

Internaljacket h.

min:−38C

max:−37C

min:−34C

max:−32.5C

min:−30C

max:−28C

min:−27C

max:−23C

Internalfluid h.

−38C −34.5C −30.5C −27C

Table 2.1: Resulting temperatures using different heating methods with different energyinputs with data from [32]

2.3 Battery Modelling

As discussed in the beginning, a simulation model is constructed as well in this work. Itis possible to capture the electrical dynamics of a battery with an equivalent circuit basedon passive components and a voltage source within certain boundaries. Many differentequivalent circuits are proposed in literature, all with different advantages. Rahmoun andBiechl [33] and He, Xiong, and Fan [34] have compared several models with respect toaccurate prediction of the batteries electrical behaviour and simplicity of the model. Theyfound that the most simple model (Rint-model) is not capable of capturing AC dynamicsproperly. Further, they both conclude that the double polarization model gives a goodbalance between simplicity and accuracy within the desired bandwidth.

RE

UOCV UBat

(a) Rint model

RE

UOCV UBat

RC1 RC2

C1 C2

(b) Double polarity model

Figure 2.1: Equivalent circtuits of a battery

15

Chapter 3

Implementation

In this chapter, the implementation of the practical work is explained. That covers theelectrochemical impedance spectroscopy, the construction of the setup and the realizationof the simulation.

3.1 Methodology

The goal is to prove that it is possible to internally heat the chosen LIBs by injecting AC.Despite several studies already have proven this concept to work (compare section 2.2.2)it is of interest to investigate further under worse conditions to generalize this proce-dures validity. Many studies used the very common cylindrical cell type 18650 (r=9.3mm,h=65.2mm) which have an approx. 20 times lower volume than the cell used in this study(see dimensions in table 3.1). Furthermore, Zhu et al. [29] found the real part of their cellimpedance varied between 0.02Ω and 0.35Ω and Zhang et al. [30] found it even to be be-tween 0.15Ω and 0.55Ω. The cells in this study show a much lower impedance as can befound in section 4.1.1. While a low battery impedance paired with high specific capacityis desirable for normal operation as it increases both energy density and operating effi-ciency, it is disadvantageous in terms of heating purposes. In this study two lithium-ioncells of the same kind are tested.

3.2 Battery Specifications

The type used in this study is LiNi 13Mn 1

3Co 1

3O2 (NMC) [18] [35]. The active material at

the positive electrode is Ni 13Mn 1

3Co 1

3O2, the material at the negative electrode is graphite

[12]. Unlike the common 18650 type batteries, the batteries in this study are of prismaticshape. The dimensions and further technical data is provided int table 3.1. More detailedinformation about the impedance is presented in section 4.1.1. Two of these batteries areused and all measurements are done on each battery individually. The batteries are calledSS18 and SS20.

16

CHAPTER 3. IMPLEMENTATION

Parameter ValueTechnology LiNi 1

3Mn 1

3Co 1

3O2

Nominal voltage 3.67 V (@ 40% SOC)Voltage range 3.45 V - 4.13 VCapacity 28 AhMax. charge current 104 A (3.7C)Max discharge current 330 A (11.8C)Weight 0.757 kgLength x width x height 148.6x 27.0x 91.6mm3

Operating temperature −40C to +70C

Table 3.1: Specifications of the lithium-ion battery used in this study

Figure 3.1: Battery with connections and part of insulation

As the electrodes are swelling when lithium intercalates into them, the batteries needto be compressed. This is done with each battery individually with two pressure platesof steel. Since steel has a high thermal capacity, two wooden press boards are insertedbetween the battery and the pressure plates to thermally insulate the battery and be ableto observe its true temperature response.

Inside the battery there are three K-type thermocouples integrated: One beneath thepositive terminal, one between the two electrodes in the center of the cell, one in thecenter of the cell but between one electrode and the outer wall of the battery. A fourthtemperature sensor is of Pt100 type and mounted outside of the battery on the negativeterminal.

The electrical connections at the terminals are, as follows, from bottom to top andcan be compared in fig. 3.1. Negative terminal: live ring cable lug, electrically isolatingmaterial, ring cable lug with Pt100, ring cable lug for voltage sensing, washer, nut. The

17

3.3. ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY

positive terminal is connected similarly but without the cable lug for temperature sensing.There are five 6mm2 stranded cables connected to the live cable lug to allow high currents.

In order to have more realistic temperature dynamics the batteries are insulated in socalled expanded poly ethylene (EPE) foam with thermal conductivity of 0.036 W

mK[36].

3.3 Electrochemical Impedance Spectroscopy

The electrochemical impedance spectroscopy is a measurement technique that measuresthe battery impedance as function of frequency. This data can either be presented inmagnitude and phase or as real and imaginary part of the impedance. Here, a ZahnerIM6 KG [37] was used to do the measurement. To achieve high accuracy, the so calledfour-terminal sensing method is applied. In this method, there are different cables forcarrying current and sensing voltage. Therefore a current induced voltage drop overthe two current carrying cables is not influencing the voltage measurement. Addition-ally, the cable pairs are twisted to minimize electromagnetic interference. The batteriesare placed inside a climate chamber with temperature control. Additionally, the cham-ber temperature is monitored, but not recorded, with a redundant thermometer insidethe test chamber. Before the measurement is started, the batteries are mounted into thepressure plates, charged to their nominal voltage of 3.67 V (+/- 0.1 V) and kept in thiscondition over the whole procedure. The frequency span is chosen from 100 mHz to 10kHz. A lower limit would have prolonged the measurements unnecessarily since severalcycles at each frequency are undergone during one measurement. The meaningfulness ofa higher upper limit would have been questionable as the battery has a very low induc-tance which is very difficult to capture, since the cables also show inductive behaviourat high frequencies, even with the taken precautions. There are seven measurements atdifferent temperatures done with each battery individually. Before one measurement isexecuted, the batteries are kept at the specific temperature for at least two hours to allowthe temperature distribution become homogeneous in the whole body.

The measurement result is presented in section 4.1.1.

3.4 Experimental Setup

The experimental setup is adapted from a more extensive study, done by Bessman et al.[38] and Soares et al. [39]. In fig. 3.2 the alignment of the following units is visualized.

• Current Ripple Generator (CRG)

– Debugger for CRG– Power Supply (24 Vdc) for CRG

• Computer with LabView

• NI cDAQ 9174 - chassis for compactDAQ I/O modules

– NI9861 - compactDAQ CAN module– NI9216 - compactDAQ Voltmeter module

18

CHAPTER 3. IMPLEMENTATION

– NI9217 - compactDAQ Thermometer for PT100 module

• Agilent 34970A - Multimeter for thermocouple temperature measurement

– Agilent 34902A - 16 channel Multiplexer card

• Power Supply (230 Vac) for Computer, cDAQ and Multimeter

• First order filter with 14.6µH inductance

• Lithium-ion battery - 28 Ah prismatic cell

– PT100, 1x - Temperature sensor, mounted at negative terminal– K-type, 3x - thermocouple temperature sensor, mounted inside the cell

• Lead-acid battery (12 V), 2x - power buffer for DC/AC converter

• TDK Lambda Z+ (0-160 Vdc, 0-4 A) - power supply for DC/AC converter

CA

NH

& C

AN

L

4 x

USB

Debugger

Lead-Acid

Battery

+ -

Li-Ion Battery

+ -

PT 100

PC

NI cDAQ 9174

CA

N –

NI9

861

12x

Vo

lt. –

NI9

215

4x T

emp

. – N

I92

17

CRG

DC/AC converter

TI µC

Power Supply 24 VDC

(adjustable)

+ T

Power Supply 24 VDC

+ T

Power Supply

230 VAC

~ T

Filter

L1 L2 L3

Agilent

34970A

K 3 x

2 x

Figure 3.2: Schematic overview of the experimental setup

The Current Ripple Generator (CRG) controls the current applied to the LIB. The currentcan be set to AC, DC or a combination of both, from 0 Hz –1.5 kHz with sine, triangleor rectangular waveform. The desired current is sent to the CRG via the controller areanetwork (CAN) protocol, together with the propotional integral (PI) controller values kpand ki. The CRG is connected to the 24 Vdc power supply, debugger, controllable powersupply TDK Lambda Z+ and lead-acid battery, filter and the NI cDAQ. On its TI micro-controller runs C-code which can be flashed with the debugger. The C-code consists oftwo loops. The faster loop, the current controller, runs at 70 kHz rate and follows theinternal current reference by controlling the pulse width modulation (PWM) duty cycle.

19

3.4. EXPERIMENTAL SETUP

It acts after the principle read-compute-act, where in this case "reading" means measur-ing the present current, "compute" means performing the control loop computations and"act" means adjusting the PWM duty cycle accordingly. There is only one PWM signalwhich all three branches of the three leg inverter follow at the same time. This simulta-neous operation is necessary to enable higher currents than in a single leg. The convertertransforms the connected constant voltage from TDK Lambda Z+ and lead-acid batteryinto an alternating current for the LIB.

