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UPTEC ES09013

Examensarbete 20 pSeptember 2009

Pre study of lead acid battery

charging for wind power

Magnus Vidmo

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Teknisk- naturvetenskaplig fakultetUTH-enheten

Besksadress:ngstrmlaboratorietLgerhyddsvgen 1Hus 4, Plan 0

Postadress:Box 536751 21 Uppsala

Telefon:018 471 30 03

Telefax:018 471 30 00

Hemsida:http://www.teknat.uu.se/student

Abstract

Pre study of lead acid battery charging for wind power

Magnus Vidmo

This thesis is a pre-study of lead acid battery charging for variable speed generatorsconnected to vertical axis wind turbines. A system that controls the turbine tooptimize the energy absorption while the batteries are charged at a healthy andefficient way is proposed.

The system is made for applications that are sited far away from the main grid, such asvacation cottages, boats, caravans and radio base stations. The system should be ableto work without maintenance for periods up to a year.

The thesis includes theory of aerodynamics, lead acid batteries and battery charging.The main subjects are the optimization of the energy absorption from the wind, howto obtain a long battery life and the integration of a battery bank in the systemwithout interfering with the consuming load. The system is going to be built and

tested with a vertical axis wind turbine in Marsta north of Uppsala.

ISSN: 1650-8300, UPTEC ES09013Examinator: Ulla Tengbladmnesgranskare: Hans Bernhoff

Handledare: Mikael Bergkvist

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Acknowledgements

Thanks to

Lars Hgberg for his enormous patienceHans Bernhoff, Mikael Bergkvist and Mats Lejion for giving me the opportunity towork with this master thesisKristina Edstrm for all the contributed battery knowledgeErik Dore for good company during late eveningsMikael Bergkvist and Olle Svensson for good advicesOscar Bernberg for the programming help

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Sammanfattning

Frnyelsebar energi r ett viktigt mne i den politiska debatten i dagslget. Densvenska regeringen har satt ett ml att bygga ut vindkraften till en produktion p 30

TWh/r innan 2020. P ngstrmslaboratoriet i Uppsala utvecklas en ny sortsvertikalaxlade vindkraftverk.

Den strsta skillnaden mellan vertikalaxlade och horisontalaxlade vindkraftverk r attvertikalaxlade inte behver vridas efter vinden, de r tystare och den tunga generatornkan sttas p marken istllet fr lngst upp vid turbinen. En del av forskningen fr devertikalaxlade vindkraftverken grs fr kraftverk som inte ska vara ntanslutna.Energin mste d lagras p ngot stt. Detta examensarbete r en del av denforskningen.

Energilagring i batterier r vl beprvat, speciellt blybatterier tack vare den storabilindustrin. Det finns en mngd olika typer av blybatterier gjorda fr olika ndaml.Dessutom finns det mnga olika stt att ladda ett batteri p.

Syftet med examensarbetet r att utveckla ett system som klarar av att kontrollera ettvindkraftverk samtidigt som en batteribank laddas p ett optimalt stt. Vindkraftverketska kontrolleras genom att batteriet laddas mer eller mindre. P detta stt kan manhlla en frutbestmd rotationshastighet p turbinen optimerat fr att utvinna smycket energi ur vinden som mjligt. Frutom detta har en fr ndamlet passande

batterityp valts ut.

Hur mycket energi som kan absorberas beror p hur snabbt turbinen snurrar relativtvindhastigheten. Fr att styra turbinen s att den ligger p en hastighet som alltidmotsvarar optimal energiabsorption tar man ut mer eller mindre effekt ur generatorn.Turbinens hastighet beror av dess rotationsenergi, om det dras mer effekt urgeneratorn s kommer turbinen att snurra lngsammare och vice versa.

Sjlva styrningen av vindkraftverket sker i en s kallad mikrocontroller, vilken kanliknas vid en primitiv dator. Mikrocontrollern r programmerad med en nskadladdningsalgoritm som talar om hur batterierna ska laddas beroende p huruppladdade de r fr tillfllet. En laddningsalgoritm r en slags karta vilken batterietska laddas efter under hela uppladdningsfrloppet. Fr att laddningen ska kunna styra

hastigheten p turbinen visar laddningsalgoritmen endast en maximal strm ellerspnning som batterierna kan laddas med fr tillfllet. Det kan allts, och kommeroftast att laddas med en styrka som ligger under den maximalt tilltna. Det betyder att

batterierna kommer att laddas under lngre tid n vad de skulle ha gjort ifrn enkontinuerlig energiklla.

Fr att ett batteri ska laddas optimalt och f lng livslngd och hg kapacitet ska detinte laddas med fr hga strmmar. Det blir drfr viktigt att dimensionera

batteribanken efter kapaciteten p vindkraftverket och de aktuella vindfrhllandena.

En annan viktig del i systemet r en DC/DC omvandlare som snker spnningen till

den nskvrda fr batteriladdning. Det r DC/DC omvandlaren som styrs av

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mikrocontrollern. Det r allts dr som sjlva effektstyrningen sker vilket leder till attturbinen hlls vid en nskad hastighet.

Fr att frtydliga laddningssttet s kan det sgas att batterierna laddas med den effektsom finns tillgnglig fr tillfllet beroende av den rdande vindhastigheten men de

laddas aldrig ver ett visst tak som r satt av laddningsalgoritmen.

Om mer energi mste dras ur generatorn fr att hlla turbinen i rtt hastighet n vadladdningsalgoritmen r satt till vid ett visst tillflle s mste energin dumpas.Dumpningen av energi kan t.ex. gras med vrmeelement eller kylflktar.

I kontrollsystemet som innefattar elektronik samt batteribanken r batteribanken enstor kostnad. Eftersom investeringskostnaden fr batterierna blir en mycket stor del avden totala kostnaden s r det viktigt att batterierna drivs p ett stt som ger en lnglivslngd. Parametrar som pverkar ett batteris livslngd r temperatur,laddningshastighet, urladdningshastighet, antal genomgngna cykler samt hur djupa

cyklerna r. En cykel r ett laddnings och urladdningsfrlopp. Djupet p en cykelbeskriver hur mycket energi som tagits ur batteriet i frhllande till den maximalabatterikapaciteten. Det r allts mnga parametrar som pverkar ett batteris livslngd,dessutom pverkar de flesta parametrarna varandra. Allt detta mste tas hnsyn till nrladdningsalgoritmen skapas.

Det finns mnga olika batterityper. I detta arbete r endast blybatteriet omnmnt.Blybatteriet r ett mycket robust och prispressat batteri med en lng historia bakomsig. Den lnga bakgrunden och det frhllandevis lga priset gr blybatteriet till ettsjlvklart val i ett samanhang som detta. Det forskas mycket p andra batterityper somhar bttre egenskaper fr t.ex. livslngden, men n s lnge r dessa alldeles fr dyra.

Fr mnga anvndningsomrden r det viktigt att det krvs s lite underhll sommjligt. Batterier som inte krver mer underhll n cirka en tillsyn per r r nskvrt.En batterityp har tagits fram under de senaste ren som gr att kontinuerligvattenpfyllning inte lngre behvs. Dessa blybatterier kallas fr VRLA batterier(ventilreglerade blysyra batterier). De r slutna batterier med en innesluten processsom gr att det vatten som frgasas terfrvandlas till vatten igen och drfr behlls i

batteriet. Dessa batterityper blir drfr ett lmpligt val fr applikationer som ska klaras lite underhll som mjligt.

Det stlls ven vissa krav p batteriernas egenskaper nr det gller cykling och att tladjupa urladdningar. Fr dessa ndaml r truckbatterier vl anpassade. Denna typ avblybatterier klarar bde djupa och mnga cykler innan de mste bytas ut. De rbyggda fr att cyklas en gng om dagen och fr att kunna laddas snabbt tillnstkommande dag. De r ofta ihopsatta med separata tvvoltsceller. Detta gr att det

blir billigare att gra ett byte ifall att en cell skulle g frlorad.

