EE152 Green Electronics

Batteries 11/5/13

Prof. William Dally Computer Systems Laboratory

Stanford University

Course Logistics •  Tutorial on Lab 6 during Thursday lecture •  Homework 5 due today •  Homework 6 out today •  Quiz 2 next Thursday 11/14

AC Input/Output Summary •  AC is just slowly changing DC

–  But need to store energy during the “nulls”

•  Power factor = Preal/Papparent –  Want power factor very close to 1 –  Requires current proportional to voltage

•  PF correcting input stage –  Controls input current – sine x error

•  Grid connected inverter –  Controls output current

•  Independent inverter –  Control output voltage –  Use a full-bridge to generate a PWM Sine Wave

•  Pulse width proportional to sin(x)

–  LC Filter to reject high frequencies

Anti-Islanding •  Grid-connected inverters need to turn off when the grid

goes down. •  Safety issue for firemen, linemen, etc… •  How do you detect when the grid goes down?

Anti-Islanding •  Line monitoring

–  Voltage limits, frequency limits. –  Rate of change of frequency –  Rapid phase shift

•  Active detection –  Impedance measurement –  Forced phase shift/frequency shift

Batteries

Batteries •  Many Green Electronic systems require energy

storage •  Batteries are widely used to store energy in chemical

bonds •  Model as dependent voltage source •  Care required in charging and discharging

Energy Density

Device Energy Density MJ/kg

Gasoline 44 Lithium Ion Battery 1.7 Lead Acid Battery 0.15

Two Dimensions of Energy Storage

electrodes exhibit power characteristics similar tothose of supercapacitors. Moreover, there are alsohybrids such as metal/air batteries (or, in otherwords, metal/air fuel cells), which contain a batteryelectrode (metal anode) and a fuel cell electrode (aircathode). Finally, Figure 3 also shows that no singleelectrochemical power source can match the charac-teristics of the internal combustion engine. Highpower and high energy (and thus a competitivebehavior in comparison to combustion engines andturbines) can best be achieved when the availableelectrochemical power systems are combined. In suchhybrid electrochemical power schemes, batteries and/or supercapacitors would provide high power and thefuel cells would deliver high energy.

Figure 4 shows the theoretical specific energies[(kW h)/t] and energy densities [(kW h)/m3)] ofvarious rechargeable battery systems in comparisonto fuels, such as gasoline, natural gas, and hydrogen.The inferiority of batteries is evident. Figure 5,showing driving ranges of battery-powered cars incomparison to a cars powered by a modern combus-tion engine, gives an impressive example of why fuel

cells, and not batteries, are considered for replace-ment of combustion engines. The theoretical valuesin Figure 4 are an indication for the maximum energycontent of certain chemistries. However, the practicalvalues differ and are significantly lower than thetheoretical values. As a rule of thumb, the practicalenergy content of a rechargeable battery is 25% ofits theoretical value, whereas a primary batterysystem can yield >50% of its theoretical value indelivered energy. In the future, fuel cells might beable to convert the used fuels into electrical energywith efficiencies of >70%. The difference between thetheoretical and practical energy storage capabilitiesis related to several factors, including (1) inert partsof the system such as conductive diluents, currentcollectors, containers, etc., that are necessary for itsoperation, (2) internal resistances within the elec-trodes and electrolyte and between other cell/batterycomponents, resulting in internal losses, and (3)limited utilization of the active masses, as, forexample, parts of the fuel in a fuel cell leave the cellwithout reaction or as, for example, passivation ofelectrodes makes them (partially) electrochemicallyinactive. However, as batteries and fuel cells are notsubject to the Carnot cycle limitations, they mayoperate with much higher efficiencies than combus-tion engines and related devices.