The two lead-acid batteries and the TDK Lambda Z+ power supply are connected in par-allel. One reason is to be able to allow backward currents that can flow back into thelead-acid battery and would not be allowed by the power supply. The other reason is tobe able to temporarily provide high currents while still maintaining a stable voltage. Thepower supply is connected to the PC via USB. This supports remote control of currentand voltage limitation.

The 1st order low-pass filter transforms the rectangular, high-frequency PWM signalinto the actually desired sine waveform. It is made of an in-line inductor. Initially, a 2ndorder filter with additional capacitor after the inductor was planned to have even betterfiltering and smoother waveform. That was not possible, since the current in the capacitoracted as additional load for the inverter and caused a current error in the CRG during atest run.

The rack NI cDAQ exchanges data with the computer during the experiment via USB.Three modules are mounted into the rack: NI9861 for CAN communication with theCRG, NI9216 for voltage measurement and NI9217 for temperature measurement withthe PT100 on the negative terminal of the LIB.

CAN communication is supplied with the same 24 Vdc as the CRG is. The two wiresterminated with a 120Ω resistance at each end.

The voltage measurement is done with two channels, one for each battery. In this wayit is not necessary to switch the cables when switching between the batteries.

The temperature measurement is done with a four terminal sensing in two channels.The current source is internal and supplies 1 mA.

The Agilent 34970A is a high precision multimeter and used for temperature measure-ment in this study. It is necessary to use a high precision multimeter since the signal of thethermocouples is in the range of microvolts which is several magnitudes less than the er-ror of the NI9216 voltmeter unit. Furthermore it delivers a temperature directly instead ofa voltage. The 16 channel multiplexer Agilent 34902A allows simultaneous measurementof all six thermocouples from the two LIBs and accounts for the voltage error inducedfrom the cable to the clamp of the multiplexer.

The computer (PC) has two purposes in this setup. Software for the CRG can be mod-ified with Code Composer and flashed on the CRG. Additionally, the correct functioningof the CRG can be monitored in the debugger mode. Further it runs LabView which isthe interface to the setup. All experimental parameters can be monitored, adjusted andrecorded. When started, it initializes the USB communications to power supply TDK

20

CHAPTER 3. IMPLEMENTATION

Lambda Z+, rack NI cDAQ and multimeter Agilent 34970A. After initialization it runsin an endless loop until stopped. In this loop it performs several functions which can besummarized as follows. When the program is stopped, the loop is exited and the com-munication ports are closed.

• Read

– from NI cDAQ– from Agilent 34970A– TDK Lambda Z+– input values from user interface

• Check

– if batteries are inside voltage limitation– for CAN-error (error in CRG)

• Decide if operation is safe to continue

• Compute necessary values

• Send

– to NI cDAQ– to TDK Lambda Z+

The recorded data is:

• Date, time

• CAN data

• Battery Temperature from thermocouples

• Battery voltage

• Battery temperature from Pt100

The LIBs are placed in a climate chamber during the entire experiment. The climatechamber runs during the whole experiment and maintains −20C. Since the batteries areinsulated, at least 10 h passed between end of one experiment and start of the next. Thealignment can be seen in fig. 3.3. The temperature has to be set manually and is crosschecked with a separate thermometer inside the chamber.

3.5 Simulations

It is of general interest to have a simulation model to reduce the effort of building a testsetup. In this case the heating effect of the battery shall be predicted under differentcircumstances. Therefore, an electrical and a thermal model of a LIB need to be designedand connected. In this way the dissipative power of the ohmic elements in the electricalmodel are used as input for the thermal model.

21

3.5. SIMULATIONS

Figure 3.3: Insulated batteries inside climate chamber

3.5.1 Electrical ModelRE

UOCV UBat

RC1 RC2

C1 C2

L

Figure 3.4: Applied equivalent circuit model

The modelled impedance Zmodel is con-structed of one ohmic resistor RE , one in-ductor LE and two R/C-branches RC1/C1

and RC2/C2 in series as shown in fig. 3.4.Further there is an open circuit voltage(OCV) UOCV and a battery voltage UBat.The battery voltage and OCV are equal ifthere is no load connected to the battery. Ifthere is a load connected then UOCV > UBat

and if the battery is charged then UOCV < UBat. As presented in section 4.1.1 the measure-ments show an inductive part. That is why an inductor is added to the equivalent circuitfrom section 2.3. The measured impedance in (3.1) consists of a real and imaginary partR and jX . The imaginary part can be positive or negative if the impedance is partlyinductive or partly capacitive.

ZEIS = R + jX (3.1)

Zmodel = RE + jωLE +RC1

1 + jωC1RC1

+RC2

1 + jωC2RC2

(3.2)

The electrical model is obtained with help of an optimization function. In contrast toLi et al. [40], in this study the simple absolute difference instead of the squared differencebetween measured and modelled impedance is taken into account, as it is presented in(3.3).

22

CHAPTER 3. IMPLEMENTATION

min fopt(ω) =ωmax∑ωmin

|Zmodel − Zeis| (3.3)

With fopt(ω) as optimization function to be minimized within the frequency rangefrom the lower frequency ωmin to the upper frequency ωmax, Zmodel as impedance of theequivalent circuit and Zeis as measured impedance with real and imaginary part fromthe EIS. This procedure is implemented in Matlab with the fmincon() function [41] andrepeated 14 times, once for each battery at each temperature, with the limitation, that thecomponent values can only be positive. Each of the 14 iterations produces a set of thoseelements which fit the measured impedance of one cell at a specific temperature. The fitis done in a range between 10 Hz and 10 kHz. This is to improve matching of the modelwith the measured impedance by looking only at a limited frequency range.

3.5.2 Thermal Model

In order to model the temperature behaviour, the following thermal model consistingof heat flow (ohmic losses in the battery), thermal capacity (battery body) and thermalresistor (insulation). The heat flow splits into two streams: one to heat the battery body(thermal capacity) and one that flows over the thermal resistor to the cold ambient air(thermal resistor) [31].

q = Cthd∆T

dt+

1

Rth

∆T (3.4)

With q as heat generation rate, Cth as total thermal capacity of one battery which isconstant over temperature, ∆T as temperature difference between ambient and cell andRth as thermal resistance of the insulation. To determine the thermal resistance, finaltemperature, applied RMS-current and mean real impedance from the experiment areinserted in (3.4) and the dynamic part neglected. This can be verified by analyticallycomputing the thermal resistance from material properties of the insulation material andthe dimensions of the box built from it.

For a state space model the temperature difference is assumed as state. The input isheat flow and equal to the dissipated electric losses which originate solely from the realpart of an impedance. This real part of the impedance is a function of frequency andtemperature.

q = Pdiss = Re(Z) · i2rms (3.5)

With Pdiss as dissipated power from a circuit, Re(Z) as ohmic part of an impedance andirms as RMS current.

Combining and rearranging (3.4) and (3.5) leads to the state space model as presentedin (3.6).

d∆T

dt= − 1

CthRth

∆T +1

Cth

Re(Z) · i2rms (3.6)

With A = − 1CthRth

and B = 1Cth

.Both measured and modelled impedance are implemented in two different electro-

thermal models. One approach is to build the circuit in the PLECS blockset in simulink.

23

3.5. SIMULATIONS

In this blockset it is possible to include passive components in a thermal environment toobtain the dissipated power. The model automatically transforms these electrical lossesinto a heat generation rate. Another approach is to create a state space model as done in(3.6) and simulate in simulink. The two different simulation environments under usageof two different circuits lead to four individual simulation models. They are comparedwith respect to accuracy and computational effort in section 4.1.3.

3.5.3 Temperature Controller

The most accurate and efficient simulation model is extended with a simple PI tempera-ture controller. Two types of batteries are compared: a single battery and a battery pack.Data for the battery pack is extrapolated from the single battery. It is assumed that all cellswith mean voltage of 3.2 V are connected in series to reach 600 V. This 186 cells wouldhave 16.7 kWh storage capacity which is comparable to the installed capacity in the fullyelectric e.GO Life with 120 - 170 km range [42] 1. All cells reach a combined thermal ca-pacity of 130.2 · 103 J

K. The cells are assumed to be aligned such to fit in a car and to reach

the minimum possible surface area as a pack.From (3.4) follows the transfer function G(s) in (3.7) with heat flow which is equal to

ohmic losses (compare (3.5)), as input and temperature increase as output. This meansthat the PI-controller gives power as a control variable. This can again be controlled withan additional current controller which ensures the correct current for the desired power.The entire block diagram can be found in appendix A. Normally the current controller isfast and accurate enough to be neglected in slow dynamic systems. Therefore, only thetemperature controller is implemented here. Both, transfer function (3.7) and controller(3.8) are connected in a closed loop system Gc as in (3.9).

