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Traction Batteries for EV and HEV Applications

 

Battery Requirements for Typical Traction Applications

Traction applications have traditionally been jobs for Lead Acid batteries but the limitations of Lead Acid batteries, together with the high cost of alternatives, have in turn limited the range of potential battery powered traction applications. A typical family car would need a battery capacity of about 40 KWh to provide a one way range of 200 miles and a 40 KWh Lead Acid battery weighs 1.5 tons.

 

The situation is changing however as new battery chemistries and supporting technologies have brought with them new technical and economic benefits making battery power viable for traction applications that were previously uneconomic or impractical. In particular, the use of light weight Nickel Metal Hydride and Lithium batteries instead of the heavy and bulky Lead Acid batteries has made practical electric vehicles and hybrid electric vehicles possible for the first time.

 

General Requirements

It goes without saying that low cost, long life (more than 1000 cycles), low self discharge rates (less than 5% per month) and low maintenance are basic requirements for all applications. Traction batteries generally operate in very harsh operating environments and must withstand wide temperature ranges ( -30°C to +65°C) as well as shock, vibration and abuse. Low weight however is not always a priority since heavy weight provides stability for material handling equipment such as fork lift trucks and the grip needed by aircraft tugs for pulling heavy loads. Low weight is however essential for high capacity automotive EV and HEV batteries used in passenger vehicles and this rules out Lead Acid for these applications.

Protection circuits are also essential for batteries using non-Lead Acid chemistries.

 

Purchasing Speciications

Traction batteries are very expensive and like all batteries they deteriorate during their lifetime. Customers expect a minimum level of performance even at the end of the battery's life, so the buyer is likely to specify the expected performance at the end of life (EOL) rather than the beginning of life (BOL). Under normal circumstances for EV applications the EOL capacity is specified as not less than 80% of BOL capacity. For HEV applications change in internal impedance is often used as an indicator of lifetime. In this case the EOL internal impedance may be specified as not more than 200% of BOL internal impedance.

This is shown graphically below.

End of Life

 

The following outlines the special performance goals and operating requirements for specific automotive applications in addition to the general requirements above.

 

12 Volt Automotive SLI (Starting, Lighting and Ignition) Battery Operating 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).

 

History

 

PowerNet 36/42 Volt Battery Operating Requirements

  • One deep discharge followed by intermittent high current loads.
  • No prolonged operation with deep discharge.
  • High energy throughput and high cycle life essential, especially if stop/start launch assist function used.
  • Tolerant to repeated high current pulses.are n
  • Typical capacity over 1 kWh.
  • Peak power 5 to12 kW.
  •  

History

 

The above two applications are not true traction applications though they may be used in mild hybrids which incorporate a start/stop mode (see below).

 

EV, HEV and PHV Battery Specifications

The diagram below compares the battery power and capacity requirements for a vehicle of the the same size and weight when configured as an EV, an HEV or a PHEV. Battery designs may be optimised for power or for capacity (energy content) but not both (see Energy/Power Tradeoffs in the section on Cell Construction) and so the type of cells used, not just the size, must be selected to suit the application.

 

Traction Battery Requirements

In the case of the EV, the battery is the sole source of power so the battery must be sized to deliver that power on a more or less continuous basis. The EV capacity has to be sufficient to achieve the required range but in addition, since it is not desireable to fully discharge the battery, a margin of about 20% is needed so that the depth of discharge will not exceed 80%. A further margin of about 5% is also required the accept any regenerative braking charge when the battery has just been charged. In othe words the battery should dimensioned to provide the required capacity when the maximum SOC is 95% and the maximum DOD is 80%. The continuous discharge rate for batteries optimised for capacity is typically about 1C although some cells may tolerate pulse currents of up to 3C or more for short periods. An EV battery will usually have one deep discharge per day with some intermediate topping up from regen braking and a typical Lithium EV battery lifetime may be from 500 to 2000 cycles.

 

The battery for an equivalent series hybrid must also be able to deliver the same power as the EV battery because the vehicles are the essentially the same size and weight and for intermittent periods the battery will be the sole source of power. However, because the energy requirement is shared with an internal combustion engine (ICE) the battery capacity required is much smaller. Parallel hybrids may have different power sharing arrangements and so their power requirements could be accommodated by lower power batteries. HEVs thus have the added burden and complication of carrying around two power sources each of which is big enough to power the vehicle on its own.

