Traction Batteries for EV and HEV Applications
BatteryRequirements for Typical Traction Applications
Traction applications have traditionally been jobs for Lead Acidbatteries but the limitations of Lead Acid batteries, together withthe high cost of alternatives, have in turn limited the range ofpotential battery powered traction applications. A typical familycar would need a battery capacity of about 40 KWh to provide a oneway range of 200 miles and a 40 KWh Lead Acid battery weighs 1.5tons.
The situation is changing however as new battery chemistries andsupporting technologies have brought with them new technical andeconomic benefits making battery power viable for tractionapplications that were previously uneconomic or impractical. Inparticular, the use of light weight Nickel Metal Hydride andLithium batteries instead of the heavy and bulky Lead Acidbatteries has made practical electric vehicles and hybrid electricvehicles possible for the first time.
Traction Batteries for EVTraction Batteries for EV Traction Batteries for EV
It goes without saying that low cost, long life (more than 1000cycles), low self discharge rates (less than 5% per month) and lowmaintenance are basic requirements for all applications. Tractionbatteries generally operate in very harsh operating environmentsand must withstand wide temperature ranges ( -30°C to +65°C) as well as shock, vibration and abuse. Low weight however is notalways a priority since heavy weight provides stability formaterial handling equipment such as fork lift trucks and the gripneeded by aircraft tugs for pulling heavy loads. Low weight ishowever essential for high capacity automotive EV and HEV batteriesused in passenger vehicles and this rules out Lead Acid for theseapplications.
Protection circuits are also essential for batteries using non-Lead Acidchemistries.
Traction batteries are very expensive and like all batteries theydeteriorate during their lifetime. Customers expect a minimum levelof performance even at the end of the battery's life, so the buyeris likely to specify the expected performance at the end of life(EOL) rather than the beginning of life (BOL). Under normalcircumstances for EV applications the EOL capacity is specified asnot less than 80% of BOL capacity. For HEV applications change ininternal impedance is often used as an indicator of lifetime. Inthis case the EOL internal impedance may be specified as not morethan 200% of BOL internal impedance.
This is shown graphically below.
The following outlines the special performance goals and operatingrequirements for specific automotive applications in addition tothe general requirements above.
12 Volt Automotive SLI (Starting, Lighting and Ignition)BatteryOperating Requirements
One short duration deep discharge (50% Depth of Discharge (DOD)with at least 5C rate) followed by trickle charging.
Battery is essentially constantly fully charged.
No prolonged operation with deep discharge.
Typical capacity 0.4 - 1.2 kWh (33 Ah - 100Ah.)
Peak power 2.4 -3.6 kW (200 - 300 Amps).
PowerNet 36/42 Volt BatteryOperating Requirements
One deep discharge followed by intermittent high currentloads.
No prolonged operation with deep discharge.
High energy throughput and high cycle life essential, especially ifstop/start launch assist function used.
Tolerant to repeated high current pulses.are n
Typical capacity over 1 kWh.
Peak power 5 to12 kW.
、The above two applications are not true traction applicationsthough they may be used in mild hybrids which incorporate astart/stop mode (see below).
EV, HEV and PHV BatterySpecifications
The diagram below compares the battery power and capacityrequirements for a vehicle of the the same size and weight whenconfigured as an EV, an HEV or a PHEV. Battery designs may be optimised for power or for capacity (energy content)but not both and so the type of cells used, not just the size, mustbe selected to suit the application.
In the case of the EV, the battery is the sole source of power sothe battery must be sized to deliver that power on a more or lesscontinuous basis. The EV capacity has to be sufficient to achievethe required range but in addition, since it is not desireable tofully discharge the battery, a margin of about 20% is needed sothat the depth of discharge will not exceed 80%. A further marginof about 5% is also required the accept any regenerative brakingcharge when the battery has just been charged. In othe words thebattery should dimensioned to provide the required capacity whenthe maximum SOC is 95% and the maximum DOD is 80%. The continuousdischarge rate for batteries optimised for capacity is typicallyabout 1C although some cells may tolerate pulse currents of up to3C or more for short periods. An EV battery will usually have one deepdischarge per day with some intermediate topping up from regenbraking and a typical Lithium EV battery lifetime may be from 500to 2000 cycles.
The battery for an equivalent series hybrid must also be able todeliver the same power as the EV battery because the vehicles arethe essentially the same size and weight and for intermittentperiods the battery will be the sole source of power. However,because the energy requirement is shared with an internalcombustion engine (ICE) the battery capacity required is muchsmaller. Parallel hybrids may have different power sharingarrangements and so their power requirements could be accommodatedby lower power batteries. HEVs thus have the added burden andcomplication of carrying around two power sources each of which isbig enough to power the vehicle on its own.
