Electric Vehicle Batteries

North Bay Chapter of the Electric Auto Associationwww.nbeaa.org

Updated 8/14/09Posted at: http://www.nbeaa.org/presentations/batteries.pdf

NBEAA 2009 Technical Series

1. EV Drive SystemsTODAY >>2. EV Batteries 3. EV Charging Systems 4. EV Donor Vehicles

Agenda

What is a Battery?Battery HistoryEV Battery RequirementsTypes of EV BatteriesEV Battery Temperature ControlEV Battery ChargingEV Battery ManagementEV Battery ComparisonEV Record HoldersFuture EV BatteriesEV Drive System Testimonials, Show and Tells and Test Drives

What is a Battery? electrolyteanode +cathode -chargercurrentDuring Chargevoltage and energy increasesenergyheatheatchemical reaction

What is a Battery? electrolyteanode +cathode -loadcurrentDuring Dischargevoltage and energy decreasesworkheatheatchemical reaction

Battery HistoryRechargeable batteries highlighted in bold.

EV Battery Requirements

SafeHigh PowerHigh CapacitySmall and LightLarge FormatLong LifeLow Overall Cost

EV Battery Requirements: Safe

Examples of EV battery safety issues:Overchargingexplosive hydrogen outgassingthermal runaway resulting in melting, explosion or inextinguishable fireShort Circuitexternal or internalunder normal circumstances or caused by a crashimmediate or latentDamageliquid electrolyte acid leakage

EV Battery Requirements: High Power

Power = Watts = Volts x AmpsTypically rated in terms of C the current ratio between max current and current to drain battery in 1 hour; example 3C for a 100 Ah cell is 300A Battery voltage changes with current level and direction, and state of charge1 Horsepower = 746 WattsCharger efficiency = ~90%Battery charge and discharge efficiency = ~95%Drive system efficiency = ~85% AC, 75% DCbatteriesmotor controllermotorheatheatheatshaftchargerheat100% in60% - 68% out32% - 40% lost to heat

EV Battery Requirements: High PowerExample

Accelerating or driving up a steep hillMotor Shaft Power = ~50 HP or ~37,000 WBattery Power = ~50,000 W DC, ~44,000 W AC Battery Current~400A for 144V nominal pack with DC drive ~170A for 288V nominal pack with AC drive

Driving steady state on flat groundMotor Shaft Power = ~20 HP or ~15,000 WBattery Power = ~20,000 W DC, ~18,000 ACBattery Current~150A for 144V nominal pack with DC drive ~70A for 288V nominal pack with AC drive

ChargingDepends on battery type, charger power and AC outlet ratingExample: for 3,300 W, 160V, 20A DC for 3,800 W, 240V, 16A AC

EV Battery Requirements: High Capacity

Higher capacity = higher driving range between chargesEnergy = Watts x Hours = Volts x Amp-HoursWatt-hours can be somewhat reduced with higher discharge current due to internal resistance heating lossAmp-Hours can be significantly reduced with higher discharge current seen in EVs due to Peukert EffectAmp-Hours can be significantly reduced in cold weather without heaters and insulation

Example: 48 3.2V 100 Amp-Hour cells with negligible Peukert Effect and 95% efficienciesPack capacity = 48 * 3.2 Volts * 100 Amp-Hours * .95 efficiency = 14,592 Wh340 WattHours per mile vehicle consumption rateVehicle range = 14,592 Wh / 340 Wh/mi = 42 miles

EV Battery Requirements: Small and Light

Cars only have so much safe payload for handling and reliabilityCars only have so much space to put batteries, and they cant go anywhere for safety reasons

Specific Power = power to weight ratio = Watts / KilogramSpecific Energy = energy capacity to weight ratio = Watt-Hours / KilogramPower Density = power to volume ratio = Watts / literEnergy Density = energy to capacity to volume ratio = Watt-Hours /liter 1 liter = 1 million cubic millimeters

Example: 1 module with 3,840 W peak power, 1,208 Wh actual energy, 15.8 kg, 260 x 173 x 225 mm = 10.1 litersSpecific Power = 3,840 W / 15.8 kg = 243 W/kg Specific Energy = 1,208 Wh / 15.8 kg = 76 Wh/kg Power Density = 3,840 W / 10.1 l = 380 W/lEnergy Density = 1,208 Wh / 10.1 l = 119 Wh/l