The result is severe design constraints on the weight and size of the battery which can be accommodated and HEV batteries are typically less than one tenth the size of EV batteries used in the same size vehicle. The unavoidable consequence is that to get the same power out of a battery one tenth the size, HEV batteries must be capable of delivering continuous currents of 10C or more. Fortunately the power requirement is intermittent (but much longer than short pulsed demands) since it is shared with the ICE. Battery capacity is thus less important than power delivery in an HEV because the range can be extended by use of the engine. HEV batteries are therefore optimised for power.

The downside is that because of its low capacity, an HEV battery is continually being charged and discharged during normal operation and can undergo the equivalent of a hundred charge-discharge cycles per day. With deep discharges the battery would unfortunately be worn out in a few weeks. We know however that battery cycle life is increased exponentially as the the DOD is reduced (See Cycle Life and DOD in the section on Battery Life) so HEV batteries must be run at partial DOD in order to extend the cycle life. This means that the battery capacity must be increased accordingly to allow for lower DODs even though the full capacity is almost never used. In the example above the HEV battery operates between 40% and 80% SOC. Longer life can be achieved by using even larger capacity batteries so that the desired capacity can be delivered between SOC limits between 60% and 75%.

 

Plug in hybrids need to operate part of the time as an EV in the charge depletion mode and part of the time as an HEV in charge maintenance mode. See more detailed PHEV Requirements below. The PHEV battery requirement must therefore be a compromise between an energy storage and power delivery.

This is a major challenge for cell makers.

 

More detailed operating requirements are outlined below.

 

Electric Vehicle (EV) Battery Operating Requirements

Large capacity batteries are required to achieve reasonable range. A typical electric car uses around 150 to 250 Watt-hours per mile depending 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 power even with deep discharge to ensure long range.
  • A range of capacities will be required to satisfy the needs of different sized vehicles and different usage patterns.
  • Must accept very high repetitive pulsed charging currents (greater than 5C) if regenerative braking required.
  • Without regenerative braking, controlled charging conditions and lower 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 Battery Management System (BMS).
  • Needs thermal management.
  • Typical voltage > 300 Volts.
  • Typical capacity > 20 - 60 kWh.
  • Typical discharge current up to C rate continuous and 3 C peak for short durations.
  •  

Because these batteries are physically very large and heavy they need custom packaging to fit into the available space in the intended vehicle. Likewise the design layout and weight distribution of the pack must be integrated with the chassis design so as not to upset the vehicle dynamics. These mechanical requirements are particularly important for passenger cars.

 

History

 

Hybrid Electric Vehicle (HEV) Battery Operating Requirements

Capacity is less important with HEVs compared with EVs since the engine also provides capacity therefore the the battery can be much smaller, saving weight. However the battery may still be required to provide the same instantaneous power as the EV battery from time to time. This means that the smaller battery must deliver much higher currents when called upon.

A very wide range of batteries is required to accommodate the range of HEV configurations as well as vehicle performance requirements. Some examples are:

  • Series Hybrid - The engine is used only to charge the battery. The electrical system provides a variable speed transmission and the electric motor provides the full driving power. Battery requirements similar to EV batteries but lower capacity needed since the charge is kept topped up by the engine.
  • Parallel Hybrid - Both the engine and the electric motor provide power to the wheels. Various configurations possible to satisfy different operating conditions. The share of the load taken by the electric motor can range from zero to 100% depending on the operating conditions and the design goals. The battery capacity may be as low as 2 KWh but it must deliver short duration power boosts requiring very high currents of up to 40C for acceleration and hill climbing.

Some examples of different EV and HEV design goals which affect the battery specification are:

  • Efficiency Optimisation - This allows the engine to run at its most efficient constant speed simply to keep the battery charged. The electrical drive eliminates the gearbox and provides the variable power output required. This type of drive was first used on Diesel Electric Locomotives. Improved efficiency reduces the fuel consumption which in turn automatically reduces exhaust emissions.
  • Efficiency Boost - This uses the battery simply to capture the energy, which would otherwise be lost, from regenerative braking. The captured energy is used to provide a power boost for acceleration and hill climbing.
  • Range Extender - This is basically an EV which uses the engine to top up the battery to prevent excessive depth of discharge.
  • Stop/Start Mode - This allows the engine to be switched off to save fuel when the vehicle is temporarily stationary at traffic lights or in traffic jams etc. The vehicle moves off under battery power and the engine is 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 in heavy traffic where it is most suited, and to be used as a normal internal combustion engined vehicle for high speed or long distance highway driving to avoid the range limitations of the EV.
  • Multi-mode - Increased versatility is possible by using combinations of the above modes.
  • Capacity and Power - In addition to the above operating modes, different batteries will be required to accommodate a range of performance requirements such as economy, top speed, acceleration, load carrying capacity, range and noxious emissions.