The result is severe design constraints on the weight and size ofthe battery which can be accommodated and HEV batteries aretypically less than one tenth the size of EV batteries used in thesame size vehicle. The unavoidable consequence is that to get thesame power out of a battery one tenth the size, HEV batteries mustbe capable of delivering continuous currents of 10C or more. Fortunately the power requirement is intermittent (butmuch longer than short pulsed demands) since it is shared with theICE. Battery capacity is thus less important than power delivery in an HEVbecause the range can be extended by use of the engine. HEVbatteries are therefore optimised for power.
The downside is that because of its low capacity, an HEV battery iscontinually being charged and discharged during normal operationand can undergo the equivalent of a hundred charge-discharge cyclesper day. With deep discharges the battery would unfortunately beworn out in a few weeks. We know however that battery cycle life isincreased exponentially as the the DOD is reduced so HEV batteries must be run atpartial DOD in order to extend the cycle life. This means that thebattery capacity must be increased accordingly to allow for lowerDODs even though the full capacity is almost never used. In theexample above the HEV battery operates between 40% and 80% SOC.Longer life can be achieved by using even larger capacity batteriesso that the desired capacity can be delivered between SOC limitsbetween 60% and 75%.
Plug in hybrids need to operate part of the time as an EV in thecharge depletion mode and part of the time as an HEV in chargemaintenance mode. See more detailed PHEV Requirements below. The PHEV battery requirement must therefore be a compromisebetween an energy storage and power delivery.
This is a major challenge for cell makers.
More detailed operating requirements are outlined below.
Electric Vehicle (EV) BatteryOperating Requirements
Large capacity batteries are required to achieve reasonable range.A typical electric car uses around 150 to 250 Watt-hours per miledepending on the terrain and the driving style.
The battery must be capable of regular deep discharge (80% DOD) operation
It is designed to maximise energy content and deliver full powereven with deep discharge to ensure long range.
A range of capacities will be required to satisfy the needs ofdifferent sized vehicles and different usage patterns.
Must accept very high repetitive pulsed charging currents (greaterthan 5C) if regenerative braking required.
Without regenerative braking, controlled charging conditions andlower charging rates are possible. (At least 2C desirable).
Routinely receives a full charge.
Often also reaches nearly full discharge.
Fuel-gauging critical near "empty" point.
Needs a BatteryManagement System (BMS).
Needs thermal management.
Typical voltage > 300 Volts.
Typical capacity > 20 - 60 kWh.
Typical discharge current up to C rate continuous and3 C peak for short durations.
Because these batteries are physically very large and heavy theyneed custom packaging to fit into the available space in theintended vehicle. Likewise the design layout and weightdistribution of the pack must be integrated with the chassis designso as not to upset the vehicle dynamics. These mechanicalrequirements are particularly important for passenger cars.
Hybrid Electric Vehicle (HEV) BatteryOperating Requirements
Capacity is less important with HEVs compared with EVs since theengine also provides capacity therefore the the battery can be muchsmaller, saving weight. However the battery may still be requiredto provide the same instantaneous power as the EV battery from timeto time. This means that the smaller battery must deliver muchhigher currents when called upon.
A very wide range of batteries is required to accommodate the rangeof HEV configurations as well as vehicle performance requirements.Some examples are:
Series Hybrid - The engine is used only to charge the battery. The electricalsystem provides a variable speed transmission and the electricmotor provides the full driving power. Battery requirements similar to EV batteries but lower capacity neededsince the charge is kept topped up by the engine.
Parallel Hybrid - Both the engine and the electric motor provide power to thewheels. Various configurations possible to satisfy differentoperating conditions. The share of the load taken by the electricmotor can range from zero to 100% depending on the operatingconditions and the design goals. The battery capacity may be as lowas 2 KWh but it must deliver short duration power boosts requiringvery high currents of up to 40C for acceleration and hill climbing.
Some examples of different EV and HEV design goals which affect thebattery specification are:
Efficiency Optimisation - This allows the engine to run at its most efficient constantspeed simply to keep the battery charged. The electrical driveeliminates the gearbox and provides the variable power outputrequired. This type of drive was first used on Diesel ElectricLocomotives. Improved efficiency reduces the fuel consumption whichin turn automatically reduces exhaust emissions.
Efficiency Boost - This uses the battery simply to capture the energy, which wouldotherwise be lost, from regenerative braking. The captured energyis used to provide a power boost for acceleration and hillclimbing.
Range Extender - This is basically an EV which uses the engine to top up thebattery to prevent excessive depth of discharge.
Stop/Start Mode - This allows the engine to be switched off to save fuel when thevehicle is temporarily stationary at traffic lights or in trafficjams etc. The vehicle moves off under battery power and the engineis restarted when a predetermined speed is reached.
Town and Country Mode - This allows the vehicle to be used in EV mode while in town or inheavy traffic where it is most suited, and to be used as a normalinternal combustion engined vehicle for high speed or long distancehighway driving to avoid the range limitations of the EV.