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Abbreviations

VAWT Vertical Axis Wind TurbineHAWT Horizontal Axis Wind Turbine

AC Alternating CurrentDC Direct CurrentOCV Open Circuit VoltageTSR Tip Speed RatioIGBT Insulated Gate Bipolar TransistorMOSFET Metal Oxide Semiconductor Field Effect TransistorPWM Pulse Width ModulationSOC State Of ChargeSLI Start, Lighting, IgnitionVRLA Valve regulated lead acidAGM Absorbent Glass MattGEL Gelified Electrolyte

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Table of contents

1. Background ....................................................................................... 6

1.1. Research projects for Wind turbines at Uppsala University thatwill be involved in this thesis........................................................... 71.2. Earlier thesis for the project............................................................ 81.3. Purpose............................................................................................ 101.4. Method ............................................................................................. 101.5. Problem formulation....................................................................... 101.6. The thesis structure ....................................................................... 10

2. Theory.............................................................................................. 122.1. Aerodynamic theory for wind power............................................ 122.2. Load control of a VAWT ................................................................ 152.3. Consequences of variable speed VAWTs for stand alone

consumers....................................................................................... 172.4. Battery theory.................................................................................. 18

2.4.1. The chemical reaction in the lead acid cell ................................ 182.4.2. The construction of the lead acid battery ................................... 192.4.3. Important parameters for the battery operation......................... 212.4.4. Charging and discharging for lead acid batteries...................... 302.4.5. Cycling and lifespan of the battery .............................................. 382.4.6. Different available battery types................................................... 392.4.7. Connections of batteries for a battery bank ............................... 41

3. Result............................................................................................... 423.1. Summary of the battery theory..................................................... 42

3.2. The system configuration.............................................................. 433.3. Choice of charging algorithm for the final solution.................... 463.4. Choice of battery type.................................................................... 493.5. Battery bank dimensioning ........................................................... 50

4. Discussion ....................................................................................... 524.1. Future work ..................................................................................... 52

5. Conclusion....................................................................................... 53References ................................................................................................................. 55Appendix 2.................................................................................................................. 59Appendix 3.................................................................................................................. 60Appendix 4.................................................................................................................. 62

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1. Background

Nowadays renewable energy is growing strongly on the energy market. In Sweden thegovernment has a development goal to install wind power that gives 30TWh/year

before 2020. [1] At the ngstrm laboratory, Uppsala University, new strategies forwind power are being developed. The red thread in the research is to use simple androbust constructions to optimize the total performance of the entire system. Thatmeans usages of minimal amount of different parts and minimal amount ofmechanical parts to reduce maintenance. The main ideas to achieve simple andreliable generators are to use directly driven permanent magnet generators withvariable speed, thats a slow rotating generator with many poles. This techniquedoesnt include a gearbox. At Uppsala University the main line for wind energyconverters are vertical axis wind turbines VAWT, see Figure 1. The big difference

between a VAWT and a Horizontal axis wind turbine HAWT is that the VAWTdoesnt need to be adjusted after the direction of the wind and the heavy generator can

be put on the ground. Besides this the VAWT has a lower noise level than the moreconventional HAWT.

Figure 1. Vertical axis wind turbine [22]

This thesis is a part of a project for wind power where grid connection isnt possible.Instead of using a diesel aggregate for power source, the VAWT will be used togetherwith a large battery bank where the received energy will be stored.

There is a large difference in the shape of the entire system if the system is gridconnected or not. The biggest difference is that the energy must be stored somehow.The storages are also a very expensive part of the system and it is therefore veryimportant to maximize the batteries capacity.

Some users for these products could be isolated cottages in offside areas, radio basestations, smaller applications for boats and caravans and many more areas.

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1.1. Research projects for Wind turbines at UppsalaUniversity that will be involved in this thesis

In 2006 a VAWT was built in Marsta north of Uppsala. Marsta has been a place formeteorological studies for many years, and now it also serves as a research place forVAWTs.

The first VAWT that was built at Uppsala University is called Lucia, see Figure 2.Lucia is six meter high with three 5 meter long fixed blades. It generates 12 kW at awind speed of 12 m/s. The generator is a permanent magnet generator placed on theground with the axis connected to the turbine. The prototype is available for differenttests and research projects in the search for the best working technique for an entirewind power station. [2]

Figure 2. The Lucia turbine in Marsta. [2]

In 2008 another research turbine was built in Marsta named Birgit see Figure 3. Its a10 kW VAWT with a four-bladed H-rotor, made for Ericssons new radiocommunication tower, the tower tube. The generator is a permanent magnetgenerator placed as a ring on the tower with the blades directly mounted to the rotor.Ericsson wants to use renewable energy sources, e.g. wind power, instead of diesel forradio base stations where grid connection isnt possible. [3]

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Figure 3. The Tower tube turbine in Marsta. 1

1.2. Earlier thesis for the project

In an earlier thesis for the same project a control system circuit was made by LarsHgberg. This thesis is among others based on the former work done in [4]. Theearlier circuit below (Figure 4) has been reformed to operate in desired way for the

battery charging system.

1The picture is taken from www.verticalwind.se

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Figure 4. Schematic overview of the control system [4]

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1.3. Purpose

The purpose of this master thesis is to develop a cost-effective method to run a battery

bank in the best possible way for off grid consumers.

1.4. Method

To meet the purpose, a study of the earlier built system and reconfiguration of it havebeen done. To know how the new system should be built in the best way to keep thecost down and to maintain a long life for the batteries, theories for the followingsubjects have been investigated:

-load control for the VAWT-DC/DC step down converters-Parts and their functions of the lead acid battery-Different available lead acid batteries on the market today and which one that willsuit this project best-Different charging methods available for lead acid batteries-Conditions affecting battery life time

Search to find and buy a suitable DC/DC converter and other needed parts for theconfiguration have been made. This thesis will later result in a built and tested systemfor the Tower tube wind mill in Marsta north of Uppsala.

Apart of this, a program have been made in Matlab that calculate the storages capacityneeded for a special area only using wind data of the specific area. For different sizesof the battery bank the amount of diesel to maintain the energy needed can also becalculated in the program. Results of the program can be seen in appendix 4.

1.5. Problem formulation

The major issues to solve for the project are:

-How is it possible to control the load for the turbine while its connected to a batterybank?-How will the batteries be affected of the new load control system?

1.6. The thesis structure

To make a good conclusion in the result there is a lot of theory to be aware of. Thetheory section starts with the aerodynamic theory of the turbine. This theory isimportant considering the efficiency of the load control. After that section the load

control is explained. The PWM of the load control is described in the appendix in theback. Then the battery theory is described. All the different battery parts and

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parameters that are important for the battery charging, discharging and the effects ofthe load control are mentioned. Different types of batteries that are of interest for the

purpose are mentioned and also different ways of charging and discharging.

In the result all the theory is used to make a good solution for an appropriate scheme

of the entire load control system. The finally charge algorithm and the batterycharging microcontroller is described. A choice of a specific battery type is made.Finally the dimensioning of the battery bank is done. The dimensioning includes a

program made in Matlab that gives the battery dimension for a specific wind site. Theprogram is further described in chapter 3.5 and in appendix 4.

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2. Theory

2.1. Aerodynamic theory for wind power

To extract the most possible power from the wind the airfoils should have a specialdesign. The design is important but the winds angle of attack at the blades is also veryimportant. The angle of attack is the angle between the chord line and the relativewind, see Figure 5. When the blade is moving it will feel a wind that is caused by itsown movement. The relative wind is the resultant of the actual wind speed and thewind speed caused by the blades movement. The relative wind will then change whenthe speed of the blade is changing.

To optimize the extraction of the wind a desired angle of attack should be kept at alltime. The wind results in a resultant force on the blades. The resultant force can bedivided in a lift force and a drag force, see Figure 5. The tangential force that is in thedirection of the blades movement is the force that is contributing to the energyconversion. The angle of attack that gives the highest tangential force is the desiredangle to keep all along.

Figure 5. Angle of attack, Lift force and Drag force for an airfoil.

For a wind turbine there are several different ways to maintain the desired angle ofattack. One common way is to have an inbuilt system in the blades that angle the

blades in the desired position. This is called pitching. Another way is to keep theturbine at a desired speed which keeps the angle to the relative wind constant. Thismeans that for a specific wind speed there is a specific turbine speed causing thedesired angle of attack. In fact there is a typical number relation between theundisturbed wind and the speed of the tips of the blades called , the tip speed ratio.

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Cp is a value of the efficiency of the rotor which depends directly on the angle ofattack of the airfoil. To achieve the highest efficiency for the turbine Cp should be ashigh as possible. Cp is the ratio between turbine power and the existing power in thewind.

windtheinPowerpowerRotorCp (1)

It is impossible for the turbine to extract all power in the wind. A part of the incomingwind will not be absorbed. If all the power in the wind would be extracted by theturbine, all of the incoming air would be accumulated behind the turbine which ofcourse would be impossible (see [5] for a deeper understanding). Because of this thereis a maximum value of Cp= 16/27 for a HAWT. This Value is called the Betz limit.