 

The battery has become an important product differentiator, just like the engine is.

Because of the very wide range of HEV operating requirements there are no standard batteries available to match the resulting range of specifications for battery voltage, capacity and power handling and batteries must be custom designed specifically for the intended application.

 

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 recharging rates.
  • Very long cycle life 1000 deep cycles and 400,000 - 1,000,000 shallow cycles.
  • Operating point is between 15% and 50% DOD to allow for regenerative braking.
  • Never reaches full discharge.
  • Rarely reaches full charge.
  • Needs thermal management.
  • Fuel-gauging and complex BMS necessary to regulate battery energy management 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 the battery pack must be tailored to the vehicle.

 

History

 

Plug in Hybrid Electric Vehicle (PHEV) Battery Operating Requirements

Batteries for plug in hybrid vehicles must satisfy conflicting performance requirements.

Traction batteries are usually optimised for high capacity in the case of pure electric vehicles of for high power in the case of hybrid vehicles. The EV battery operates down to a deep depth of discharge (DOD) for long range whereas the HEV operates at a shallow DOD for long life.

The plug in hybrid is designed to be used both as an EV for city driving and as an HEV when the charge is depleted or for highway driving. The dual requirements for an extended all electric range, typically forty miles, as well as maintaining high power availability at low state of charge, (see below), impose very stressful conditions on the battery.

 

The PHEV battery is thus expected to perform both as an EV and as an HEV.

The all electric range requirement can only be satisfied by using larger capacity batteries which adds considerably to the cost and because the high cost, consumers have high expectations about battery lifetime.

 

Bicycle Battery Operating Requirements

In China where the bicycle is a workhorse, batteries are typically 36 Volt units.

In Europe and USA where bicycles are more often used for recreation, lighter, 24 Volt batteries are more popular.

  • Designed as removable modules for convenient indoor charging and as anti theft precaution.
  • Should give 5 Amps for 2 hours (240 to 360 Wh depending on the voltage) to allow one hour travel to work. Higher capacity not feasible with Lead Acid because the weight puts limits on portability.
  • Peak current 15 Amps.
  • Long lifetime minimum 500 cycles or two years.
  •  

Marine Battery Operating Requirements

  • Requires deep cycle batteries.
  • Wide range of capacities and powers required.
  • Low weight.
  • Must be tolerant to wide range of charging conditions.
  • Special environmental conditions.

 

Materials Handling Equipment Battery Operating Requirements

Similar to EV applications but normally no weight restrictions.

 

Practical Traction Batteries

For over a century Lead Acid batteries have been the prime source of energy for traction applications because they are both robust and relatively inexpensive. For fork lift trucks, milk floats and similar applications Nickel Iron batteries, which are almost indestructible and have a lifetime of up to ten years, have also been used successfully. The high weight and bulk of these batteries however has precluded their use in passenger cars.

 

In the 1970s work started on Sodium Nickel Chloride (Zebra) batteries designed for traction applications since they offer the possibility of very high energy densities which could overcome this problem. Unfortunately these are high temperature batteries which must 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 operating temperature problems has encouraged several automotive manufacturers to introduce EVs or HEVs using NiMH batteries. NiMH cells operate at normal ambient temperatures. They have a higher energy and power density than Lead Acid cells but not as good as the Zebra cells.

 

Recently high power Lithium Ion cells which have an even higher energy density than NiMH cells, on a par with Zebra cells, have become available. They also operate at normal temperatures and are just being introduced into new electric vehicle designs.

These new high energy cells however are more vulnerable to abuse and need the support of electronic Battery Management Systems to provide protection and ensure long cycle life.

 

Traction Battery Chargers

High capacity batteries also require high power chargers to achieve reasonable charging times and the chargers must be compatible with the cell chemistry and should be able to interface with the cell protection circuitry. Just as the battery is matched to the vehicle, the charger must be custom designed and matched to the battery. More information can be found in the section on Chargers.

 

See also Traction Battery Charging Infrastructure

 

Traction Applications - Electric Drives

 

Market Trends

Information on battery sales volumes and industry trends is given on the History pages.

 

 

 

 

 

 

 

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