G(s) =Rth

sCthRth + 1(3.7)

F (s) =kps

+ ki (3.8)

Gc(s) =F (s)G(s)

1 + F (s)G(s)=

α

s+ α(3.9)

α is the bandwidth and corresponds directly to the rise time tr: α = ln(9)tr

. With α chosento be 3 min, following relation for the control gains are found.

kp =ln(9)Cth

180

ki =ln(9)

180Rth

These of course vary if a single battery or a battery pack is considered. Furthermore, tohave a more realistic scenario, the current is limited to 100 A as otherwise cabling wouldbecome too bulky in a real application [43]. In order to maintain proper control, evenif the current is limited for a long time, anti-windup functionality is included, too. Thesimplified model without current loop is presented in fig. 3.5.

1Compact model Life from the German startup E.GO mobile AG, located on the campus of the RWTHAachen

24

CHAPTER 3. IMPLEMENTATION

Battery

Controller

anti-windup

dT/dt deltaTheatpowerProduct

1s

Integrator

-K-

thermalcapacity

-K-

thermalinsulation1s

Integrator1

-K-

K_p

-K-

K_i

T_ref

T_ref

1-DT(u)

EIS:Re(Z(1kHz))

-C-

T_amb

max.100A

-K-

1/K_p

-K-

#cells

bat_temp

ToWorkspace

1s

Integrator2

power

ToWorkspace4

energy

ToWorkspace5

current

ToWorkspace1

heatflowbattery->ambient

Batterytemp.ctrlcurrent

totalohmicresistance

Figure 3.5: Model of battery pack with controller

25

Chapter 4

Results

In the first part of this chapter, the measurement results of the EIS and the experimentare presented as well as a comparison between the simulation and the experiment s pre-sented. In the second part, the results are analyzed and discussed.

4.1 Measurements

4.1.1 Electrochemical Impedance Spectroscopy

There are several ways of displaying the measured battery impedance. Each one is suitedto point out a specific detail of the measurement. Firstly, the Bode graph in fig. 4.1 pro-vides an overview of the absolute impedance’s variation with temperature with respectto frequency. Secondly, the Nyquist graphs present the complex impedance at differenttemperatures: In fig. 4.2 −30C, −20C, −10C and in fig. 4.3 0C, +10C, +20C, +30C.Finally, the surf graphs fig. 4.4 and fig. 4.5 show the isolated real and imaginary partsrespectively, with respect to both temperature and frequency.

As presented in the Bode graph in fig. 4.1, the total impedance changes differently atcertain frequencies with temperature. The largest temperature impact can be observedat frequencies below 100 Hz. Here, the impedance increases by several degrees of mag-nitude from +30C to −30C. With increasing frequency, the impact of temperature onimpedance decreases. Above 1 kHz barely any difference between low and high temper-atures can be observed. From the phase plot it can be seen that temperature impact onlytakes place for negative phases, which indicate a capacitive impedance. The zero phasehappens at lower frequencies for high temperatures ( 300 Hz at +30C) and at higher fre-quencies for low temperatures ( 1300 Hz at −30C). Furthermore, minimum impedanceshows a similar temperature behavior as the zero phase, although they do not occur at thesame frequency. The minimum impedance at +30C occurs at 1 kHz and the minimumimpedance at −30C occurs at 2 kHz.

26

CHAPTER 4. RESULTS

10-1 100 101 102 103 104

Frequency (Hz)

10-4

10-3

10-2

10-1

1

ab

s I

mp

ed

an

ce

Z (

)

10-1 100 101 102 103 104

Frequency (Hz)

-90

-60

-30

0

30

60

90

Ph

ase

(°)

SS18@30°C

SS18@20°C

SS18@10°C

SS18@0°C

SS18@-10°C

SS18@-20°C

SS18@-30°C

SS20@30°C

SS20@20°C

SS20@10°C

SS20@0°C

SS20@-10°C

SS20@-20°C

SS20@-30°C-30°C

-20°C

-10°C

0°C

10°C

20°C

30°C

Figure 4.1: Bode graph of battery impedance

The impedance increase at lower temperatures becomes more obvious on the linearscale of the Nyquist graph in fig. 4.2. For low temperatures, the bow on the negativeimaginary plane is so dominant, that only −30C, −20C, −10C can be shown properlyin this figure. The bow is similar to a parallel R/C branch half-circle, but compressed.From 0C to −10C, from −10C to −20C and from −20C to −30C, the bow triples insize with every 10C temperature decrease.

0 0.03 0.06 0.09 0.12 0.15

Re(Z) ( )

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

-Im

(Z)

()

SS18 @30°C

SS18 @20°C

SS18 @10°C

SS18 @0°C

SS18 @-10°C

SS18 @-20°C

SS18 @-30°C

SS20 @30°C

SS20 @20°C

SS20 @10°C

SS20 @0°C

SS20 @-10°C

SS20 @-20°C

SS20 @-30°C

-30°C -20°C -10°C 0°C 10°C 20°C 30°C

Figure 4.2: Nyquist graph of battery impedance for cold temperatures

A similar compressed semi-circle can be observed at higher temperatures in fig. 4.3.

27

4.1. MEASUREMENTS

It can be observed that after the imaginary part reaches a minimum on the right end ofthe bow it increases again. The mismatch of zero phase and minimum real part of theimpedance can be observed in this figure as well. At the point, where the imaginary partbecomes zero, the real part is not yet at its minimum. The minimum real has its minimumslightly in the positive imaginary plane. This minimum is in the range of 0.7mΩ at +30C

and 1.5mΩ at −30C. This is remarkably small compared to other studies, as mentionedin section 2.2.2. In the positive imaginary plane, the inductive part shows the same shapebut at different offsets.

0 1 2 3 4 5 6 7

Re(Z) (m )

-3

-2

-1

0

1

2

3

-Im

(Z)

(m)

SS18 @30°C

SS18 @20°C

SS18 @10°C

SS18 @0°C

SS18 @-10°C

SS18 @-20°C

SS18 @-30°C

SS20 @30°C

SS20 @20°C

SS20 @10°C

SS20 @0°C

SS20 @-10°C

SS20 @-20°C

SS20 @-30°C

-30°C -20°C -10°C 0°C 10°C 20°C 30°C

Figure 4.3: Nyquist graph of battery impedance for higher temperatures

The real part of the impedance shows partly high temperature dependence and partlylow temperature dependence, as shown in fig. 4.4. Similarly to the previous figures,the graph can be divided at around 1 kHz. Above this frequency, the real impedanceis nearly temperature independent. The minimum is located slightly above 1 kHz andrises marginally towards 10 kHz. Below 1 kHz, the real part shows increasing temper-ature dependence with lower frequency. From little temperature variance at 1 kHz, thereal part changes from 1mΩ at 30C to more than 100mΩ at −30C at 100 mHz.

28

CHAPTER 4. RESULTS

-30-20

-10

Tempe

ratu

re (°

C)

010

20

10-3

Frequency (Hz)

10-2

Re(Z

) (

)

10-1 301 10

1

10-1

102

103

104

Figure 4.4: Surf graph of real part of battery impedance

In fig. 4.5 capacitive (negative) and inductive (positive) imaginary part are split upto left hand and right hand side on the figure due to the zero-crossing, which cannot bedisplayed on a logarithmic scale. Above 1 kHz the imaginary part becomes inductiveand increases with increasing frequency. The inductive part does not show a temperaturedependence. However, the capacitive imaginary part on the left hand side shows evenmore temperature dependence than the real part of the impedance. Similar to the realpart, the capacitive imaginary part changes more and more with temperature, the morethe frequency is below 1 kHz. The most significant change can be observed at 1 Hz,where the absolute capacitive imaginary part rises from magnitude 10−4 at +30C to tomagnitude 10−1 at −30C.

29

4.1. MEASUREMENTS

-10-1

-10-2

30

-10-3

-10-4

Im(Z

) (

)

-10-5

2010

Tem

p. (°C

)

Capacitive

0-10

-20 104

Freq. (Hz)

103

102

101-30 1

10-1

0

1 10-3

30

2 10-3

Im(Z

) (

)

3 10-3

2010

Tem

p. (°C

)

Inductive

0-10

-20 104

Freq. (Hz)

103

102

101-30 1

10-1

Figure 4.5: Surf graph of imaginary parts of battery impedance

For a better overview, the impedance change over temperature and frequency is pre-sented qualitatively in table 4.1.

−30 to −10C −10 to +10C +10 to +30C

R XC XL R XC XL R XC XL

10−1 to 10 Hz ↓ → – ↓ – → –10 to 103 Hz ↑ – ↑ – → –103 to 104 Hz – ↑↑ – ↑↑ – ↑↑

Table 4.1: Qualitative overview of the temperature behavior of the real (R), imaginarycapacitive (XC) and imaginary inductive (XL) part of the measured impedance.

4.1.2 Experiment

Both cells are tested at nine operating points, all at −20C. Three current rates (0.5 C, 1 Cand 1.25 C), each one with three different frequencies. That was to identify which factor,frequency or current amplitude, has what kind of impact on battery internal heating.Lithium plating must be avoided, therefore the lowest AC-frequency was chosen to be100 Hz. Since, this study is designed as a critical case study, the second AC frequencywas chosen to be 1 kHz. That is, where the total impedance, but also the real part, whichcreates dissipative losses, have their minimum. For comparison, the batteries are alsodischarged at the corresponding C-rate to the applied RMS current in the AC-case.