EV Battery Requirements

Large FormatMinimize the need for too many interconnects; example 100 Ah

Long LifeMinimize the need for battery replacement effort and costExample: 2000 cycles at 100% Depth-of-Discharge to reach 80% capacity charging at C/2; 5 years to 80% capacity on 13.8V float at 73C

Low Overall CostMinimize the purchase and replacement cost of the batteriesExample: $10K pack replacement cost every 5 years driven 40 miles per day down to 80% DOD = 1825 days, 73,000 miles, 14 cents per mile

Source: Life Expectancy and Temperature, http://www.cdtechno.com/custserv/pdf/7329.pdf. Higher Temperature Reduces Shelf Life13 degrees reduces the life of lead acid batteries by half.

EV Battery ComparisonAvailable large format only considered; NiMH, small format lithium and large format nano lithium not included.

Data Source: MPS 12-75 Valve Regulated Lead Acid Battery Datasheet, http://www.cdstandbypower.com/product/battery/vrla/pdf/mps1275.pdf. Note: do not use Dynasty MPS batteries in EVs they are not designed for frequent deep cycling required in EVs

Lead Acid Battery Peukert Effect Reduces Range at EV Discharge Rates A 75 Amp Hour battery that provides 75 amp hours at the 20 hour C/20 rate or 3.75 amps only provides 42 amp-hours at 75 amps, a typical average EV discharge rate, or 57% of the nameplate rating. Nickel and lithium batteries have far less Peukert effect.

peukert

0.2346133333

0.4366666667

0.54

0.65

0.6533333333

0.7466666667

0.796

0.8533333333

0.9205333333

0.9466666667

0.9616

1

Constant Discharge Rate, Amps

Amp hours to 80% DOD (1.75 VPC, 10VP6C)

Peukert EffectDynasty AGM MPS Series 75 Ah

open ckt

112.84

0.812.6

0.7512.54

0.512.24

0.2511.94

011.64

12.721

12.480.8

12.420.75

12.120.5

11.820.25

11.520

AGM

Gel

Open Circuit Rest Voltage

State of Charge

Open Circuit Rest Voltage vs. State of Charge

data

Batteries

AGM

MPS-12-75 discharge rate vs. capacity

hourscurrent% of nameplateamp hours

0.08321223%18

0.2513144%33

0.58154%41

0.756565%49

14965%49

22875%56

319.980%60

512.885%64

88.6392%69

107.195%71

126.0196%72

203.75100%75

open circuit voltage voltage test

SOC

rest voltageAGMgel

12.8100%

12.680%

12.575%

12.250%

11.925%

11.60%

12.7100%

12.580%

12.475%

12.150%

11.825%

11.50%

voltage

Source: Dynasty VRLA Batteries and Their Application, http://www.cdtechno.com/custserv/pdf/7327.pdf. Lead Acid AGM Batteries are Better for High Current Discharge RatesGels have higher internal resistance.Higher discharge rates are typical in heavier vehicles driven harder in higher gears with smaller packs and less efficient, higher current, lower voltage DC drive systems.

Source: Impedance and Conductance Testing, http://www.cdtechno.com/custserv/pdf/7271.pdf. Source: Capacity Testing of Dynasty VRLA Batteries, http://www.cdtechno.com/custserv/pdf/7135.pdf. Lead Acid Batteries Need Heaters in Cold ClimatesThey lose 60% of their capacity at 0 degrees Fahrenheit.

Source: Dynasty VRLA Batteries and Their Application, http://www.cdtechno.com/custserv/pdf/7327.pdf. Gels Have a Longer Cycle LifeAGMs only last half as long, but as previously mentioned can withstand higher discharge rates.

Flooded Lead Acid Battery Acid Containment is Required for Safety

In addition to securing all batteries so they do not move during a collision or rollover, flooded lead acid batteries need their acid contained so it does not burn any passengers.

Flooded Lead Acid Battery Ventilation is Required for Safety

When a cell becomes full, it gives off explosive hydrogen gas. Thus vehicles and their garages need fail safe active ventilation systems, especially during regular higher equalization charge cycles that proceed watering.

