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? During Charge voltage and energy increases heat anode + electrolyte cathode - chemical reaction heat current charger energy
What is a Battery? During Discharge voltage and energy decreases heat anode + electrolyte cathode - chemical reaction heat current load work
Battery History Rechargeable batteries highlighted in bold.
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 heat heat heat heat motor controller charger batteries motor shaft 100% in 32% - 40% lost to heat 60% - 68% out
EV Battery Requirements: High Power Example Accelerating or driving up a steep hill Motor Shaft Power = ~50 HP or ~37,000 W Battery 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 ground Motor Shaft Power = ~20 HP or ~15,000 W Battery Power = ~20,000 W DC, ~18,000 AC Battery Current ~150A for 144V nominal pack with DC drive ~70A for 288V nominal pack with AC drive Charging Depends on battery type, charger power and AC outlet rating Example: 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 / Kilogram Specific Energy = energy capacity to weight ratio = Watt-Hours / Kilogram Power Density = power to volume ratio = Watts / liter Energy 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 liters Specific 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/l Energy 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
Higher Temperature Reduces Shelf Life 13 degrees reduces the life of lead acid batteries by half. Source: Life Expectancy and Temperature, http://www.cdtechno.com/custserv/pdf/7329.pdf.
EV Battery Comparison Available large format only considered; NiMH, small format lithium and large format nano lithium not included.
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. 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 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: Dynasty VRLA Batteries and Their Application, http://www.cdtechno.com/custserv/pdf/7327.pdf.
Lead Acid Batteries Need Heaters in Cold Climates They lose 60% of their capacity at 0 degrees Fahrenheit. Source: Capacity Testing of Dynasty VRLA Batteries, http://www.cdtechno.com/custserv/pdf/7135.pdf. Source: Impedance and Conductance Testing, http://www.cdtechno.com/custserv/pdf/7271.pdf.
Gels Have a Longer Cycle Life AGMs only last half as long, but as previously mentioned can withstand higher discharge rates. Source: Dynasty VRLA Batteries and Their Application, http://www.cdtechno.com/custserv/pdf/7327.pdf.
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 Power Thunder Sky LMP Valence Technologies U-Charge XP, Epoch PbA AGM BB Battery EVP Concorde Lifeline East Penn Deka Intimidator EnerSys 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.
Battery Chargers Need Voltage Regulation and Current Limiting This shortens charge time without shortening life. Source: Charging Dynasty Valve Regulated Lead Acid Batteries, http://www.cdtechno.com/custserv/pdf/2128.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. Source: Thermal Runaway in VRLA Batteries – It’s Cause and Prevention, http://www.cdtechno.com/custserv/pdf/7944.pdf.
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.
Amp-Hour Counters are More Accurate “Fuel Gages” Than Volt Meters To predict when your batteries will drop below the minimum voltage, Depth of Discharge should be monitored. 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. Ideally your fuel gage looks at all of the above plus temperature and then estimates depth of discharge. Data Source: MPS 12-75 Valve Regulated Lead Acid Battery Datasheet, http://www.cdstandbypower.com/product/battery/vrla/pdf/mps1275.pdf. Data Source: Integrity Testing, http://www.cdtechno.com/custserv/pdf/7264.pdf. 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 Cycler 2.4 battery monitoring screen capture (idle mode; 2.8 now available)
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 Phoenix Motorcars SUT: charged 50 times in 10 minutes with no degradation in 2007; 130 mile range AC Propulsion tZero: drove 302 miles on a single charge at 60 MPH in 2003, Lithium Ion batteries 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
Future EV Batteries Stanford University Silicon Nanowire electrodes have 3X capacity improvement expected for Lithium batteries Not technically a battery, but MIT Nanotube ultracapacitors have very high power, 1M+ cycle energy storage approaching Lithium battery capacity