Despite the fact, that great care was taken to create the insulation layer for the batteries,they turned out to be quite different. That was observed during cooling, when batterySS20 cooled faster than battery SS18. As a result of that, the temperature development

30

CHAPTER 4. RESULTS

is presented individually for each battery. Due to safety reasons, the batteries are notcharged and discharged to their absolute limits. Therefore, discharging at 0.5C, 1C and1.25C happens in slightly less time than the corresponding 2 h, 1 h or 48 min, whichwould result by definition of the C-rate.

Fig. 4.6 and fig. 4.7 show the warmup at 0.5 C rate, which corresponds to 14 A. NeitherSS18 nor SS20 heated up with AC, independent if 100 Hz or 1 kHz was injected. How-ever, pure discharge at the same current rate increases the temperature in both batteriesidentically by 12.5C from −20C to −7.5C in approx. 110 min.

0 20 40 60 80 100 120 140 160

Time (min)

-25

-20

-15

-10

-5

0

5

Ba

tte

ry T

em

pe

ratu

re (

°C)

SS18 - 0.5C

0 Hz

100 Hz

1k Hz

Figure 4.6: SS18 warmup during experiment at 0.5 C

31

4.1. MEASUREMENTS

0 20 40 60 80 100 120 140 160

Time (min)

-25

-20

-15

-10

-5

0

5

Ba

tte

ry T

em

pe

ratu

re (

°C)

SS20 - 0.5C

0 Hz

100 Hz

1k Hz

Figure 4.7: SS20 warmup during experiment ay 0.5 C

The heating effect of 1 C current (28 A) is presented in fig. 4.8 for SS18 and in fig. 4.9for SS20. 1 kHz raises the temperature by 3.0C and 2.3C in SS18 and SS20 respectively.100 Hz works slightly better. In SS18 the temperature rises by 4.2C to −15.8C and inSS20 by 3.0C to −17C. All after approx. 120 min. Discharge heats the batteries slightlyabove (SS18: 1.0C) or slightly below (SS20: −0.8C) 0C within 50 min.

0 20 40 60 80 100 120 140 160

Time (min)

-25

-20

-15

-10

-5

0

5

Ba

tte

ry T

em

pe

ratu

re (

°C)

SS18 - 1C

0 Hz

100 Hz

1k Hz

Figure 4.8: SS18 warmup during experiment at 1 C

32

CHAPTER 4. RESULTS

0 20 40 60 80 100 120 140 160

Time (min)

-25

-20

-15

-10

-5

0

5B

att

ery

Te

mp

era

ture

(°C

)SS20 - 1C

0 Hz

100 Hz

1k Hz

Figure 4.9: SS20 warmup during experiment at 1 C

The highest temperature increase can be observed with 1.25 C current (35 A), as shownin fig. 4.10 for SS18 and in fig. 4.11 for SS20. As in the previous cases SS18 warms upbetter. Its temperature increases by 5.3C to−14.7C with 1 kHz and by 6.6C to−13.4C

with 100 Hz within 140 min. In the same time, the temperature in SS20 increases by3.3C to −16.7C with 1 kHz and by 4.5C to −15.5C with 100 Hz. Discharging rises thetemperature within 40 min to +4C in SS18 and to +2.3C in SS20.

33

4.1. MEASUREMENTS

0 20 40 60 80 100 120 140 160

Time (min)

-25

-20

-15

-10

-5

0

5

Ba

tte

ry T

em

pe

ratu

re (

°C)

SS18 - 1.25C

0 Hz

100 Hz

1k Hz

Figure 4.10: SS18 warmup during experiment at 1.25 C

0 20 40 60 80 100 120 140 160

Time (min)

-25

-20

-15

-10

-5

0

5

Ba

tte

ry T

em

pe

ratu

re (

°C)

SS20 - 1.25C

0 Hz

100 Hz

1k Hz

Figure 4.11: SS20 warmup during experiment at 1.25 C

34

CHAPTER 4. RESULTS

4.1.3 Simulation Model

Equivalent circuit

All components of the equivalent circuit vary greatly with temperature except for theinductive part and the second capacitor. As shown in fig. 4.12 the inductance is within30− 45nH . The series resistor RE increases from 0.9mΩ at +30C to 2.2mΩ at −30C. It isworth mentioning that the series resistance increases exponentially, which causes a fasterincrease below 0C.

-30 -20 -10 0 10 20 30

Temperature (°C)

0

0.5

1

1.5

2

2.5

Re

sis

tan

ce

(m

)

RE

SS18

SS20

mean

-30 -20 -10 0 10 20 30

Temperature (°C)

0

10

20

30

40

50

Ind

ucta

nce

(n

H)

L

SS18

SS20

mean

Figure 4.12: Series resistance and inductance

Fig. 4.13 shows the behavior of the second RC-branch. The parallel resistance RC1,similarly to RE , increases with decreasing temperature, the parallel capacity C1 increasesmoderately from−30C to +10C and dramatically above. At +30C,RC1 is at 0.3mΩ andat −30C it is at 0.135Ω. Also similar to RE , RC1 increases exponentially, which meansslow resistance change at positive temperatures and fast at negative temperatures. C1

increases on a much higher scale. At −30C it is at 2.3F and at +30C it is at 13.5103F .The behavior above +10C might seem irregular but is out of the focus of this study,where the LIBs are studied at low temperatures.

35

4.1. MEASUREMENTS

-30 -20 -10 0 10 20 30

Temperature (°C)

0

0.03

0.06

0.09

0.12

0.15R

esis

tan

ce

()

RC1

SS18

SS20

mean

-30 -20 -10 0 10 20 30

Temperature (°C)

100

101

102

103

104

Cp

acity (

F)

C1

Figure 4.13: First parallel R-C branch

While the second parallel resistance RC2 again shows similar behavior to the otherresistances, the second capacitor remains absolutely unchanged over temperature (seefig. 4.14). In addition, an exponential increase in resistance can be observed with de-creasing temperature. This means in numbers: 1.3mΩ at +30C, 58mΩ at 0C and 1.4Ω at@− 30C. The capacitance is 67.2 · 103F .

-30 -20 -10 0 10 20 30

Temperature (°C)

0

0.3

0.6

0.9

1.2

1.5

Re

sis

tan

ce

()

RC2

SS18

SS20

mean

-30 -20 -10 0 10 20 30

Temperature (°C)

104

6.72 104

105

Ca

pa

city (

F)

C2

Figure 4.14: Second parallel R-C branch

With the above mentioned components included in the equivalent circuit ((3.2)) it ispossible to create a Nyquist graph. In this way the modelled and measured impedance

36

CHAPTER 4. RESULTS

can be compared.In fig. 4.15, where the focus is on lower temperatures (−30C and −10C) it can be

seen that, with respect to the real axis, the imaginary part rises faster into the negativeimaginary plane than the actual impedance. Furthermore, the modelled impedance alsoturns earlier and decreases thereafter. This creates a more roundly shaped bow in thenegative imaginary part of the modelled impedance compared to the actual impedance.Anyhow, the half circle of a single RC-branch is compressed. The modelled impedance inthe −30C-case does not follow the dent of the measured impedance. The zero crossingat 1 kHz matches well in both cases.

Nyquist graphs with focus on higher temperatures (+10C and +20C) are shown in4.16. Here, the bows of the modelled impedance show as well a more round shape thanthe bows of the actual impedance. Even so, the difference at higher temperature can beneglected, since the focus is on low temperatures in this study.

0 1 2 3 4 5 6 7

Re(Z) (m )

-3

-2

-1

0

1

2

3

-Im

(Z)

(m) SS18 @0°C

SS18 @-30°C

SS20 @0°C

SS20 @-30°C

eq. circuit @0°C

eq. circuit @-30°C

Figure 4.15: Nyquist graph with both measured data and equivalent circuit I

37

4.1. MEASUREMENTS

0 1 2 3

Re(Z) (m )

-1

-0.5

0

0.5

1

-Im

(Z)

(m) SS18 @20°C

SS18 @10°C

SS20 @20°C

SS20 @10°C

eq. circuit @20°C

eq. circuit @10°C

Figure 4.16: Nyquist graph with both measured data and equivalent circuit II

4.1.4 Heat Simulation

Computation of the Thermal Resistance

The thermal resistance is computed as described in section 3.5. Eq. 4.1 shows the proce-dure for the purpose of computing the thermal resistance. The data is taken from the casecell SS18 - 1.25C - 1 kHz. Reason for choosing this particular case is that SS18 has betterinsulation, 1.25C caused the highest temperature increase and at 1 kHz the impedancechanges fewest over temperature.

Rth =∆T

q=

∆T

i2rmsRe(Z(T ))(4.1)

The experimental data ∆T = 5.3K, Re(Z(T )) = 1.17mΩ (mean ohmic resistance), iRMS =

35A gives the thermal resistance Rth = 3.7WK

. To verify this value the analytic method isapplied ((4.2)).