High Power, High Capacity Deep Cycle Large Format Batteries Used in EVs:

LiFePO4Hi PowerThunder Sky LMPValence Technologies U-Charge XP, EpochPbAAGMBB Battery EVPConcorde Lifeline East Penn Deka IntimidatorEnerSys Hawker Genesis, OdysseyExide Orbital Extreme Cycle Duty Optima Yellow Top, Blue TopGelEast Penn Deka DominatorFloodedTrojan Golf & Utility VehicleUS Battery BB SeriesNiCdFloodedSaft STMNiZnSBS EvercelLi PolyKokam SLPB

Note: LiFePO4 are recommended, having the lowest weight but highest initial purchase price. But they have similar overall cost, and the rest have safety, toxicity or power issues.

EV Battery Charging

Source: Charging Dynasty Valve Regulated Lead Acid Batteries, http://www.cdtechno.com/custserv/pdf/2128.pdf. Battery Chargers Need Voltage Regulation and Current LimitingThis shortens charge time without shortening life.

Source: Thermal Runaway in VRLA Batteries Its Cause and Prevention, http://www.cdtechno.com/custserv/pdf/7944.pdf. EV Charger Temperature Compensation is Required for SafetyExcess voltage at higher temperatures can lead to thermal runaway, which can melt lead acid modules, explode nickel modules, and ignite thermally unstable lithium ion cells. Battery cooling systems are typically employed with nickel and unstable lithium ion packs to maintain performance while providing safety.

EV Battery Management

EV Batteries Need to be Monitored

All batteries need to be kept within their required voltage and temperature ranges for performance, long life and safety. This is particularly important for nickel and thermally unstable lithium ion batteries which can be dangerous if abused.

Ideally each cell is monitored, the charge current is controlled, and the driver is alerted when discharge limits are being approached and then again when exceeded.

For high quality multi-cell modules without cell access, module level voltage monitoring is better than no monitoring.

For chargers without a real time level control interface, a driven disable pin or external contactor will suffice for battery protection, but may result in uncharged batteries in time of need.

Dashboard gages and displays are good, but combining them with warning and error lamps is better.

Data Source: Integrity Testing, http://www.cdtechno.com/custserv/pdf/7264.pdf. Data Source: MPS 12-75 Valve Regulated Lead Acid Battery Datasheet, http://www.cdstandbypower.com/product/battery/vrla/pdf/mps1275.pdf. Amp-Hour Counters are More Accurate Fuel Gages Than Volt MetersOpen circuit voltage drops only 0.9V between 0 and 80% depth of discharge. Voltage drops up to 2.7V at 600 amps discharge, and can take a good part of a minute to recover.Ideally your fuel gage looks at all of the above plus temperature and then estimates depth of discharge. To predict when your batteries will drop below the minimum voltage, Depth of Discharge should be monitored.Note: do not use Dynasty MPS batteries in EVs they are not designed for frequent deep cycling required in EVs

peukert

0.2346133333

0.4366666667

0.54

0.65

0.6533333333

0.7466666667

0.796

0.8533333333

0.9205333333

0.9466666667

0.9616

1

Constant Discharge Rate, Amps

Amp hours to 80% DOD (1.75 VPC, 10VP6C)

Peukert EffectDynasty AGM MPS Series 75 Ah

open ckt

112.84

0.812.6

0.7512.54

0.512.24

0.2511.94

011.64

12.721

12.480.8

12.420.75

12.120.5

11.820.25

11.520

AGM

Gel

Open Circuit Rest Voltage

State of Charge

Open Circuit Rest Voltage vs. State of Charge

resistance

12.8

12.575

12.35

12.125

11.9

11.675

11.45

11.225

11

10.775

10.55

10.325

10.1

Dynasty 12-75 AGM (4.5 milliohm)

discharge rate, amps

battery voltage

Internal Resistance Effect

data

Batteries

AGM

MPS-12-75 discharge rate vs. capacity

hourscurrent% of nameplateamp hours

0.08321223%18

0.2513144%33

0.58154%41

0.756565%49

14965%49

22875%56

319.980%60

512.885%64

88.6392%69

107.195%71

126.0196%72

203.75100%75

open circuit voltage voltage test

SOC

rest voltageAGMgel

12.8100%

12.680%

12.575%

12.250%

11.925%

11.60%

12.7100%

12.580%

12.475%

12.150%

11.825%

11.50%

voltage vs amperage at full charge

ampsvolts

012.8

5012.6

10012.4

15012.1

20011.9

25011.7

30011.5

35011.2

40011.0

45010.8

50010.6

55010.3

60010.1

peukert

0.2346133333

0.4366666667

0.54

0.65

0.6533333333

0.7466666667

0.796

0.8533333333

0.9205333333

0.9466666667

0.9616

1

Constant Discharge Rate, Amps

Amp hours to 80% DOD (1.75 VPC, 10VP6C)

