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Electric Vehicle Batteries
North Bay Chapter of the Electric Auto Association
www.nbeaa.org
Updated 8/14/09
Posted at: http://www.nbeaa.org/presentations/batteries.pdf
NBEAA 2009 Technical Series
1. EV Drive Systems
TODAY >> 2. EV Batteries
3. EV Charging Systems
4. EV Donor Vehicles
Agenda
What is a Battery?
Battery History
EV Battery Requirements
Types of EV Batteries
EV Battery Temperature Control
EV Battery Charging
EV Battery Management
EV Battery Comparison
EV Record Holders
Future EV Batteries
EV Drive System Testimonials, Show and Tells and Test Drives
What is a Battery?
electrolyteanode + cathode -
charger
current
During Charge
voltage and energy increases
energy
heat
heat
chemical reaction
What is a Battery?
electrolyteanode + cathode -
load
current
During Discharge
voltage and energy decreases
work
heat
heat
chemical reaction
Battery HistoryRechargeable batteries highlighted in bold.
First battery, “Voltaic Pile”, Zn-Cu with NaCl electrolyte, non-rechargeable, but short shelf life
1800 Volta
First battery with long shelf life, “Daniel Cell”, Zn-Cu with H2SO4 and CuSO4 electrolytes, non-rechargeable
1836 England John Fedine
First electric carriage, 4 MPH with non-rechargeable batteries
1839 Scotland Robert Anderson
First rechargeable battery, “lead acid”, Pb-PbO2 with H2SO4 electrolyte
1859 France Gaston Plante
First mass produced non-spillable battery, “dry cell”, ZnC-Mn02 with ammonium disulphate electrolyte, non-rechargeable
1896 Carl Gassner
Ni-Cd battery with potassium hydroxide electrolyte invented
1910 Sweden Walmer Junger
First mass produced electric vehicle, with “Edison nickel iron” NiOOH-Fe rechargeable battery with potassium hydroxide electrolyte
1914 US Thomas Edison and Henry Ford
Modern low cost “Eveready (now Energizer) Alkaline” non-rechargeable battery invented, Zn-MnO2 with alkaline electrolyte
1955 US Lewis Curry
NiH2 long life rechargeable batteries put in satellites 1970s US
NiMH batteries invented 1989 US
Li Ion batteries sold 1991 US
LiFePO4 invented 1997 US
EV Battery Requirements
Safe
High Power
High Capacity
Small and Light
Large Format
Long Life
Low Overall Cost
EV Battery Requirements: Safe
Examples of EV battery safety issues:
Overcharging
explosive hydrogen outgassing
thermal runaway resulting in melting, explosion or inextinguishable fire
Short Circuit
external or internal
under normal circumstances or caused by a crash
immediate or latent
Damage
liquid electrolyte acid leakage
EV Battery Requirements: High Power
Power = Watts = Volts x Amps
Typically 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 charge
1 Horsepower = 746 Watts
Charger efficiency = ~90%
Battery charge and discharge efficiency = ~95%
Drive system efficiency = ~85% AC, 75% DC
batteries motor controller
motor
heat heat heat
shaftcharger
heat
100% in 60% - 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 charges
Energy = Watts x Hours = Volts x Amp-Hours
Watt-hours can be somewhat reduced with higher discharge current due to internal resistance heating loss
Amp-Hours can be significantly reduced with higher discharge current seen in EVs due to Peukert Effect
Amp-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% efficiencies
Pack capacity = 48 * 3.2 Volts * 100 Amp-Hours * .95 efficiency = 14,592 Wh
340 Watt–Hours per mile vehicle consumption rate
Vehicle 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 reliability
Cars only have so much space to put batteries, and they can’t 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 Format
Minimize the need for too many interconnects; example 100 Ah
Long Life
Minimize the need for battery replacement effort and cost
Example: 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 Cost
Minimize the purchase and replacement cost of the batteries
Example: $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 Life
13 degrees reduces the life of lead acid batteries by half.
EV Battery Comparison
Type Power Energy Stability
Max temp Life Toxicity Cost
LiFePO4 + + + ~ ~ + -
LiCO2 + + - - - + -
NiZn ~ ~ ~ ~ - + ~
NiCd - ~ ~ ~ + - +
PbA AGM + - + ~ - - +
PbA gel ~ - + ~ - - +
PbA flooded ~ - - ~ - - +
Available 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
Peukert EffectDynasty AGM MPS Series 75 Ah
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 50 100 150 200 250
Constant Discharge Rate, Amps
Am
p h
ou
rs t
o 8
0%
DO
D (
1.7
5 V
PC
, 10
VP
6C
)
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.
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 Rates
Gels 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 Climates
They 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 Life
AGMs 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:
LiFePO4 Hi PowerThunder Sky LMPValence Technologies U-Charge XP, Epoch
PbA AGM BB Battery EVPConcorde Lifeline East Penn Deka IntimidatorEnerSys Hawker Genesis, Odyssey
Exide Orbital Extreme Cycle Duty Optima Yellow Top, Blue Top
Gel East Penn Deka Dominator
Flooded Trojan Golf & Utility Vehicle
US Battery BB Series
NiCd Flooded Saft STM
NiZn SBS Evercel
Li Poly Kokam 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.
Source: Charging Dynasty Valve Regulated Lead Acid Batteries,
http://www.cdtechno.com/custserv/pdf/2128.pdf.
Battery Chargers Need Voltage Regulation and Current Limiting
This shortens charge time without shortening life.
Source: Thermal Runaway in VRLA Batteries – It’s Cause and Prevention,
http://www.cdtechno.com/custserv/pdf/7944.pdf.
EV Charger Temperature Compensation is Required for Safety
Excess 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 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.
Internal Resistance Effect
10.0
10.5
11.0
11.5
12.0
12.5
13.0
0 100 200 300 400 500 600
discharge rate, ampsb
att
ery
vo
lta
ge
Dynasty 12-75 AGM (4.5 milliohm)
Data Source: MPS 12-75 Valve Regulated Lead Acid Battery Datasheet,
http://www.cdstandbypower.com/product/battery/vrl
a/pdf/mps1275.pdf.
Amp-Hour Counters are More Accurate “Fuel Gages” Than Volt Meters
Open 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.
Open Circuit Rest Voltage vs. Depth of Discharge
10.0
10.5
11.0
11.5
12.0
12.5
13.0
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Depth of Discharge
6 c
ell
Re
st
Vo
lta
ge
AGM
Gel
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
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.
Optima Blue Top AGM Sealed Lead Acid Batteries with PCHC-12V-2A Power Cheqs Installed in Don McGrath’s Corbin Sparrow
Valence battery monitoring results: maximum charge voltage vs. target
Troubleshooting 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: charge and discharge
Troubleshooting bad cell that abruptly went from >90 Ah to 25 Ah in less than 1 week
EV Record Holders
AC Propulsion tZero: drove 302 miles on a single charge at 60 MPH in 2003, Lithium Ion batteries
Phoenix Motorcars SUT: charged 50 times in 10 minutes with no degradation in 2007; 130 mile range
Solectria Sunrise: drove 375 miles on a single charge in 1996, NiMH batteries
DIT Nuna: drove 1877 miles averaging 55.97 MPH on solar power in 2007, LiPo batteries