Rth =d

λAbat.box

=d

λ · 2(hl + hw + lw)(4.2)

With d as thickness of the insulation material, λ as thermal conductivity,Abat.box as surfacearea of the battery box built from the insulation material with the dimensions l - length,w - width and h - height. The box has the dimensions d = 2.5cm, l = 22cm,w = 12cm, h =

18cm. The material is EPE-foam with the thermal conductivity λ = 0.036 Wm·K . This results

in a thermal resistance of Rth = 3.96WK

.

38

CHAPTER 4. RESULTS

Simulation Results

Below, the results of the different simulations are compared. Fig 4.17 shows the temper-ature development of the experiment compared to the different simulation methods at0.5C (14Arms and 100Hz. Below the final temperature of each simulation is presented.

• experiment: −19.59C

• state space equivalent circuit: −18.93C

• state space EIS Re(Z): −18.74C

• PLECS equivalent circuit: −19.94C

• PLECS EIS Re(Z): −18.77C

0 20 40 60 80 100 120 140 160

Time (min)

-25

-20

-15

-10

Tem

pera

ture

(°C

)

0.5C @ 100 Hz

experiment

stsp eq c

stsp eis re(z)

plecs eq c

plecs eis re(z)

Figure 4.17: Model comparison

Fig. 4.18 shows the temperature development of the experiment compared to the dif-ferent simulation methods at 0.5C (14Arms and 1kHz. Below the final temperature of eachsimulation is presented.

• experiment: −19.42C

• state space equivalent circuit: −18.94C

• state space EIS Re(Z): −19.15C

• PLECS equivalent circuit: −19.95C

• PLECS EIS Re(Z): −19.15C

39

4.1. MEASUREMENTS

0 20 40 60 80 100 120 140 160

Time (min)

-25

-20

-15

-10

Tem

pera

ture

(°C

)0.5C @ 1k Hz

experiment

stsp eq c

stsp eis re(z)

plecs eq c

plecs eis re(z)

Figure 4.18: Model comparison

Fig 4.19 shows the temperature development of the experiment compared to the dif-ferent simulation methods at 1C (28Arms and 100Hz. Below the final temperature of eachsimulation is presented.

• experiment: −15.84C

• state space equivalent circuit: −16.0C

• state space EIS Re(Z): −15.33C

• PLECS equivalent circuit: −19.78C

• PLECS EIS Re(Z): −15.23C

40

CHAPTER 4. RESULTS

0 20 40 60 80 100 120 140 160

Time (min)

-25

-20

-15

-10T

em

pera

ture

(°C

)1C @ 100 Hz

experiment

stsp eq c

stsp eis re(z)

plecs eq c

plecs eis re(z)

Figure 4.19: Model comparison

Fig 4.20 shows the temperature development of the experiment compared to the dif-ferent simulation methods at 1C (28Arms and 1kHz. Below the final temperature of eachsimulation is presented.

• experiment: −16.9C

• state space equivalent circuit: −16.03C

• state space EIS Re(Z): −16.81C

• PLECS equivalent circuit: −19.8C

• PLECS EIS Re(Z): −16.74C

41

4.1. MEASUREMENTS

0 20 40 60 80 100 120 140 160

Time (min)

-25

-20

-15

-10

Tem

pera

ture

(°C

)1C @ 1k Hz

experiment

stsp eq c

stsp eis re(z)

plecs eq c

plecs eis re(z)

Figure 4.20: Model comparison

Fig 4.21 shows the temperature development of the experiment compared to the dif-ferent simulation methods at 1.25C (35Arms and 100Hz. Below the final temperature ofeach simulation is presented.

• experiment: −13.43C

• state space equivalent circuit: −14.03C

• state space EIS Re(Z): −12.99C

• PLECS equivalent circuit: −19.66C

• PLECS EIS Re(Z): −12.85C

42

CHAPTER 4. RESULTS

0 20 40 60 80 100 120 140 160

Time (min)

-25

-20

-15

-10T

em

pera

ture

(°C

)1.25C @ 100 Hz

experiment

stsp eq c

stsp eis re(z)

plecs eq c

plecs eis re(z)

Figure 4.21: Model comparison

Fig 4.22 shows the temperature development of the experiment compared to the dif-ferent simulation methods at 1.25C (35Arms and 1kHz. Below the final temperature ofeach simulation is presented.

• experiment: −14.7C

• state space equivalent circuit: −14.09C

• state space EIS Re(Z): −15.15C

• PLECS equivalent circuit: −19.69C

• PLECS EIS Re(Z): −15.05C

43

4.1. MEASUREMENTS

0 20 40 60 80 100 120 140 160

Time (min)

-25

-20

-15

-10

Tem

pera

ture

(°C

)1.25C @ 1k Hz

experiment

stsp eq c

stsp eis re(z)

plecs eq c

plecs eis re(z)

Figure 4.22: Model comparison

There are three major trends, which can be observed in every figure. First trend is theequivalent circuit in PLECS goes in the right direction but with mean error of 3.16C thefinal values are far from the measured data. Furthermore, it shows the wrong dynamics,since the temperature is only increasing in the beginning and stays constant thereafter.

The second trend is the real part from the impedance spectroscopy, which producesthe same result, independent if simulated in PLECS or the state space model. Moreover,the impedance spectroscopy data produces the most accurate results compared to theexperiment with a mean error of only 0.44C and 0.47C for the state space and PLECSsimulation respectively.

The third trend is the result of the state space model with usage of the equivalentcircuit. It results in the same temperature, independent if 100 Hz or 1 kHz is used. Onone hand this means for 1 kHz the state space model with equivalent circuit producesslightly too high temperatures and for 100 Hz it produces slightly too low temperatures.On the other hand the state space model with equivalent circuit produces good resultswith a mean error of 0.56C, considering the time frame of more than 2.5 hours.

Enormous differences were encountered in terms of the simulation time. The mostinaccurate simulation, the PLECS model with equivalent circuit, took between 17 - 18hours. With the simple lookup table of the EIS data this time could be reduced to 2 hours.Major achievements in reducing simulation time were made with usage of the state spacemodel. Independent if equivalent circuit or EIS data was used, the simulation took onlyfew seconds.

44

CHAPTER 4. RESULTS

environmentimpedance

equivalent circuit EIS Re(Z)

PLECSsim. time: 17h sim. time: 2h

accuracy: Worst accuracy: Best

state spacesim. time: <5s sim. time: <5s

accuracy: Good accuracy: Best

Table 4.2: Comparison of the four electro-thermal battery simulations

4.1.5 Temperature Controller

Fig. 4.23 shows the temperature increase of a battery pack compared to a single bat-tery. All data originates from simulations of the differential equation with usage of realimpedance from the EIS. The reference temperature is selected to be +5C and the cur-rent limit is set to 100Arms. The reference is chosen such that there is a safety marginto the minimum suggested operating temperature found in the literature (compare sec-tion 2.1.4). It can be seen that the battery pack reaches the reference temperature after 40min, if the ambient temperature is −10C or after 60 min if the ambient temperature is−30C. On the other side it is difficult to reach the reference temperature with a singlebattery. Even after 120 min it is not possible to reach the desired +5C if the ambienttemperature is −30C.

0 20 40 60 80 100 120

Time (min)

-30

-20

-10

0

5

10

Tem

pera

ture

(°C

)

Pack -30°C -> +5°C

Pack -20°C -> +5°C

Pack -10°C -> +5°C

Cell -30°C -> +5°C

Cell -20°C -> +5°C

Cell -10°C -> +5°C

Figure 4.23: Temperature increase with controller in either battery pack or single battery

45

4.1. MEASUREMENTS

0 20 40 60 80 100 120

Time (min)

0

20

40

60

80

100

120

Curr

ent (A

)

Pack -30°C -> +5°C

Pack -20°C -> +5°C

Pack -10°C -> +5°C

Cell -30°C -> +5°C

Cell -20°C -> +5°C

Cell -10°C -> +5°C

Figure 4.24: Controlled current for temperature increase in either battery pack or singlebattery

Table 4.3 shows how much time and energy is needed to heat the battery pack froma certain temperature, how much power is necessary to maintain +5C and up to whichtime it is more energy efficient to maintain the battery temperature instead of letting itcool down and heating it up again. It is interesting to see that the time threshold, wheremaintaining the battery temperature at +5C is more efficient than letting the battery cooldown and heating it up again, is between 6.4 h and 8.4 h. Considering that it takes sometime for the battery to cool down from its field operating temperature to +5C, it wouldbe most energy efficient to maintain a battery temperature of +5C.

+5C +5C +5C

↑ ↑ ↑−30C −20C −10C

Energy to heat 1.12 kWh 0.74 kWh 0.36 kWhTime to heat 33 min 24 min 13 minPower to maintain 133 W 95 W 57 WTime threshold 8.4 h 7.8 h 6.4 h

Table 4.3: Time and power consumption to heat a battery pack

Overall, the state space simulations reproduce the experimental results quite well inall operating points. Therefore, it can be assumed to be correct and it can be used furtheron.