For a turbine that has fixed blades (without a pitching system) the tip speed ratio is

an important parameter to achieve high efficiency. is a number of how much fasterthe tip of the blade is moving than the undisturbed wind.

speedwinddUndisturbe

bladetheofspeedTip (2)

Note that for a VAWT the speed is the same for the whole blade.

A specific will give a specific Cp for a specific turbine. The optimum value ofcanbe measured and calculated for a specific turbine, see Figure 6.

Figure 6. Measured Cp points for different for the Lucia turbine. [21]

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This means that when the undisturbed wind speed increases, the tip speed of the blademust increase to keep an optimum ensuring an optimum Cp.

For the Lucia turbine =4 is the optimum tip speed ratio to achieve highest Cp. Thatmeans that the tip speeds of the blades should always be four times faster than the

actual wind, but above a given wind speed this is not possible (see Figure 7).

Change of the rotors tip speed

0

5

10

15

20

25

30

35

40

45

0 5 10 15 20 25

The undisturbed wind [m /s]

Therotortipspeed[m/s]

Figure 7. The rotors tip speed changing with the wind speed.

A wind energy converter has limits depending on the construction. The maximumtolerance will be reached at a given wind speed, e.g. the bearings have a maximumtolerance speed. After the point where the maximum tolerance wind speed is achievedthe turbine is set to continue to rotate at the same speed. After this point willdecrease and Cp will follow causing a lower efficiency. In Figure 6 the Cp value willslide down to the left of the top.

The power in the wind that the turbine can absorb is

3

2

1vACP p (3)

where is the density of the air, A is the swept turbine area and v is the wind velocity.[5]

Figure 8 shows the achieved power for a certain wind speed for Lucia. This graph

shows what would happen with the power outtake if there were no construction limits

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of the wind power station. The diagram also shows when the Lucia turbine will startto extract power. This will be done when the wind reaches 4 m/s. [6]

Figure 8. The theoretical power outtake of the Lucia turbine in kW. The red dotted line shows

the power absorbed if there were no construction limits of the wind power station. Between 4-10

m/s the tip speed ratio (TSR) is 4. After 10 m/s the turbine speed is fixed. [6]

2.2. Load control of a VAWT

As said above the turbine should be kept at a specific speed specified for a certainwind speed to achieve a high Cp and also not to destroy the turbine. The system thatcontrols this is called the load control.

The tip speed of the blade is measured adjuvant with the voltage from the generator.The voltage is proportional to the rotor speed,

dt

dNU

(4)

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where U is the generated voltage, N is the number of turns of the windings, anddt

d

is the time derivate of the alternating flux.

The power for a rotating body is dependent of the torque and the angular velocity

P=M* (5)

where P is the power, M the torque and the angular velocity. The power outtake isthe voltage multiplied with the current

P=U*I (6)

where I is the current. Equation 4 and equation 5 gives that as said before U isproportional to and I is proportional to M.

The control system is made to take out more or less power from the turbine to controlthe turbine speed. If the turbine is rotating too fast, above the desired speed, a higher

power outtake must be made to slow down the turbine. When the turbine is rotating toslow less power is taken out, and the absorbed wind power will be stored in therotating turbine causing it to speed up.

The control is made to keep the turbine at fixed rotation speed during short timeperiods. During longer periods of several minutes the controlling mechanism is set tokeep the turbine at a mean value of.

To understand the controlling mechanism better the function pulse widthmodulation (PWM) must be understood.

PWM is a tool that gives the opportunity to control a parameter to approach a desiredvalue. For example a DC/DC converter gets an in value of Uin=100 Volts and theoutput Uout should be 48 Volts. The desired value is then 48 V. A control value theduty cycle D is then multiplied with Uin to get Uout, as in equation 6. The duty cycleyields a signal to a switching device. D is a value of how much of the time the switchshould be open or closed.

Uout=D*Uin (7)

D is set with a control signal set by typically a triangle wave. Uin will pass the trianglewave in some height. Depending on where Uin passes the triangle wave D will get adifferent value. A deeper explanation can bee seen in Appendix 1. PWM can bemanaged in many ways and the one that is mention is just one of many different types.[4] [7]

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2.3. Consequences of variable speed VAWTs forstand alone consumers

Intermediate energy sources like wind power have one major issue. They produce afluctuating power. At the main grid this is solved by regulating power as e.g. hydro

power. However, this will be a problem for island-operated systems. The load demandwill not be the same as the power from the VAWT.

To solve this problem the excess energy has to be stored one way or another, or bedumped as heat. When the power is too low, some other energy source has to supportthe load. The support and storage device could e.g. be a battery.

A load has regulation for its input voltage. A variable speed machine gives differentvoltages depending on the present wind speed. These types of machines are supported

by power electronics to give a desired voltage and frequency when connected to the

main grid. This is also the case when the machines work for stand alone consumers,see Figure 9.

The fluctuating voltage from the VAWT is controlled by a DC/DC converter thatsupplies the load with the desired voltage. If the power from the VAWT is too low,the external source must be able to supply the desired power for the load. A batterygives a higher voltage when fully charged compared to when partly discharged. The

battery voltage range must be in the desired range of the load.

Figure 9. An example for how a scheme for an island operated VAWT could look like. At a

specific time the VAWT gives 2 kW, the load demand is 1 kW, 0,5 kW can be accepted by the

storages device and 0,5 kW must be dumped as heat. The voltage is predetermined for a specific

wind speed to keep the desired and the current will therefore be fluctuating depending of the

power produced. The current is proportional to the torque and the torque will therefore also

fluctuate.

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The wind is constantly changing. The optimum would be to regulate the turbine speedaccording to the exact wind speed at all time. This is not possible in reality. Thecontrol system that regulates the power outtake has to have some kind of time step.

Its neither possible nor necessary to regulate the turbine after all the rapidly changing

gusts. Its very difficult to make accurate measurements of the wind speed duringshort time steps. The turbine also has a large moment of inertia which reduces the

possibility of fast regulations.

A mean wind over a longer period is used which will keep the frequency of thecontrol system low. The result of this is that will not be at optimum during strongshort gusts and Cp will be lower, but the turbine will run smother. Frequencies that areharmful for the load should be avoided or filtered.

2.4. Battery theory

Battery theory in this thesis only concerns the lead acid battery.

2.4.1. The chemical reaction in the lead acid cell

The lead acid battery has lead dioxide (PbO2)and metallic lead (Pb) as active materialon the positive and negative electrode, see Figure10. The electrolyte is a sulfuric acid(H2SO4) solution of typically 37 weight percent of acid. During the discharging statethe lead and lead dioxide are consumed with the sulfuric acid solution to produce

water and lead sulphate (PbSO4). The lead sulphate will be formed at the electrodesurfaces. When the battery is charged the process reverses, the lead sulphate is brokendown resulting in lead dioxide and lead recovered to the positive electrode respectiveto the negative electrode. The chemical reaction for discharging is as follows:

For the negative anode: Pb Pb2+ + 2e

Pb2+ + SO42- PbSO4

For the positive cathode: PbO2 + 4H+ + 2e Pb2+ + 2H2O

Pb2+

+ SO42-

PbSO4

Total reaction: Pb + PbO2 + 2H2SO4 2PbSO4 + 2H2O

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Figure10. Schematic picture of the lead acid battery being discharged.

2.4.2. The construct ion of the lead acid battery

The batterys equivalent model is shown in Figure 11. The inductive reactance of thebattery is neglected for low frequencies, while the capacitive reactance is increasingwith lower frequencies. More about the battery parameters will be explained later. [8][9]

Figure 11. Model of a battery according to Randles battery model. C1 is considered to be the

main charge storage, R3 is the self discharge resistance, R1 is the resistance of the batterys

terminals and inter-cell connections and C2 and R2 are results of shifting electrolyte

concentrations and plate current densities. [8]

The battery is built up by individual cells that are serial connected, see Figure 12.Each cell is of two volts. The common 12 volt battery is therefore divided in 6 cells.Each cell consists of a positive and a negative plate with a separator between them.The end positive plate is connected to a positive terminal and the negative end plate isconnected to the negative terminal. The plates of same polarity are connected to eachother with straps. See Figure 12 for a cross section overview of a battery.

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The capacity of the cell is proportional to the surfaces area of the plates that are incontact with the electrolyte. The plates are often made as a paste with a lot of poresthat increases the active plate area. The active material is fixed by a grid which worksas a mechanical support and current conductor. This construction is called the flat

pasted plate and is very common for the negative plate. The positive plate iscommonly either a flat plate or a tubular plate. The tubular plate has many advantagesfor heavy cycled operations. The paste are there held in micro porous tubes that areconnected in series. To increase the capacity even further plates can be parallelconnected to achieve a larger active area.