1.2. DefinitionsThe following definitions are used during the

course of discussions on batteries, fuel cells, andelectrochemical capacitors.A battery is one or more electrically connected

electrochemical cells having terminals/contacts tosupply electrical energy.A primary battery is a cell, or group of cells, for

the generation of electrical energy intended to beused until exhausted and then discarded. Primarybatteries are assembled in the charged state; dis-charge is the primary process during operation.A secondary battery is a cell or group of cells for

the generation of electrical energy in which the cell,after being discharged, may be restored to its originalcharged condition by an electric current flowing inthe direction opposite to the flow of current when thecell was discharged. Other terms for this type ofbattery are rechargeable battery or accumulator. Assecondary batteries are ususally assembled in the

Figure 3. Simplified Ragone plot of the energy storagedomains for the various electrochemical energy conversionsystems compared to an internal combustion engine andturbines and conventional capacitors.

Figure 4. Theoretical specific energies [(kW h)/tonne] andenergy densities [(kW h)/m3] of various rechargeable bat-tery systems compared to fuels, such as gasoline, naturalgas, and hydrogen.

Figure 5. Comparison of the driving ranges for a vehiclepowered by various battery systems or a gasoline-poweredcombustion engine.

Editorial Chemical Reviews, 2004, Vol. 104, No. 10 4247

Energy Density vs Battery Chemistry

250

200

150

100

50

50 100 150 200 250 300 350 400 450

WattHours/Litre

WattH

ours/K

ilogram

Lead Acid

Nickel Cadmium

Cylindrical

Cylindrical

Cylindrical

Prismatic

Prismatic

Prismatic

Prismatic

Aluminum Cans

Nickel Metal Hydride

Lithium Ion

Lithium Phosphate

Lithium Polymer

Photo of Pack

18650 Cell

Tesla Pack

Battery Model

LB RB

VBS VB

+

-

VBS depends on state of charge and temperature

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4.2VStd.1625mACharging Current

Rated Capacity (at 20䉝)

Nominal Voltage

Charging Method

Charging Voltage

Nominal Capacity (at 25䉝)

Cell Type NCR18650BSpecifications

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Std.1625mA4.0hrs.

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47.5g

(D) 18.25mm

(H) 65.10mm

676Wh/l243Wh/kgGravimetric Energy Density

Charging CurrentCharging Time

Ambient Temperature

Weight (Max.)

Dimensions (Max.)

Volumetric Energy Density

H 64.93mmD 18.2mmd 7.9mm

Dimensions(Typ.)of

Bare CellMaximum size without tube

Discharged State after Assembling

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3.5

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Discharge Temperature Characteristics for NCR18650B1Scell-1 Charge:CC-CV:1.625A-4.20V(65.0mA cut)

Discharge:CC:3.25A(E.V.:2.50V)1.-20Υ 2.-10Υ 3.0Υ 4.25Υ 5.40Υ 6.45Υ 7.60Υ

Discharge Rate Characteristics for NCR18650BCharge:CC-CV:1.625A-4.2V (65.0mA cut)Discharge:CC:Variable Current (E.V.:2.50V)Temp:25Υ

2G23X0KYKU

0 500 1000 1500 2000 2500 3000 3500 4000Discharge Capacity / mAh

2.0

2.5

3.0

3.5

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VDischarge Temperature Characteristics for NCR18650B1S

cell-1 Charge:CC-CV:1.625A-4.20V(65.0mA cut)Discharge:CC:3.25A(E.V.:2.50V)1.-20Υ 2.-10Υ 3.0Υ 4.25Υ 5.40Υ 6.45Υ 7.60Υ

Discharge Temperature Characteristics for NCR18650B

2G23X0KYKU

0 500 1000 1500 2000 2500 3000 3500 4000Discharge Capacity / mAh

2.0

2.5

3.0

3.5

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3000

4000

5000

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3000

4000

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3.5

4.0

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3000

4000

5000

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Charge Characteristics for NCR18650B1S

No.1 Charge:CC-CV:1.625A-4.20V(65.0mA cut)