46

CHAPTER 4. RESULTS

4.2 Analysis

4.2.1 Electrochemical Impedance Spectroscopy

The EIS shows great impedance increase with reduced temperature. This is counter intu-itive if compared with metal conductors, which show better electrical conductivity withreduced temperature. Better electrical conductivity at low temperature is also happeningin LIBs. Anyhow, the proportion of the parts with better conductivity at low temperaturesbecomes irrelevant when looking at the whole impedance. The measurement clearly re-veals that the proportion of the ion conductivity and the mass transfer ability to the wholeimpedance is dominating. This is also the reason why LIBs have a reduced power outputat low temperatures. However, for the purpose of heating, high impedance, especiallyhigh ohmic impedance, is favorable.

4.2.2 Experiment

There is a different heating effect depending on both frequency and current amplitude.The difference between 100 Hz and 1 kHz can be explained by looking into the equiv-

alent circuit. The impedance of a capacitor follows xc = 1ωC

. That means with higherfrequency, the capacitor has lower impedance. Since the capacitor is in parallel with anohmic resistance with fixed impedance, more current will flow over the capacitor withincreasing frequency. In exchange for that, less current will flow over the ohmic resistor,where it would cause dissipative heat. The reason for the much higher heat generationat 0 Hz discharge is the higher charge transfer overpotential and mass transfer over po-tential. At 1 kHz, time is too short for Li+ ions to leave or join an electrode. Therefore,the chemical reaction cannot contribute any heat generation. As a result of high frequencyflow of Li+ in the electrolyte, which would cause frictional heat. At 0 Hz discharge, chargeand mass transfer overpotential is fully established with the result that the cell heats at amuch higher rate than at 100 Hz or 1 kHz.

Different heating at different current levels can also be explained with the equivalentcircuit and the charge and mass transfer over potential. The dissipated power followsPdiss = i2rmsR, which means that doubled current results in four times increased dissipatedpower. Higher current also means higher chemical reaction rate at the electrode and thushigher flow rate of Li+ ions through the electrolyte.

There is also a different temperature increase between the two batteries. This is simplydue to deviating thermal insulation. Despite striving for making the battery insulationsas equal as possible, they turned out to have slightly different thermal insulation.

In some figures, at some points, unexpected temperature progress can be observed.Independent of time, the curve shows a linear progression instead of a typical step re-sponse or even a dent, as i.e. in fig. 4.10. The green curve (100 Hz) shows linear behavioruntil 60 min. The blue curve (1 kHz ) shows a dent around 100 min. This behavior isdue to the cooling of the climate chamber. The climate chamber regulates the tempera-ture with a so called bang-bang controller. It heats or cools the air if the temperature isdetected to be at the lower or upper boundary, respectively. At all time it maintains highrate of air convection. Since the battery, even if insulated, acts as a heat source, the climatechamber dominantly cools to compensate for the heat generated by the battery. Since the

47

4.2. ANALYSIS

cooling happens at a high rate, it also affects the battery’s temperature in return.

4.2.3 Simulation

In the Nyquist graph the equivalent circuit shows a rounder shape than the actual batteryimpedance. Any parallel RC branch produces a half circle. A second RC-branch cancompress that half circle but only to a certain extent. Even more elements could form amore sophisticated shape. Anyhow, the chosen circuit is in good accordance with currentliterature and reproduces the electrical behavior of the battery properly for temperaturesabove −10C. Below −10C the dent is not captured. This means that the heat causingreal part has the same value at 100 Hz and 1 kHz. That is the reason why the differentbehavior at 100 Hz and 1 kHz cannot be reproduced in simulations.

There is also a subtle deviation for the value of the thermal resistance computed withexperimental data or material properties. This deviation is caused by the cabling, whichwas not taken into account for the computation from material properties. Here, a flawlesssurface was assumed, while the real insulation has several openings through which thecables are routed. These cables have a good thermal conductivity and are the reason forthe lower thermal insulation.

The cooling phenomenon of the climate chamber and how it affects the temperatureprogress has been discussed in section 4.2.2. This feedback coupling from climate cham-ber to battery is not taken into account in the simulation. Therefore, some discrepanciescan be observed during the battery is heated. However, at steady state, the simulationsagree very well with the experiment with only about 0.5C error.

4.2.4 Controller

The battery pack simulation of course employs extensive simplifications. Firstly, it isnot known how a battery pack is insulated. Here it was assumed that the encapsulationmaterial would have the same thermal insulating properties as EPE foam. Secondly, thereis much more material (BMS circuit board, compression frame, fans) inside the batterypack than just the cells. This increases the thermal capacity. Finally, not all cells heatin the same way. Cells located at the edge of the pack will be cooler than cells entirelysurrounded by other cells. Here it is assumed that all cells would unite to a huge body,with high thermal conductivity, so that the whole body has a uniform temperature.

Nevertheless, this simplified simulation of a battery pack with a controller showedthat even though it is not possible to heat a single lithium-ion cell it is possible to heata battery pack. In this fictional battery pack all batteries are connected in series, whichincreases the battery impedance by the number of series connected batteries. As a resultof that, the heating power increases at the same rate, even with unchanged current. How-ever, the surface area facing the ambient and therefore the heat flow towards the ambient,only increases approx. 15 times. This relation makes it possible to produce more heat inthe battery pack than is lost towards the ambient and therefore increase the temperaturesufficiently.

48

Chapter 5

Conclusion

In this chapter the results are summarized. The summary is presented such that the re-search questions are answered. The experiment agrees well with the presented litera-ture, which stated that low frequency and high current are favorable for the AC heatingmethod. On top of that, it was possible to construct a valid simulation model, that com-bines electric and thermal properties in order to predict heating of the LIBs, which wereused in this study. An extended model of a battery pack finally proved that it is possi-ble to properly heat LIBs, even if they show low impedance, high thermal capacity, largesurface area and even if the AC frequency is in a range, where the cell impedance is at itsminimum.

5.1 Answers to Research Questions

5.1.1 What are the existing methods to heat LIBs?

Several heating methods, which are currently discussed are reviewed in detail in sec-tion 2.2.2 and section 2.2.1. They can be categorized into four groups: internal and ex-ternal heating, depending on whether the source of heat is inside or the outside of thebattery; each method can be battery self-powered or externally powered. An overview isgiven in table 5.1. In literature external heating methods are considered as self-powered.It is, however, also possible to power them externally from a charger, if the car is con-nected to one.

49

5.1. ANSWERS TO RESEARCH QUESTIONS

Internal heating External heating

Selfpowered

•Additional foil as ohmicheater [25]

•Refrigerant heating withair-conditioning systemand additional ohmicelement [22]

•Buck-boost converterfrom electric motor andfilter capacitor [26]

•Heat pipes with heatblock [23]

•Mutual pulse heating [31] •Direct cabin air blow, PCScycle, refrigerant heating[24]

Externallypowered

•CVM, PCM [27]

-•AC heating 10 - 20 kHz(NiMH) [28]•AC sinusoid & rectangu-lar 1 - 600 Hz [29]•AC sinusoid 0.1 - 10 Hz[30]

Table 5.1: Overview of battery heating methods

External heating methods have the disadvantage of requiring additional components.They can either be mounted inside the battery pack or somewhere else in an electric vehi-cle. Yet, they all make the vehicle heavier and more complex. Internal heating methods onthe other side do not require additional components, but often depend on external powersources. This would make battery heating only available as pre-conditioning, if the caris connected to a charger. However, hardware in battery chargers is already equippedwith a current controller, which might be adjustable for injecting AC, too, by upgradingthe software functionality. Only few methods discussed in literature combine internalheating with self powering. Anyhow, since internal heating gives advantages for reduc-ing both complexity and and weight of vehicle batteries, self-powered internal heating isnecessary to cover both scenarios: when the car is connected to a charger and when it isnot.

5.1.2 To what extent is it possible to heat LIBs with current?

Heating is mostly correlates with the current level. Therefore, faster and more effectiveheating will come with higher current. Furthermore, frequency has also an influenceon the heating effect. As the experiments in this study show, LIBs have higher realimpedance with lower frequencies and therefore generate more dissipative heat at lowfrequencies at the same current. However, too low frequencies bear the risk of perma-nent capacity loss through lithium plating and further even safety hazards. Therefore thelowest frequency has to be identified where risk of lithium plating is low and safety isguaranteed. At this frequency the applied AC will be most effective. For faster response,the current needs to be increased. Nonetheless, not only current frequency and ampli-tude affect effective heating. Attention has to be payed to compact packaging, proper

50

CHAPTER 5. CONCLUSION

thermal insulation as well as little thermal conductivity through cabling. Another aspectis the heating strategy, which compares the energy used for heating up a battery and theenergy used for maintaining a certain temperature over a limited time. Simulations showthat in many cases, maintaining the battery temperature is more efficient than letting itcool down and heating it up again. On one hand, the experiments in this study showedthat it is possible to increase temperature even in LIBs with low impedance and high ther-mal capacity at a frequency, where the battery’s real impedance is at its minimum. On theother hand, it was not possible to increase the temperature sufficiently in the existingsetup. However, simulations that take a battery pack and a more powerful source intoaccount show that it is possible to heat LIBs even under the worst conditions.