The grids are made of lead alloys. The lead is not strong enough as a support for theactive material. Metals as antimony, calcium, selenium or tin can be alloyed toimprove the grids. The alloys change the properties of the plates in different ways.

The antimony alloys can be deep cycled more times than the calcium alloys.

Tin added to lead-calcium alloys improves the cycling capability. The calcium alloys have lower self discharge rate. If the cells are long time overcharged the positive calcium alloyed plates will

grow due to oxidation which can cause the cell to be damaged Adding Selenium gives plate properties between the antimony and the calcium

alloys.

The most used alloys are antimony and calcium.

The separators main function is to electrically insulate the plates from each other. The

electrolyte which makes the electron transport possible has to be able to pass throughthe separator. The separators construction is therefore very important. [10] [11]

Figure 12. Cross section of the lead acid battery.2

2 The picture is taken from www.reuk.co.uk/Lead-Acid-Batteries.htm

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2.4.3. Important parameters for the battery operation

Parameters mentioned here are different specified voltages, battery losses, internal

resistance, battery capacity, C-rate, SOC, the electrolyte density and the temperature.

Voltage

The theoretical voltage of a battery is a function of its construction and thesurrounding temperature. The cathode and anode materials and the structure of theelectrolyte are parameters that all matters for the voltage value.

The open circuit voltage OCV is a function of the electrolyte concentration and thetemperature. The OCV is a close approximation of the theoretical voltage which is

2.125 V per cell for a fully charged cell with a 1.28 kg/dm

3

electrolyte concentration.The OCV is the voltage that is given when the current is zero.

The cut off voltage where the battery is said to be fully discharged is around 1.75V/cell. The cut off voltage is specified by the manufacturer and is a function of thedischarge rate and the temperature.

During charge the charge voltage is between 2.3 and 2.8 V/cell depending of thecharge algorithm.

The gassing Voltage is the voltage when the temperature is so high so the electrolyte

will start to gas. The cell gassing voltage is specified in Figure 13 for differenttemperatures.

Figure 13. Cell gassing voltage at different temperatures. [10]

These values mention above are none specific values of lead acid batteries. Differenttypes of lead acid batteries have slightly different values. [10] [11]

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Losses of the battery cell and the internal resistance

Losses from the battery are dependent of the polarization and the internal impedance.The losses are given as waste heat. A lead acid battery has a typical efficiency of 85%from charging to discharging.

There are two different polarization phenomena. The activation polarization drives theelectrochemical reactions at the electrode surfaces. The concentration polarization isdue to the concentration difference of the products and reactants at the electrodesurface.

The internal impedance also consumes energy and reforms it to waste heat. Theinternal impedance causes a voltage drop which is proportional to the current drawnfrom the system. The batterys total internal resistance which follows ohms law is asum of the characteristics of the electrolyte, the active mass, the plates, the straps, theterminals and the contacts between them. The internal resistance is an indication ofhow deteriorated a battery is. The resistance will rise when the battery is aging.

The internal impedance rises during discharge and decreases during charge due to theelectrolytes changing chemistry.

The losses due to polarization and internal impedance rise with higher currents, whichcan be seen in Figure 14. The power losses are dependent of the current in quadrate.

onpolarizatiRIP 2 (8)

To get most possible theoretic capacity out of the battery a low current should bedrawn from the battery. Although there is a limit for how low the current should be. Ifthe current is as low as the polarization current then the losses will remarkable big.

Figure 14. Losses with increasing discharge current. [10]

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A comparison to the conventional car engine can be made. An engine that will bedriven at low speed will consume less gasoline than an engine that will be driven athigh speed, covering the same distance.

Battery capacity, C-rates and state of charge (SOC)

The battery capacity can be described in many different ways. Therefore is itimportant to specify exactly what kind of capacity that is being discussed to avoidmisunderstandings. The maximum capacity available in a battery is determined by the

batterys quantity of active material, the surface area of the plates and the amount ofelectrolyte. These factors only depend on how the battery is constructed, but the

battery capacity also depends on how the battery is used.

The battery capacity is often rated after the current rate. The capacity is differentdepending of the current rate during discharge and charge, see Figure 15.

Figure 15. The discharge rate effect on the capacity for traction batteries with tubular plates and

flat pasted plates. [10]

The rate is called the C-rate. A battery that is discharged with a rate C1 is totallydischarged with a constant current during one hour. If its a C5 rate the battery will bedischarged with constant current during 5 hours. The C5 rate has a lower current rate

but the C5 rate will also deliver more energy than the C1 rate out of the same battery.The nomenclature of C1 is equal to 0.1C or C/1. The charging current rate ismentioned in the same way. A C20 battery is charged during 20 hours.

An extension of the nomenclature is e.g. 0.1C5, which means that a battery that is

rated to e.g. X Ah at the C5 rate is discharged with a rate of C10.

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Its important to notice that the capacity is specified for a specific C-rate, whileretailers have different standard rates when they specify the battery capacity. ASonnenschein 602 battery can e.g. deliver 12 A during 100 hours (C100 rate) giving atotally capacity of 1200 Ah at 20 C. If the same battery delivers a constant current of100 A it can only be discharged for 10 hours (C10 rate) before it reaches the cut of

voltage, the total capacity is then only 1000 Ah. Note that the battery that is rated to1000 Ah during a constant current can still give more energy if the current will belowered, see Figure 17. Even though, it can never give the same amount of energy asthe one which was discharged with a lower C-rate. There will always be larger lossesfor the higher C-rates. [10] [11] [12]

When a battery is totally discharged it has reached its cut of voltage. After this pointthe voltage will continue to fall but there is not much more energy left to drain and the

battery will only be damaged. The cut of voltage is different dependent off the C-rateand the type of battery. Higher C-rates has a lower cut off voltage as can be seen inFigure 16.

Figure 16. The discharge time in relation to the discharge current. The discharge time is

measured until the cut off voltage is reached. The cut off voltage is following the dotted line in the

figure and is dependent of the discharge current. The figure is only an example of a specific

battery and will differ for different batteries.

If the battery is discharged to its cut off voltage at a specific C-rate it can still givemore energy at a lower C-rate as can be seen in Figure 17. When the discharge currentis less the battery voltage will get higher and the battery can be discharged again. Ifthe battery in a Mp3 player is empty it can e.g. still be used in a remote control to theTV that uses a lower current.

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Figure 17. The voltage for a battery discharged at sequentially lower C-rates.

The battery capacity left to use at a given time for a specific C-rate is often stated asthe state of charge SOC. The SOC is a percent value where the Ah discharged andcharged from the battery is divided with the nominal capacity of the battery at aspecific C-rate. If the battery will be discontinued discharged with moments ofcharging the SOC formula will be

NomCap

dtIdtINomCap

SOCech

ech

edisch

edisch

arg

arg

arg

arg

11

(9)

where NomCap is the nominal capacity of the battery at a specific C-rate, Idischarge isthe discharge current, Icharge is the charge current, discharge is efficiency of thedischarge and charge is the efficiency of the charge. The overall efficiency of the

battery charged and discharged is a quota between the energy delivered during chargeand the energy received during discharge. Its difficult to measure discharge andchargeindividually.

The usage of SOC can be a bit confusing while the nominal battery capacity is statedat a specific C-rate. The C-rate that the SOC is stated for is typically the C-rate close

to the optimum C-rate where the battery can deliver a maximum possible amount ofAh. This means a current rate so low that there will be minimum possible losses.

If a battery is rated to 200 Ah at a specific C-rate and its at 80% SOC then it can stilldeliver 160 Ah before it reaches the cut of voltage at that specific rate.

To make it even more confusing the SOC can be stated for the nominal capacity assaid above or it can be stated after the last charge completed. If the SOC value isstated after the latest charge it will be a higher percentage than for the comparisonwith the nominal capacity when the battery was unused. A battery can then have aSOC that is 100% but its only 80% of the nominal capacity when the battery was

new. This is due to the batteries ageing parameters that deteriorate the cells. To avoidconfusion lets call the SOC stated in comparison to the new battery qualities for only

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SOC and the aging batteries SOC for SOCage. The different SOCs are goodparameters for different things for e.g. the charge algorithm. [10] [13]

The SOC is either measured with the assistance of the electrolyte density, the OCV orwith the equation 9. Note that the density measurement can not be done with VRLA

batteries. One way to know when the battery will be fully charged is to measure whenthe current decreases till its gets constantly small over a longer period.