Peukert EffectDynasty AGM MPS Series 75 Ah

open ckt

12.8412.72

12.612.48

12.5412.42

12.2412.12

11.9411.82

11.6411.52

AGM

Gel

Depth of Discharge

6 cell Rest Voltage

Open Circuit Rest Voltage vs. Depth of Discharge

resistance

12.8

12.575

12.35

12.125

11.9

11.675

11.45

11.225

11

10.775

10.55

10.325

10.1

Dynasty 12-75 AGM (4.5 milliohm)

discharge rate, amps

battery voltage

Internal Resistance Effect

data

Batteries

AGM

MPS-12-75 discharge rate vs. capacity

hourscurrent% of nameplateamp hours

0.08321223%18

0.2513144%33

0.58154%41

0.756565%49

14965%49

22875%56

319.980%60

512.885%64

88.6392%69

107.195%71

126.0196%72

203.75100%75

open circuit voltage voltage test

rest voltage

SOCDODAGMgel

100%0%12.812.7

80%20%12.612.5

75%25%12.512.4

50%50%12.212.1

25%75%11.911.8

0%100%11.611.5

change1.21.2

voltage vs amperage at full charge

ampsvolts

012.8

5012.6

10012.4

15012.1

20011.9

25011.7

30011.5

35011.2

40011.0

45010.8

50010.6

55010.3

60010.1

2.7

EV Batteries Need to be Balanced

All batteries will drift apart in state of charge level over time. This is due to differences in Peukert effect and internal leak rates. This will be detected during monitoring as early low voltages during discharge, and early high voltages and not high enough voltages during charge.

Sealed batteries need to be individually balanced, whereas flooded batteries can be overcharged as a string, then watered.

Individual balancing can be done manually on a regular basis with a starter battery charger, or with a programmable power supply with voltage and current limits, but the latter can be expensive. And it can be a hassle, and it can be difficult if the battery terminals are hard to get to.

Automatic balancing maximizes life and performance. Ideally balancing is low loss, switching current from higher voltage cells to lower voltage cells at all times. Bypass resistors that switch on during finish charging only is less desirable but better than no automatic balancing.

EV Battery Pictures

Optima Blue Top AGM Sealed Lead Acid Batteries with PCHC-12V-2A Power Cheqs Installed in Don McGraths Corbin Sparrow

Valence Module

Valence BMU

Valence batteries and BMU connected via RS485

Valence battery monitoring via CANBus and USB to laptop

Valence Cycler 2.4 battery monitoring screen capture (idle mode; 2.8 now available)

Valence battery monitoring file list

Valence battery monitoring file example

Valence battery monitoring results: maximum charge voltage vs. targetTroubleshooting unbalanced cell (dropped from >90 Ah to 67 Ah after balancing disabled for 3 months due to late onset RS485 errors due to missing termination resistor and unshielded cables)

Valence battery monitoring results: discharge

Valence battery monitoring results: charge and dischargeTroubleshooting bad cell that abruptly went from >90 Ah to 25 Ah in less than 1 week

EV Record HoldersAC Propulsion tZero: drove 302 miles on a single charge at 60 MPH in 2003, Lithium Ion batteriesPhoenix Motorcars SUT: charged 50 times in 10 minutes with no degradation in 2007; 130 mile rangeSolectria Sunrise: drove 375 miles on a single charge in 1996, NiMH batteriesDIT Nuna: drove 1877 miles averaging 55.97 MPH on solar power in 2007, LiPo batteries

Future EV BatteriesStanford University Silicon Nanowire electrodes have 3X capacity improvement expected for Lithium batteriesNot technically a battery, but MIT Nanotube ultracapacitors have very high power, 1M+ cycle energy storage approaching Lithium battery capacity