5.1.3 What are the limitations of the AC heating method

As previously described it is possible to heat LIBs with alternating current. However, thatis if three conditions are fulfilled:

1. The batteries are connected in series, so that the impedance adds up and the powerincreases by the number of series connected batteries (see section 4.2.4).

2. The batteries are aligned such that they form a compact body with the least possiblesurface area, that can fit into a vehicle.

3. The batteries are isolated properly

Furthermore, if the whole system is taken into account, more factors need to be con-sidered. Generally, battery chargers are designed for DC. However, since they have tosupport both constant voltage and constant current charging, they have a built-in currentcontroller. This current controller needs to be able to provide high enough frequency inorder to make the AC heating method available in real applications. Depending on thetype of battery this frequency can vary. More precisely the frequency needs to be highenough to ensure safe operation. Next to frequency, the charger should also be able toprovide high enough current amplitude. This again varies from battery to battery and isdependent on thermal insulation of the pack, connection and impedance of the cells. Inaddition to the importance of the charger, the functionality of the car’s drive train playsan important role, too. Additional heating components can only be left out if battery heat-ing can be ensured also with disconnected charger and missing heating elements. Thisself-powered internal heating requires again additional components i.e. a DC/DC con-verter for mutual pulse heating or a relay for building a buck-boost converter. However,the effort for self-powered internal heating is expected to be less than for self-poweredexternal heating.

5.2 Ethical Aspects

As discussed, the AC heating method can help to lift EV’s popularity. In return, a highershare of electric vehicles make the total traffic less polluting and more silent. However,more EVs demand more LIBs and therefore more raw materials. It is important to payattention to how these raw materials are extracted. Not only environmental pre-cautions

51

5.2. ETHICAL ASPECTS

have to be installed in order to not pollute the environment at the extraction sites. More-over, safety pre-cautions and fair treatment for the workers need to be established. En-vironmental and social standards are generally met well in the automobile industry. Ifthose standards can be applied to the whole value chain of the vehicle production, alsosouthern American countries, where many of the raw materials are extracted, can benefitfrom the electrification of transportation.

52

Chapter 6

Outlook

6.1 Usability in Real Applications

Experimental data and further simulation show, that it is possible to pre-heat EV-batterieswith AC. Even under unfavorable conditions such as low battery impedance and largethermal capacitance it is possible to heat a single battery from −20C to −15C in theexperiment and a battery pack to from−20C to +5C in simulations. These results showthat a battery charger could be used to pre-heat an EV’s battery, before it is unplugged fordriving. This capability makes additional ohmic heating elements inside the battery packobsolete, leading to fewer components inside the battery pack. Fewer components in thebattery pack give the opportunity to reduce weight, size and complexity of the batterypack. In return, these advantages can help to increase the driving range and cut costs ofEVs at the same time.

However, there are some limitations to pay attention to in order to be able to installthis functionality and remove additional heating elements from the battery pack. Firstly,the battery pack has to be insulated properly. This is necessary, to limit the heat lossif the battery has a high temperature difference compared to the ambient as well as tolimit the current draw from the power supply if the battery has a low inner impedance.Secondly, the arrangement of the cells inside the battery pack needs to be such that theresulting impedance is as high as possible and the resulting surface area of the pack isas little as possible. This reduces the heating time as well as the necessary power toheat the battery. Finally, self-powered internal heating needs to be installed, too. If not,additional heating elements are still necessary to heat the battery if the car is not pluggedto a charger. Solutions for this scenario are already being discussed in literature.

6.2 Future work

Further research should be conducted in order to explore the potential of the AC heatingmethod for real applications.

Developing a more detailed thermal model of a battery pack is necessary to see theheat distribution among the cells. This detailed thermal model should take into account,that the cells in the center of the pack are easier to heat than the surrounding cells. Further,the detailed model should also take into account the thermal capacity of the cooling liquid

53

6.2. FUTURE WORK

and the thermal capacity of the electronics of the battery management system.

Figure 6.1: Circuit with power supply, res-onance capacitor and battery equivalent cir-cuit

Identifying the lowest AC frequencywhere the chemical reactions still do nothappen gives the opportunity to maximizethe heat dissipation at a certain AC am-plitude. Since the real part of the batteryimpedance increases at lower frequencies,it would be desirable to apply the lowestfrequency possible, that does not cause ag-ing in the battery.

The other way to increase the heatingeffect is to increase the current. Since thecurrent output of battery chargers is lim-ited, another approach could be to buildup a resonance circuit, as shown in fig. 6.1.

This resonance circuit, consisting of acharger as power supply, a resonance ca-pacitor and the battery, requires only little current from the power supply but wouldmaximize that current in the battery.

0 0.1 0.2 0.3 0.4 0.5

Time (ms)

-150

-120

-90

-60

-30

0

30

60

90

120

150

Curr

ent (A

)

Supply current

Resonance Current

Figure 6.2: Current from power supply and current in resonance circuit

This is simulated with the equivalent circuit model, as presented in section 3.5 withthe parameter as presented in table 6.1. The result is presented in fig. 6.2.

54

CHAPTER 6. OUTLOOK

Parameter ValueBattery inductor Lbat = 45nH

Resonance capacitor Cres = 200µF

Resonance frequency fres = 53.1kHz

Supply current isupply = 40App

Battery current ibat = 282App

Table 6.1: Parameters of the equivalent circuit

In this example, a current of only 40App (0.5C) from the power supply is necessary,to generate a current of 282App (3.5C) in the battery. To recall, the experimental showedthat a current level which corresponds to 0.5C did not have any heating effect. However,the resonance circuit would increase that current by seven times, which leads to 49 timeshigher heating effect.

55

Bibliography

[1] Kraftfahrt-Bundesamt, ed. Jahresbilanz der Neuzulassungen 2016. German. 2017. URL:https://www.kba.de/DE/Statistik/Fahrzeuge/Neuzulassungen/neuzulassungen_node (visited on 07/07/2017).

[2] ADAC-Schutzbrief Versicherungs-AG, ed. Autokatalog VW e-Golf. German. 2017.URL: https://www.adac.de/infotestrat/autodatenbank/autokatalog/detail.aspx?mid=266575&bezeichnung=vw-e-golf (visited on 07/07/2017).

[3] ADAC-Schutzbrief Versicherungs-AG, ed. Autokatalog VW Golf 1.0 TSI BMT Trend-line. German. 2017. URL: https://www.adac.de/infotestrat/autodatenbank/autokatalog/detail.aspx?mid=266199&bezeichnung=vw-golf-1-0-tsi-bmt-trendline (visited on 07/07/2017).

[4] ADAC-Schutzbrief Versicherungs-AG, ed. Autokatalog Ford Focus Electric. German.2017. URL: https://www.adac.de/infotestrat/autodatenbank/autokatalog/detail.aspx?mid=268521&bezeichnung=ford-focus-electric (visitedon 07/07/2017).

[5] ADAC-Schutzbrief Versicherungs-AG, ed. Autokatalog Focus 1.6 Ti-VCT Ambiente.German. 2017. URL: https://www.adac.de/infotestrat/autodatenbank/autokatalog/detail.aspx?mid=242611&bezeichnung=ford-focus-1-6-ti-vct-ambiente (visited on 07/07/2017).

[6] ADAC-Schutzbrief Versicherungs-AG, ed. Autokatalog smart fortwo coupé electric drive.German. 2017. URL: https://www.adac.de/infotestrat/autodatenbank/autokatalog/detail.aspx?mid=265463&bezeichnung=smart-fortwo-coup-electric-drive (visited on 07/07/2017).

[7] ADAC-Schutzbrief Versicherungs-AG, ed. Autokatalog smart fortwo coupé 1.0. Ger-man. 2017. URL: https://www.adac.de/infotestrat/autodatenbank/autokatalog/detail.aspx?mid=244178&bezeichnung=smart-fortwo-coup-1-0 (visited on 07/07/2017).

[8] Carl H. Hamann, A. Hamnett, and Wolf Vielstich. “Electrochemistry”. In: 2nd ed.Wiley-VCH, 2007. Chap. 1. ISBN: 978-3-527-31069-2.

[9] M. Rosa Palacín. “Recent advances in rechargeable battery materials: a chemist’sperspective”. In: Chemical Society Reviews 38 (2009), pp. 2565–2575.

[10] Carl H. Hamann, A. Hamnett, and Wolf Vielstich. “Electrochemistry”. In: 2nd ed.Wiley-VCH, 2007. Chap. 9. ISBN: 978-3-527-31069-2.

[11] David Linden and Thomas B. Reddy. “Linden’s Handbook of Batteries”. In: ed. byThomas B. Reddy. 4th ed. McGraw-Hill, 2011. Chap. 1. ISBN: 978-0-07-162419-0.