The electrolyte density and the surrounding temperature influence

Its important for the life span of the battery to know what effect the reaction betweenthe different substances in the battery construction will have. The sulfur acid solutionin the electrolyte is aggressive to some separators and some other components in the

battery. Higher concentration of the sulphur makes the electrolyte more aggressive.Lower concentration makes the solution less conductive. Therefore a density valuethat isnt to aggressive but still gives a good conductivity is preferable.

The solution also makes the lead corrode. The amount of corrosion is due to thesulphur concentration. In temperate climates the electrolyte solution has a weight

percent of 37 % and a density of about 1.26-1.28 kg/dm3. A matter of fact is that anelectrolyte density of 1.28 is least corrosive for the lead. Therefore to enlarge the totallifespan it is very important to avoid having a battery discharged longer thannecessary.

The electrolyte density is designed after the sector of application. When the battery isfully charged, the electrolyte density is set after the conductivity needed to get a

desired capacity out of the battery. The heavily cycled batteries have the highestdensity and the stationary that are low cycled have less density. For stationarybatteries with small high rate demands and larger proportional electrolyte volumes theconcentration can be held lower and be less aggressive. These batteries can have aconcentration as low as 1.21 kg/dm3.

The density is also important considering the freezing point of the battery. Thefreezing point will be lower for higher sulphur concentrations up to a density of 1.3kg/dm3 where the lowest freezing point at -70 C is reached. A battery that is fullycharged at a density of 1.28 kg/dm3 has a freezing point around -65 degrees and whenthat type is fully discharged at a density of 1.16 kg/dm3 the freezing point is around

-17 C. A battery that is fully charged at a density of 1.21 kg/dm3

will freeze at around-27 C and that type of battery can fully discharged have a freezing point at almost 0C. Therefore the electrolyte density is held a little bit higher in colder areas than inwarmer areas. [10]

Higher density also results in a higher boiling point.

Lower temperatures cause a reduction in chemical activity. For a specific density ofthe electrolyte the internal resistance increases with lower temperatures. The internalresistance losses will therefore get higher with lower temperatures. The capacity ofthe battery will be lower at lower temperatures. The capacity of the battery changing

with temperature can be seen in fig Figure 18. The battery capacity is usually

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measured at 20 or 25 C. Note that the higher C-rates are more affected by thetemperature.

Figure 18. Available capacity in relation to the surrounding temperature at different C-rates.

The values are measured at a Sonnenschein A600 battery. [14]

Low temperatures are good for batteries that stand unused for long times. The selfdischarge will be low when the chemical reduction is held low. A high temperature istherefore not always a good thing, see Figure 19. Thermodynamically the discharged

state is most stable and the self discharge will be a problem at all time when thebattery is unloaded or not at charge.

Figure 19. Self discharge in order to the surrounding temperature. The values are measured at a

Sonnenschein A600 battery. [14]

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Although the capacity is increasing with higher temperatures the total age of thebattery will be less with higher temperatures. The battery will hold for fewer cycleswith higher temperatures. Batteries that are constantly used at very high temperaturesshould be built with another electrolyte density for longer life. [10] [11]The density of the electrolyte is also dependent of the surrounding temperature. The

density increases with lower temperatures as the electrolyte contracts by the coldertemperature. The density can be calculated with the equation

)15(10)15()( 5 tCt (10)

where is the density, t is the temperature in C where the density is calculated at and is a temperature coefficient.

The internal resistance of the battery is dependent of the density of the electrolyte.The internal resistance is both dependent of the temperature directly as said earlierand also as a function of the density that change with the temperature. The densityfluctuation due to a temperature shift is however relatively small, (see equation 10)and the internal resistance dependency will therefore be much smaller than the directdependency of a temperature shift.

The open circuit voltage is dependent of the electrolyte density and the surroundingtemperature, see Figure 20 for the density dependence at 25C. Its almost a linearrelation between them down to a concentration of about 1.10 kg/dm3. For an accuratemeasurement the battery should stand alone for 4-8 hours before the measurementscan be done. The approximation equation for the linear relation is

OCV = (25C) + B (11)

where B is a constant. B=0,845V. The temperature dependence is about +0.2 mV/Cfor most batteries. [10] With the temperature dependence the equation will be

OCV = (tC)*A + B + t*0.0025V/C (12)

Where A is a constant. A=1 [Vdm3/kg].

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Figure 20. The cells open circuit voltage as a function of the electrolyte concentration at 25C.

The AGM batteries have slightly higher OCV than the curve in Figure 20 shows.Calcium doped batteries often VRLA batteries have 5-8 % higher OCV. [9] [10]

The density of the electrolyte is an indication of the state of the battery, see Figure 22.When the battery is discharged the electrolyte density decreases in proportion to theamount of Ah that is discharged from the battery. The state of charge for different

types of lead acid cells can be seen in Figure 21. While there is a linear relationbetween the OCV and the electrolyte density from 1.10 kg/dm3 the OCV can also bean indicator of the SOC.

Figure 21. Electrolyte density at different SOC, for different types of lead acid batteries. More

about the different types of batteries can be read about in 2.3.6.

When the battery is being charged there is not a similar linear behavior between thedensity and the SOC. The electrolyte is not completely mixed during charge, thiscauses a lag between the density and the Ah charged. When the battery has reached itsgas voltage the electrolyte will be mixed again, see Figure 22. [10][15]

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Figure 22. The electrolyte and SOC dependence during constant current discharge and charge.

From the chapters above a conclusion can be made that all the parameters aredependent of each other. Its good to know all this parameters dependency whilemeasure any of them. See Figure 31 in the result chapter for an overview of all the

parameters impact on each other.

2.4.4. Charging and discharging for lead acidbatteries

Charging

There are some easy but important rules for battery charging.

If the battery has been deeply discharged it should be carefully charged at thebeginning with a proportionally low current.

In the end when the battery is charged until 100% SOCage the battery should also becharged with a low current, normally below the C20 rate.

The most important rule when charging is to avoid the gassing voltage. Manychargers include rectifying equipment that causes an AC ripple with the direct current.The ripple causes further heating of the battery. Its important to minimize the rippleespecially in the end of the charge where the margin to the gas voltage is less. A wayto minimize the ripple is to implement a filter of suitable size. The current thattheoretical can be applied to the battery without reaching the gas voltage is aninversed exponential function

t

eddischech eAhI

argarg (13)

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where Ahdischarged is the ampere hours discharged at any chosen time, t is the time ande is the exponential function. This means that the charge current can be the samevalue as the ampere hours discharged from the battery, e.g. a 100Ah battery that have

been discharged with 80Ah can be charged with 80 A. This is the theoretical values.In reality there are other factors that decrease the charge current. To high currents will

change the morphology of the electrode and the conducting material will be heated.Side reactions will occur for batteries that will get to warm particularly above 55 Ce.g. the corrosion rate will increase. The losses would be really high, see Figure 14. Inreality the charger often have a current limit in the beginning of the chare algorithm.Its common that the chargers give a constant current up to 80 % of the SOCage. [10][11]

The current that a battery will be charged at is a proportion of the difference of thebatterys open circuit voltage and the charge voltage, see Figure 23.

ernal

OCVech

echR

UU

Iint

arg

arg

(14)

Where Ucharge is the charge voltage and UOCV is the open circuit voltage. The internalresistance will fall during charge as the open circuit voltage is rising. The chargevoltage is e.g. set by the power given from a generator.

Figure 23. Charging scheme. Ucharge = Uload = Ubattery = UOCV + Icharge Rinternal = Icustomer Rload.

(Rpolarization is included in Rinternal).

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Constant voltage charging

The old fashion conventional method of charging lead acid batteries, often for carbatteries has been to put a constant voltage over the terminals. The voltage is thensafely set a bit under the gassing voltage. The current in the beginning will then bevery high, see equation 13. However there is a current limit set by the electronics in

the charger or by the thermal characteristics of the battery. The charge current willthen fall as the open circuit voltage rises. When the open circuit voltage starts toapproach the charge voltage the charge current will get remarkable low and it will bevery time consuming to charge the remaining capacity to reach 100% SOCage. Thelast percentage of the SOC is important for the batterys service time. These old timechargers are therefore not suitable for charging a battery bank to achieve highstandard and a good efficiency. Although if the charge voltage is set just under the gasvoltage the battery will reach 100 % SOCage faster. Often the chargers stop at a finishrate with a current at around C20. [15] [16]

Figure 24. Constant voltage charging to the left in the figure. When the charge voltage is kept

constant while the OCV is rising the current will taper. Constant current charging to the right in

the figure. To keep a constant current the charge voltage must rise simounsly with the OCV rise.

Float charging

Float charging is a type of constant voltage charging held at a low potential. The Floatcharge state is often done when the batteries are fully charged. The low potential is

held just enough to cover for the self discharge.