Cell Voltage

Charge Characteristics for NCR18650B

2G23X0KYKU

0 60 120 180 240Charge Time / min

2.0

2.5

3.0

3.5

Cel

l Vo

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0

1000

2000

3000

Cur

rent

/ m

1000

2000

3000

Cap

acity

/ m

0 60 120 180 240Charge Time / min

2.0

2.5

3.0

3.5

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0

1000

2000

3000

Cur

rent

/ m

Capacity

45Υ 25Υ 0Υ

Current

Lead-Acid Charge Cycle

IB

IT

IC

VT

VC

VF

t0 tB tC tF

Bat

tery

Vo

ltag

eB

atte

ry C

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ent

Time

trickle bulk charge completion float

High Charge Voltage Reduces Life

Deep Discharge Reduces Life

High Temperature Reduces Life Particularly at high SOC

Lead-Acid Charge Cycle

IB

IT

IC

VT

VC

VF

t0 tB tC tF

Bat

tery

Vo

ltag

eB

atte

ry C

urr

ent

Time

trickle bulk charge completion float

Layered Control

State Machine

V or IControl

State

Vbat

Ibat

PWMCurrentControl

Imax

IM1 G

Lead-Acid Charging States

Off Trick Bulk Comp Float

Off I = IT I = IB V = VC V = VF

VT>V≥0 V≥VT

VB>V≥VT

V≥VC

VMax>V≥VC

Reset, V<0,

V>VMax I<IF

Charging Power Path

Vac

RCSVbat

L1

CoD2

D1

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CT

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50 H

.005

1mF

1mF

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F1

400V

Vi

GND

Vo

input stage switched inductor output filter

Cells

One Cell ~2.0V for lead-acid ~3.2V for LiFePO4 ~3.7V for LiCo Capacity depends on volume 0.4Ah to 200Ah or more 1.28Wh to 640Wh

Series Connection Increases Voltage Ah remains the same

100 LiFePO4 cells is about 320V (280-360) Capacity (in Ah) the same as one cell 128Wh (0.4Ah cells) to 64kWh (200Ah cells)

Series Parallel vs Parallel Series

Which is preferred (assuming same Volts and Ah)?

Battery Management Tasks •  Charge control

–  CC, CV profile –  Cell balancing –  Temperature monitoring/control

•  SOC (state of charge) estimation –  Fuel gauge –  Integrate power –  Estimate from voltage, current, and temperature

•  Lifetime extension –  Avoid deep discharge –  Avoid high-charge, high-temperature storage

Cell Balancing

Maximum cell voltage must not be exceeded during charging Voltage of each cell must be monitored Current must be “bled off” of high-voltage cells before they exceed Vmax Simple resistive balancer or flyback to recycle energy

When Things go Wrong

Battery Summary •  Batteries store energy in chemical bonds •  Model as dependent voltage source with series R and L •  Terminal voltage is a function of charge state Q

–  Also a function of temperature and charge rate

•  Area of charge-discharge curve is loss in battery •  Cells connected in series and parallel to build large batteries

(series connection of parallel cells) •  Cells must be balanced to avoid overcharge •  Battery management

–  Charge control –  Fuel gauge –  Lifetime extension

Grounding

Ground •  Exactly one point in your circuit is GND

–  An arbitrary point that we refer to as having 0 Volts potential –  All other voltages are referenced to this point

•  Connections to GND should be made in a Spider not a Daisy Chain

Good Bad

Ground Loops •  Avoid loops

Why Ground Loops are Bad

Consider The PV Lab Power Path

A Poor Way to Ground This Why?

Grounding Resistor Introduces a Loop

Grounding Summary •  Single-point ground •  Avoid loops

Debugging

Debugging •  Get one thing working at a time

–  Work from input of circuit to output –  Work with simple stimulus, then more complex

•  Form a hypothesis and test it –  Don’t just randomly change things –  Work from the schematic –  Calculate voltage on each node and then verify it.

•  If a chip isn’t doing what it should –  Check every pin of the chip –  Vdd, GND, all inputs, all outputs

•  For current monitor –  Check voltage across sense resistor –  Check “bias” voltage for op-amp (0.45V) –  Check that op-amp inputs have the expected voltage –  Verify output voltage –  Check with DC input first, then with operating MPPT –  Check AC common-mode input voltage

•  Use a scope –  Signals that look good at DC (on a multimeter) may have big AC problems –  Signals that look good on a logic analyzer (digitized) may have big analog problems

Future Lectures •  Tutorial for Lab 6 •  Quiz 2 Review •  Guest Lectures Tesla, Renovo, Enphase •  Wrapup •  Project Presentations