Multi-mode - Increased versatility is possible by using combinations of theabove modes.
Capacity and Power - In addition to the above operating modes, different batterieswill be required to accommodate a range of performance requirementssuch as economy, top speed, acceleration, load carrying capacity,range and noxious emissions.
Traction Batteries for EV Traction Batteries for EV Traction Batteries for EV Traction Batteries for EV
The battery has become an important product differentiator, justlike the engine is.
Because of the very wide range of HEV operating requirements thereare no standard batteries available to match the resulting range ofspecifications for battery voltage, capacity and power handling andbatteries must be custom designed specifically for the intendedapplication.
Some typical requirements are as follows:
Designed to maximise power delivered.
Must deliver high power (up to 40C) in repetitive shallow discharges and accept very high rechargingrates.
Very long cycle life 1000 deep cycles and 400,000 - 1,000,000shallow cycles.
Operating point is between 15% and 50% DOD to allow forregenerative braking.
Never reaches full discharge.
Rarely reaches full charge.
Needs thermal management.
Fuel-gauging and complex BMS necessary to regulate battery energymanagement as well as for driver instrumentation.
Needs interfacing with overall vehicle energy management.
Typical voltage > 144 Volts.
Typical power > 40 kW (50 bhp).
Capacity 1 to 10 kWh depending on the application.
As with EVs above, the size, shape and weight distribution of thebattery pack must be tailored to the vehicle.
Plug in Hybrid Electric Vehicle (PHEV) BatteryOperating Requirements
Batteries for plug in hybrid vehicles must satisfy conflictingperformance requirements.
Traction batteries are usually optimized for high capacity in thecase of pure electric vehicles of for high power in the case ofhybrid vehicles. The EV battery operates down to a deep depth ofdischarge (DOD) for long range whereas the HEV operates at ashallow DOD for long life.
The plug in hybrid is designed to be used both as an EV for citydriving and as an HEV when the charge is depleted or for highwaydriving. The dual requirements for an extended all electric range,typically forty miles, as well as maintaining high poweravailability at low state of charge, (see below), impose verystressful conditions on the battery.
The PHEV battery is thus expected to perform both as an EV and asan HEV.
The all electric range requirement can only be satisfied by usinglarger capacity batteries which adds considerably to the cost andbecause the high cost, consumers have high expectations aboutbattery lifetime.
Bicycle BatteryOperating Requirements
In China where the bicycle is a workhorse, batteries are typically 36 Voltunits.
In Europe and USA where bicycles are more often used for recreation, lighter, 24 Voltbatteries are more popular.
Designed as removable modules for convenient indoor charging and asanti theft precaution.
Should give 5 Amps for 2 hours (240 to 360 Wh depending on thevoltage) to allow one hour travel to work. Higher capacity notfeasible with Lead Acid because the weight puts limits onportability.
Peak current 15 Amps.
Long lifetime minimum 500 cycles or two years.
Marine BatteryOperating Requirements
Requires deep cycle batteries.
Wide range of capacities and powers required.
Must be tolerant to wide range of charging conditions.
Special environmental conditions.
Materials Handling Equipment BatteryOperating Requirements
Similar to EV applications but normally no weightrestrictions.
Practical Traction Batteries
For over a century Lead Acid batteries have been the prime source of energy for tractionapplications because they are both robust and relativelyinexpensive. For fork lift trucks, milk floats and similarapplications Nickel Iron batteries, which are almost indestructible and have a lifetime ofup to ten years, have also been used successfully. The high weightand bulk of these batteries however has precluded their use inpassenger cars.
In the 1970s work started on Sodium Nickel Chloride(Zebra) batteries designed for traction applications since they offer thepossibility of very high energy densities which could overcome thisproblem. Unfortunately these are high temperature batteries whichmust run at 270°C and this has limited their adoption.
The advent of high power Nickel Metal Hydride (NiMH) cells which have overcome both the weight and the operatingtemperature problems has encouraged several automotivemanufacturers to introduce EVs or HEVs using NiMH batteries. NiMHcells operate at normal ambient temperatures. They have a higherenergy and power density than Lead Acid cells but not as good asthe Zebra cells.
Recently high power Lithium Ion cells which have an even higher energy density than NiMH cells, ona par with Zebra cells, have become available. They also operate atnormal temperatures and are just being introduced into new electricvehicle designs.
These new high energy cells however are more vulnerable to abuseand need the support of electronic Battery Management Systems toprovide protection and ensure long cycle life.
High capacity batteries also require high power chargers to achievereasonable charging times and the chargers must be compatible withthe cell chemistry and should be able to interface with the cellprotection circuitry. Just as the battery is matched to thevehicle, the charger must be custom designed and matched to thebattery. More information can be found in the section onChargers.