Constant current charging

Another type of charging is the constant current method. The current is held at a fixvalue until the battery is fully charged. This is not a common method for charginglead acid batteries. To achieve a fast and effective charge algorithm, adjustments ofthe current rate is needed.

Trickle charging

Trickle charging is a type of constant current charging. The trickle charging is used tomaintain a fully charged battery fully charged. It gives a low current around C100 to

handle the self discharging and small intermittent discharges.

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Pulse Charging

Pulse charging is a method developed to minimize the charging time. Tests made onthis method also show increased battery life. Basically the method consists of highcurrent pulses. The current is much higher than allowed for the gassing voltage. Thespecialty with the Pulse charging is that the period of the current pulses are

controllable. The time of the pulses are set so the battery wont be able to heat up.While the battery is being charged and the SOC is getting higher the pulse time has to

be shorter. This is a result of the gas development time. The hydrogen and oxygendevelopment has a time constant that is depending of the SOC. If the current pulse isshort enough the time is not enough to produce gas. The current will then only beconsumed in the charge reaction.

In this particularly way the applied currents can be much higher. Integrating thecurrent over the time pulses shows that the total Ah charged can be applied in muchshorter time than for constant voltage or constant current charging, without harmingthe battery.

Figure 25 shows a common charging scheme where constant current charge first isused where most of the energy is charged to the battery. This is followed by a constantvoltage charge where the rest of the energy is charged.

Figure 25. Conventional charge algorithm with a constant current followed by a constant voltage.[16]

Comparing the Pulse method with the commonly used method in Figure 25 shows thatthe Pulse charge has most benefits during the later part of the charge. The reason forthis is that pulses can be very long in the beginning of the charge when the battery ismore resistant to high currents. The long pulses make no big difference to the constantcurrent charging. The high current pulses should be used carefully in the early SOCavoiding damages to the battery.

The pulses length is set by a highest OCV allowed. The OCV can be measured duringthe off time of the pulses. If the OCV is too high depending of a reference temperaturethe current pulses is held shorter. The OCV increases with the SOC and the current

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pulses length decreases. When the battery is fully charged the time duration of theOCV to decay is the same as the charge pulses and the battery is kept fully charged atall time. This can be compared with the float charging. Some newer charges have aPulse charge method in the end of the charge algorithm, to maintain the battery fullycharged. This method has shown good benefits for the capacity and battery life in

made tests, e.g. in [17].

Many of the up to date chargers have charger algorithms made of many differentsteps. Keeping the correct pulse periods during the entire charge, a Pulse charge could

be made during the entire charge, keeping the algorithm very simple.

Tests have shown that if the pulses also include a small discharge current after eachcharge current the charge time can be even lower and also giving a higher life time ofthe battery. The discharge currents equalize the concentration of the active material inthe battery. This reaction improves the charge acceptance of the next current pulse.

In the test made in [16] two VRLA GEL 28 Ah batteries were used. The dischargecurrent pulse was applied at 8 % of the charge pulse time. The test shows that theenergy being charged to the battery is of the same amount as without the dischargecurrent. Note that the charge current is applied at 8% less time than before, see Figure26.

Figure 26. Relative charging rates with and without the discharge pulses. The curve starts

around 50% where the constant current step is finished. This is because the constant currentmethod and the pulse method is slightly similar in the begging of the charge. [16]

In the figure below from the same test the conventional charge curve is shown as well.This shows the remarkable reduce of charge time that the pulse test gives.

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Figure 27. The pulse charging with and without discharging pulses and the conventional

constant current/voltage charging. [16]

Even though this test shows good features fore the pulse charging, other battery typesmight show other results. The technique should be used only with certainty that thegas voltage isnt exceeded. Modifications of the method can also be required when the

battery is aging. [10] [16] [18]

Equalization charging

The equalization charging is done if the electrolyte needs to be equalized. Cells thatare connected in series will often differ a bit in the electrolyte density, and the densitycan also differ within a cell from the bottom and up. Parts that have higher density areheavier and will sink to the bottom. The equalization is achieved by a constant currentcausing a high voltage around 2.65 V/cell. The voltage is then clearly above thegassing voltage and the electrolyte will start to bubble and it will be mixed around.This operation is clearly not good for all types of batteries while some water will belost in the electrolysis and a small part will also evaporate. For open flooded batteriesthat can be refilled with water this is however a good thing to do occasionally.

Mixed charge algorithm

To achieve a healthy charging algorithm for the battery the above mentioned methodsis mixed in a desired way. The best possible algorithm differs some depending onwhich type of battery and system its done fore. A voltage and current curve for aspecial charge algorithm for some of the different charge steps is shown in Figure 28.

The first step in the curve starts with a low current for deeply discharged batteries,(towards the cut off voltage). This could e.g. be a trickle charge. When the OCV ishigh enough the bulk phase can be applied. The Bulk phase is where the main chargeis done. Here the current is held constant only varying depending of the temperature.To get fastest possible charge during the Bulk the constant current is held as high as

possible without harming the battery life. The Bulk phase is finished when the chargevoltage have get as high as the gassing voltage. Then the charging is held at a constantvoltage just below the gassing voltage, typical at 2.39 V/cell at 25 C. This chargesection is sometimes called the Absorption phase. The current is then following thecurve set by equation 13. The charge algorithm could end here and the battery would

eventually be fully charged. The following steps in Figure 28 are mentioned above.[10][11][15]

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Figure 28. Different charge steps for charge algorithms. The red line is the charge voltage and the

blue line is the charge current. [15]

OverchargeThe battery is often charged to 110-120 % SOCage to compensate for the losses fromthe last discharge. Its important to do this overcharging to remain the capacity of the

battery, but it should be done carefully without gassing. Too much overcharge willcause the pressure to exceed above the designed venting pressure. The loss of waterand internal heating accelerate the positive grid corrosion. Overcharge have a benefit,it results in an equalization of the electrolyte. [10][19]

Discharging

The discharge of the battery will take place when the charge voltage is less

than the battery voltage, see Figure 29.

When the cells are being discharged the structure of the active material and electrolytewill change. Therefore the open circuit voltage will change during the discharge. Theinternal impedance rises during discharge due to the electrolytes changing chemistry.These parameters cause the voltage of a cell to fall during discharge. With a constantdischarge current the density of the electrolyte is proportional to the dischargedampere hours, see Figure 22.

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Figure 29. A circuit scheme over the battery cell and the connected load during discharge. Rload is

the resistance of the load. I is the current drawn from the battery. Rinternal is the internal

impedance of the battery. Uocv is the open circuit voltage. Ubattery is the battery voltage during

load. Udrop is the voltage drop over the internal impedance during load. The voltage drop caused

by the polarization is not showing in the figure, its instead included in the internal resistance. The

diode in the figure is preventing back currents.

Figure 29 shows the circuit scheme of the battery cell and the connected load duringdischarge. The equation for the battery voltage is

Ubattery = UOCV Upol Icustomer Rinternal = Icustomer Rload (15)

where Uocv is the open circuit voltage and Upol is the voltage drop for the polarization.

The discharging can be made in many different ways, with constant current, stochastic

fluctuating current or as short current pulses. A battery that is discharged with pauseswill get some time to recover during the pauses. The voltage will then be able to riseafter a heavy discharge, see Figure 30. The pauses will be more important duringheavy discharge.

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Figure 30. Effects of discharge pauses.

2.4.5. Cycling and lifespan of the battery

After each discharge and charge the battery chemistry will change slightly. Thebattery is ageing and the capacity is slowly decreasing. The reaction is not reversible.There are some parameters that are clearly changing the life of the battery.

C-rate. High C-rate gives shorter life. Temperature. High temperature shortens the battery life. Depth of discharge. To deep discharges change the battery chemistry and

shortens the battery life Significant overcharge Number of cycles Its important to charge the battery all the way up to 100 % SOCage

occasionally

Discharges that are deeper than what the battery is made for causes sulphation andgrid corrosion at the plates. Some of the sulphate will crystallize on the electrodesreducing the active area which reduces the battery capacity. To dissolve the crystalsand to recover some of the battery capacity a high voltage must be applied. Morecrystals require higher voltage. One way to recover the battery without causinggassing is to apply current interrupts as a kind of Pulse charging. A Pulse charge madein the end of the recharge reduces the sulphation and increase the restored capacity.

Increased depth of discharge will result in less number of cycles possible before thebattery is useless. More cycles per year also decreases the battery life.