56

BIBLIOGRAPHY

[12] Jeff Dahn and Grant M. Ehrlich. “Linden’s Handbook of Batteries”. In: ed. by ThomasB. Reddy. 4th ed. McGraw-Hill, 2011. Chap. 26. ISBN: 978-0-07-162419-0.

[13] Anthony Barré, Benjamin Deguilhem, Sébastien Grolleau, Mathias Gérard, FrédéricSuard, and Delphine Riu. “A review on lithium-ion battery ageing mechanismsand estimations for automotive applications”. In: Jounal of Power Sources 241 (2013),pp. 680–689.

[14] Languang Lu, Xuebing Han, Jianqiu Li, Jianfeng Hua Hua, and Minggao Ouyang.“A review on the key issues for lithium-ion battery management in electric vehi-cles”. In: Jounal of Power Sources 226 (2013), pp. 272–281.

[15] Pankaj Arora, R.E. White, and M. Doyle. “Capacity Fade Mechanisms and Side Re-actions in Lithium-Ion Batteries”. In: Journal of the Electrochemical societey 1998 (145),pp. 3647–3667.

[16] J. Vetter, P. Novák, M.R. Wagner, C. Veitb, K.-C. Möller, J.O. Besenhard, M. Winter,M. Wohlfahrt-Mehrens, C. Vogler, and A. Hammouched. “Ageing mechanisms inlithium-ion batteries”. In: Journal of Power Sources 147 (2005), pp. 269–281.

[17] M. Broussely, Ph. Biensan, F. Bonhommeb, Ph. Blanchard, S. Herreyre, K. Nechev,and R.J. Staniewicz. “Main ageing mechanisms in Li Ion Batteries”. In: Journal ofPower Sources 146 (2005), pp. 90–96.

[18] Alexander Bessman. Private Communication. 2017.

[19] Quinn Horn, Troy Hayes, Daren Slee, Kevin White, John Harmon, Ramesh Godithi,Ming Wu, Marcus Megerle, Surendra Singh, and Celina Mikolajczak. “Linden’sHandbook of Batteries”. In: ed. by Thomas B. Reddy. 4th ed. McGraw-Hill, 2011.Chap. 39 Appendix H. ISBN: 978-0-07-162419-0.

[20] Thomas Waldmann, Marcel Wilka, Michael Kasper, Meike Fleischhammer, and Mar-gret Wohlfahrt-Mehrens. “Temperature dependent ageing mechanisms in Lithium-ion batteries e A Post-Mortem study”. In: Journal of Power Sources 262 (2014), pp. 129–135.

[21] Jiang Fana and Steven Tanb. “Studies on Charging Lithium-Ion Cells at Low Tem-peratures”. In: Journal of the Electrochemical Society 153 (6) (2006), A1081–A1092.

[22] Huiming Zou, Wei Wang, Guiying Zhang, Fei Qin, Changqing Tian, and YuyingYan. “Experimental investigation on an integrated thermal management systemwith heat pipe heat exchanger for electric vehicle”. In: Energy Conversion and Man-agement 118 (2016), pp. 88–95.

[23] Q. Wang, B. Jiang, Q.F. Xue, H.L. Sun, B. Li, H.M. Zou, and Y.Y. Yan. “Experimentalinvestigation on EV battery cooling and heating by heat pipes”. In: Applied ThermalEngineering 88 (2015), pp. 54–60.

[24] Xiongwen Zhang, Xin Kong, Guojun Li, and Jun Li. “Thermodynamic assessmentof active cooling/heating methods for lithium-ion batteries of electric vehicles inextreme conditions”. In: Energy 64 (2014), pp. 1092–1101.

[25] Chao-Yang Wang, Guangsheng Zhang, Shanhai Ge, Terrence Xu, Yan Ji, Xiao-GuangYang, and Yongjun Leng. “Lithium-ion battery structure that self-heats at low tem-peratures”. In: Nature 529 (2016), pp. 515–529.

57

BIBLIOGRAPHY

[26] Hiroyasu Baba and Koji Kawasaki. “Battery Heating System for Electric Vehicles”.In: SAE International 2015-01-0248 (2015), pp. 1–7.

[27] Shankar Mohan, Youngki Kim, and Anna G. Stefanopoulou. “Energy-ConsciousWarm-Up of Li-Ion Cells From Subzero Temperatures”. In: IEEE TRANSACTIONSON INDUSTRIAL ELECTRONICS 63 (2016), pp. 2954–2964.

[28] T.A. Stuart and A. Handeb. “HEV battery heating using AC currents”. In: Journal ofPower Sources 129 (2004), pp. 368–378.

[29] Jiangong Zhu, Zechang Sun, Xuezhe Wei, and Haifeng Dai. “An alternating currentheating method for lithium-ion batteries from subzero temperatures”. In: Interna-tional Joudnal of Energy Research 40 (2016), pp. 1869–1883.

[30] Jianbo Zhang, Hao Ge, Zhe Li, and Zhanming Ding. “Internal heating of lithium-ionbatteries using alternating current based on the heat generation model in frequencydomain”. In: Journal of Power Sources 273 (2015), pp. 1030–1037.

[31] Yan Ji and Chao Yang Wang. “Heating strategies for Li-ion batteries operated fromsubzerotemperatures”. In: Electrochimica Acta 107 (2013), pp. 664–674.

[32] Andreas Vlahinos and Ahmad A. Pesaran. “Energy Efficient Battery Heating inCold Climates”. In: Future Car Congress 2002-01-1975 (2002), pp. 1–10.

[33] Ahmad Rahmoun and Helmuth Biechl. “Modelling of Li-ion batteries using equiv-alent circuit diagrams”. In: Electrical Review 7b (2012), pp. 152–156.

[34] Hongwen He, Rui Xiong, and Jinxin Fan. “Evaluation of Lithium-Ion Battery Equiv-alent Circuit Models for State of Charge Estimation by an Experimental Approach”.In: energies 4 (2011), pp. 582–598.

[35] Pontus Svens. Private Communication. 2017.

[36] FoamSearch.com Powered by Momentum, ed. EPP/EPE Foam Technical Data. En-glish. 2017. URL: https://www.foamsearch.com/products/epp- epe-foam/ (visited on 07/21/2017).

[37] ZAHNER-ELEKTRIK GmbH & Co. KG, ed. Product catalog. English. 2016. URL:http://zahner.de/pdf/catalog.pdf (visited on 07/17/2017).

[38] A. Bessman, R. Soares, M. Behm, G. Lindbergh, O. Wallmark, M. Leksell, and P.Svens. “Investigating the Aging Effect of Current Ripple in Lithium- ion Cells”. In:ECS Transactions 69 (2015), pp. 101–106.

[39] R. Soares, A. Bessman, O. Wallmark, M. Leksell, M. Behm, and P. Svens. “Design As-pects of an Experimental Setup for Investigating Current Ripple Effects in Lithium-ion Battery Cells”. In: Power Electronics and Applications 17 (2015), pp. 1–8.

[40] Shengbo Eben Li, Baojin Wang, Huei Peng, and Xiaosong Hua. “An electrochemistry-based impedance model for lithium-ion batteries”. In: Journal of Power Sources 258(2014), pp. 9–18.

[41] Stefan Skoog. Private Communication. May 2017.

[42] e.GO Mobile AG, ed. e.GO Life. German. 2017. URL: http://e-go-mobile.com/de/modelle/e.go-life/ (visited on 08/06/2017).

[43] Rudi Soares. Private Communication. Aug. 2017.

58

Appendix A

Battery pack with temperature- and cur-rent controller

59

Batte

ry p

ack

elec

tric

Cur

rent

con

trolle

rBa

ttery

pac

k th

erm

al

Tem

pera

ture

con

trolle

r

dTco

ntro

lpo

wer

dT/d

tBa

ttery

tem

pre

f.Po

wer

cont

rol

volta

geco

ntro

lcu

rrent

ref.

curre

nt

cont

rol

curre

nt

Pro

duct

1 s

Inte

grat

or

-K-

ther

mal

capa

city

-K-

ther

ma

insu

latio

n1 s

Inte

grat

or1

-K-

K_p

_th

-K-

K_i

_th

T_re

f

T_re

f

-C-

T_am

b

max

. 100

A

-K-

# ce

lls

bat_

tem

p

Bat

tery

tem

pera

ture

Div

ide1

u

Sqr

t

1 s

Inte

grat

or3

-K-

K_p

_el

-K-

K_i

_el

-K-

K_p

_el1

Re(

u)C

ompl

ex to

Rea

l-Im

ag

Freq

uenc

y

Tem

pera

ture

Impe

danc

e

Bat

tery

ele

ctric

U/Z

freq

frequ

ency

i_ba

t

curre

nt

Re(

u)C

ompl

ex to

Rea

l-Im

ag1

Re(

u)

Com

plex

toR

eal-I

mag

2

P_h

eat

Pow

er

1 s

Inte

grat

or2

E_h

eat

Ene

rgy

heat

flow

bat

tery

-> a

mbi

ent