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Significant overcharges also cause grid corrosion and if the pressure gets to high itwill cause losses of the electrolyte. [17] [19]

The battery ageing can be measured with the resistance. When the resistance of thebattery has increased with 25% from the point when the battery was new, the capacity

is reduced from 100% to 80%. The resistance can differ with 8% between VRLAbatteries of the same batch. A battery is almost useless when the SOCage at 100% is80% of the SOC. [9]

2.4.6. Different available battery types

The Lead-Acid batteries are still by far the cheapest and most robust battery on themarket providing good performance and life characteristics. For this thesis the choiceof battery type is therefore very easy. Comparison and investigation on different

battery products will not be a part of this thesis. The lead-acid battery have a marketof 40-45% of the sales value in the world including for example energy storages,emergency power, vehicles, telephone systems, power tools, communication devicesand as the power source for mining.

The electrical turnaround efficiency is about 80 % thats comparing discharge energyout with charge energy in. A single battery can have a size of thousands Ah. These arefactors together with the relatively low price that makes them suitable for energystorages use. The lead acid batteries have a typically energy density of 30-40 Wh/kgwhich makes them very heavy, although this is not a problem for systems that aresited on fixed places.

There are several different types of lead-acid batteries constructed for specificapplications. Optimizations for different parameters distinguish the different types of

batteries. Some parameters are e.g. energy density, power density, cycle life, floatservice life and cost. [10]

The SLI battery

The most common battery type is the lead acid batteries made for the car industry.These batteries are often called SLI batteries, which stands for start, lighting andignition. The batteries are often open, so gasses can freely vanish. The open batteries

are very robust and reliable batteries. The drawback of the open batteries is that theyneed maintenance and have to be refilled with water. They have a vent plug where thegases that are formed can escape. The start battery made for cars have a high powerdensity to be able to give the high current needed to start a car. After the start the

battery is float charged when the motor is running. The battery will seldom be deeplydischarged and the cycle life is not an important factor.

The Valve Regulated Lead Acid (VRLA) batteries are made to minimize the waterlosses. These types of batteries are sealed which makes them maintenance free. Thesealed environment keeps all the gases inside the jar. However if the internal battery

pressure will be too high there is a valve letting the gas out. The reaction that recreates

the water is called recombination. The sealed battery forces the oxygen and hydrogento react and to produce water. The recombination reaction has a drawback, at the same

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time when water is reproduced lead sulphate is produced at the anode. This causesproblem for batteries when they are ageing. A possible way to solve this problem is toinclude a pulse current interrupt in the charging algorithm that dissolves the leadsulphate again.

There are two different types of VRLA batteries GEL and AGM. In a GEL batterythere are some substances added to the sulphur acid solution to make it more solid.This is made to prevent the solution to spread to the surroundings. These types of

batteries are very robust and can handle deep discharging cycles very good.

In an AGM battery the separator are made of glass fiber. This special separator keepsthe sulphur steady by capillary forces. The AGM can get higher power from a smallervolume because the separator can be made very thin with a low inner resistance. Thedrawback with this type of separator is that the solution has to have higher sulphurconcentration. To handle this, AGM batteries should be charged by a higher voltage.[10][11][15]

Two other classifications other than the most common SLI batteries are the tractionand the stationary batteries. These batteries can be of open vented or of VRLA type.

The stationary battery

Stationary batteries are typical made for standby or emergency backup power. Therecan be long periods where the batteries stand unused. They are often left at floatcharge, to be fully charged when needed. The electrolyte is often exceeded tominimize maintenance and to be more resistant to gassing. This makes the batteries

capacity limited by the positive plates in comparison with the traction batteries thatare limited by the acid in the electrolyte. To make the intervals between wateringgreater a nonantimonial grid is used. To be more sustainable against grid corrosiongrowth, the positive plate is scaled so it can grove 10 % before the battery will beunsuitable for use.

The traction battery

The traction battery is used in vehicles that are driven electrically or as hybrids. Themain difference to the SLI battery is that the traction batteries have to give suitable

power through all day while the SLI batteries are built to give a high current for a

short start up time. This means that the traction battery should be able to be deepcycled many times without being depleted. The SLI batteries can only manage around10 deep cycles before they are in bad condition.

The traction batteries have to include many Ah to be able to work properly beforethere is time to recharge them, e.g. a truck that is working 8 hours per day. When theworking day is over the battery requires a high charging rate to be able be used nextday again. The cost of a failure for a battery of e.g. 24 V would be fatally high.Therefore the batteries are built as separate 2 volt cells that are serial connected.

The parameter that makes deep cycling possible is the thickness of the positive and

the negative plates. The formation of the positive plates of the cell is an importantfactor for the traction batteries. The cells are built with positive cells that are either

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flat pasted or tubular plates. The tubular have a numerous advantages during operationsuch as less grid corrosion, less self discharge, less polarization losses and longer life,

but they have a higher initial cost. [10] [11]

2.4.7. Connect ions of batteries for a battery bank

When batteries are connected together forming a battery bank, its important that allthe batteries are working equally. Therefore its also important that they are of theexact same brand to achieve the same working conditions. Although the batteries areof the same batch there will be individual differences. When batteries are serial or

parallel connected, the bank will never work better than the worst battery in theconnection. Connecting batteries together should therefore be avoided, although itsoften necessary to achieve a high energy bank or to get a desired potential. [20]

Appendix 2 shows how a parallel connection shouldnt be done and how it should be

done properly.

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3. Result

3.1. Summary of the battery theory

In Figure 31 all the operation parameters of the battery is connected to each other. Thearrows points at the boxes that are influenced by a parameter. In this way the schemealso shows secondary dependencies. The battery capacity increases with highertemperatures as a direct dependency, but it also decreases with higher temperaturewhile the battery life will be reduced with higher temperatures. The amplitude of thedependency is not showing, this can be read about in the theory chapters.

Figure 31. An overview of the parameters impact on each other. The parameter that another

parameter is dependent of is always rising in the figure. The + and symbols indicates if there is

a negative or positive dependency. E.g. the internal resistance is decreasing when the

temperature is increasing. The internal resistance can either increase or decrease dependent ofthe electrolyte density. The smallest internal resistance is achieved at densities around 1.28 kg/m

3.

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3.2. The system conf iguration

The system is built to keep the turbine at an optimal speed as much as possible. Thecharging of the battery bank should not affect the efficiency of the turbine. Thecharging of the batteries will not be optimal in regard to the conventional chargingtime. The wind energy is simply not enough at all times, although the chargingalgorithm can be the same as for conventional chargers. When the energy from thegenerator is less than what the charging algorithm is demanding in regard to thecurrent or the voltage amplitude the charging will just take longer time. When the

battery is fully or almost fully charged the energy from the generator may be largerthan what the battery and the load can receive. The excess energy is then simplydumped as heat.

For the vacation cottages etc. the heat can be used to heat the house or the watersupply. For the Tower tube the area inside will be heated which can be good for the

batteries during winter to maintain a high capacity from them. In warmer countries theenergy can be dumped with cooling fans to maintain a longer battery life.

The entire load and charge control system scheme is drawn in Figure 32.

The AC-Voltage from the generator is first rectified to DC-voltage. For the Towertube VAWT the AC-Line to line voltage is kept at 200 V at a wind speed of 10 m/swith =4, and at wind speeds above 10 m/s. 200 V is the highest line to line voltage ata frequency of 75 Hz. At a wind speed of 4 m/s and =4 the line to line voltage is 80V and the frequency is 30 Hz. The DC-voltage lies between 108 V and 270 Vdependent of the wind speed.

The dump load is placed after the AC/DC step. The dump load is a resistive loadwhich is switched with an IGBT. If there is more power available than the customerand the battery can absorb, energy will be stored in the turbine causing it to speed up.When the turbine rotation speed is above a decided value the IGBT will start to switchand energy will be dumped to keep the turbine at the optimal speed. See appendix 1for a detail declaration of the PWM for the dump control.

Next in line in the scheme in Figure 32 is the DC/DC converter. The DC/DCconverter is needed to get a desired voltage. The output voltage should have typicallyvalues for charging a lead acid battery and the DC output must therefore be

adjustable. The voltage level is set by the energy available from the generator and thecharge algorithm.

The voltage span from full till no charge current that is applied when charging abattery is small. The span for a 48 volt battery bank is about 42-57.6 volts dependingof the depth of discharge of the battery and if its a VRLA or a flooded battery. Thisdoesnt include the equalization charge. The same voltage span will not be of greateffect of the customer current. While the battery current will differ highly dependentof the charge voltage, the customer current will only differ by the load fluctuations.

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Figure 32. Overview scheme of the load and charge system. The customer current Ic is constant

for a specific load and will only differ slightly with the charge voltage. The battery charge

current Ib will fluctuate and is only dependent of the charge voltage, the internal resistance and

the OCV.

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When the charge voltage will be lower than the OCV of the battery the battery will bein the discharge mode. The entire load current will then be taken from the battery.

When searching after appropriate DC/DC converters we found it very hard to findconverters that were adjustable. It was also hard to find converters above 2 kW. The

reason for the low power is that most converters are made for the wall outlets whichhave a voltage of 230 volts. The fuse is often at 10 Ampere which gives 2.3kW in

power. This problem can be solved by serial or parallel connecting the converters.

A serial connection is proper if the batteries are charged individually, which wouldgive good individual charge properties. A parallel connection would be good if a unitwould crash. If a 10 kW system would include five 2 kW units and one would crashthere would still be 8 kW left to use. A system that is properly built for the chosenwind site would then still run at optimum until very high wind speeds would occur.

For the case that will be built and tested we have obtained six 2 volt cells to build a 12

volt battery bank. The DC/DC converter chosen for that project is a Cosel PBA1500F-153. The ripple from the converter should be considered if it is at a reasonablelevel. If the ripple is too high a filter is required after the converter. The ripple is toohigh when it affects the charge voltage with about 0.005 volts.

To be able to control the DC/DC converter in a way that give desired values for thebattery and the turbine speed at all time, a battery charging microcontroller isconnected to the DC/DC converter. The quota between the wind speed and therotation speed indicates how much power that is available for battery charging and theload. The microcontroller is programmed with a charging algorithm which is steered

by measured inputs such as the customer and battery current, the battery voltage andthe battery temperature. The charge algorithm sets how much of the available powerthat is going to be used to charge the battery and how much that will be dumped. Thecharging microcontroller gives a signal to the DC/DC converter of 0-5 volts. For a 12volt battery e.g. 2.5 volts means that the DC/DC converter should give a voltage of 15V to the battery.

When the battery is discharged and cant deliver the desired load current a dieselaggregate will be used. When the diesel aggregate is used the battery bank can becharged again from the VAWT and the gate to the customer will be open. It will becharged until a level where its able to give a suitable load current again for a longer

time. This will prevent that the diesel aggregate will turn on and off over short timeperiods.

The aggregate should also be able to charge the batteries in case of bad windconditions during a longer time. The batteries life time is shortened much faster ifthey stand completely discharged. The gate from the diesel aggregate should becontrolled by the charging microcontroller. When the battery voltage is constant

below a specified voltage during a specified time dependent of the battery type thegate will be closed.

3 Data sheet of the Cosel PBA 1500F-15 can be found athttp://www.trcelectronics.com/Cosel/pdf/pba1500f.pdf

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When maintenance is done of the system, the state of the batteries can be measuredwith a rapid battery tester such as the Spectro CA-12.4

3.3. Choice of charging algorithm for the final

solution

The charge algorithm chosen in this result is made to extract as much as possible ofthe wind energy in the turbine optimized for a high Cp. The wind is a fluctuatingsource, its important to always take as much as possible out of the wind energy.

Another way to form the charge algorithm would be to make a load control optimizedfor the battery charging. The WAVT could then be controlled by an optimum C-ratefor the battery charging, instead of a specified for optimum Cp.

When the specified C-rate for the optimum battery charging is higher than what thegenerator can deliver at a specific moment the battery C-rate will simply be lower.This is exactly the same case as for the charge algorithm made to keep an optimumCp.

When the specified C-rate for batteries is lower than what the generator can give at aspecific moment the extra energy must either be dumped or be saved as rotationenergy in the turbine. If the extra energy is dumped the VAWT can never give ahigher power than the optimum C-rate for the batteries multiplied with the DC voltagee.g. 48 volts multiplied with 50 amperes gives 2400 W.

Energy saved as rotation energy can only be saved for very short moments dependingof the moment of inertia of the turbine. For both the Lucia and the Tower tube theturbine would have to be shut off in just a few seconds not to destroy the constructiondue to the high speed and the Cp would decline very fast see Figure 6. This type ofcharge algorithm would give much higher loses and the overall efficiency would bemuch lower. See appendix 3 for calculations of the saved extra rotation energy.

To extract most energy possible, equation 13 should be used, although when thebatteries capacity is lower than the nominal power output of the WAVT the batterywill be damaged due to high currents. E.g. if the VAWT can give 200 amperes at 48volts and the battery capacity is only 400 Ah the battery will be damaged very fast.

This shows the importance of big battery banks to be able to take care of all theincoming wind energy.

Proposal one for a charge algorithm

The first step of the algorithm will be set by a maximum current level. The level is setby the batteries specifications of highest acceptable current. Normally the rate is aboutC3. The high current will provide high losses, but its important to use all the availableenergy in the wind.

When the OCV has become so high so the charge current will be lower than thehighest specified C-rate of the battery, the charge current can follow the exponential

4 More information about rapid battery testers can be read about at www.cadex.com

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function in equation 13. This is set by a voltage limit, set just below the gassingvoltage to achieve a high charging rate, see the absorption phase in Figure 28. Thecharge voltage should be temperature compensated by the values in Figure 13.

When the current is constant low for a longer time the battery is fully or almost fully

charged. The charge current is then set to C20 to get the last few but important percentof the charge. During conditions when there will be an excess of energy and the

battery is fully charged the battery should be float charged at a voltage of about2.25V/cell at 20 C dependent of the battery type.

In the end of a discharge when the battery is totally discharged the battery voltage willdecline very rapidly and there is almost no energy left to drain. To deep dischargeswill harm the battery and shorten the life to a great extent. An emergency tool can beimplemented to the microcontroller to stop the discharge before the critical point. Tosee if the battery is fully discharged the derivate of the voltage can be calculated andwhen the graph is getting steep the battery is close to the cut off voltage and the

battery should be disconnected, see Figure 16. If the battery is discharged to low, atrickle charge should be done before the C3 current can be applied when the battery isrecharged.

During normal conditions the discharging should be disconnected before the rapidlyvoltage drop, to save the battery. To measure SOC of the battery equation 9 can beused. To get an accurate value from this equation the losses should be calculated. Thisis not a simple task and the losses are often template calculated. While there are somedifficulties with this method some reference points should be measured while the

battery is new. The reference points should be measured during a total discharge andcharge cycle. Parameters to measure are the OCV, battery voltage and dischargecurrent when fully charged and discharged. The internal resistance of the battery forthese points can then be calculated. The temperature must be measured as well whilethe parameters are temperature dependent.

A special condition will appear when the power from the VAWT is lower than theload power demand (customer current needed multiplied with the charge voltage).The load will consume the current needed to operate properly and the charge voltagewill be reduced. If the power from the VAWT is to low, the charge voltage will getlower than the OCV. The load will then have to take the power from the battery andthe system will be in the discharge mode. This will cause a fluctuation behavior of the

discharge and charge. When the battery is being discharged the turbine will speed upcausing the voltage to rise again and more energy will be stored in the turbine. Someof the energy will also be stored in the filter capacitors. When the power of theVAWT is high enough the system will be in charge mode again. If the wind is thesame or decline, the procedure will go around again and the charge voltage will soon

be below the OCV. This causes a pulse behavior of the discharge and charge modes.The pulses duration depends on how close the charge voltage is to the OCV.

This special behavior when the system is switching between the charge mode and thedischarge mode causes the turbine to run with fluctuating speed. This behavior couldcause higher maintenance of the VAWT.

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How often this behavior will occur is dependent of how close the load power demandis to the nominal power of the power plant and what kind of wind site its stated on.

To prevent this pulsating behavior of the turbine, a pulse charge should be applied inthe charge algorithm with faster frequency than the naturally pulsating frequency that

appears when the load demand is higher than the turbine power.

The pulse discharge gives the battery time to recuperate, see Figure 30, and thebattery will be able to give more energy than without the pulses.

During the discharge interrupts the battery will be charged but the OCV and thecharge voltage will be so close to each other so the charge current will be of minorimportance, see equation 14.

The pulsating charge and discharge is proved to be very good for battery capacity andlife, see the charge and discharge chapters in the theory.

Proposal two for a charge algorithm

A second approach to solve the charge algorithm problem is to use Pulse charging atall time through the charge. Pulse charging through the entire charge have been testedin [16] and shown very good results. If this method is better than the other onesuggested above must be tested in reality on the specific battery bank. The problemwith the Pulse charging is to modify the pulses to the right length, avoiding gassing.The OCV is measured during the off time of the pulses and is compared with areference temperature. To high OCV shortens the pulse length and to low OCVextend the pulse length.

A pulse charge method that is working properly wi