Analyzing Battery Performance Characteristics Part 2: Different Batteries for Different Applications

There are not only different types of battery families (NiMH, Li-Ion, Lead-Acid), but within each family is a number of different types of chemistry variations that will yield different performance characteristics. Each battery type has different applications based on the specific needs for hybrids, plug-in vehicles, and full battery electric vehicles.

In part 1 of the battery performance characteristics series, I talked about the primary performance characteristics of batteries for hybrid and electric cars.

We’ll start with where we are today – the standard hybrid vehicle. These use NiMH batteries because of the comparatively low cost (Toyota cut Prius battery replacement costs last year) and sufficient power density to drive the small 50kW electric motor despite the small pack size (1-2kWh). The energy density of these batteries is low since you’re not expected to drive the car using only batteries for an extended period, so it isn’t necessary to have a large battery capacity. Since battery replacements due to mileage have been few and far between, the cycle counts and depth of discharge of the current packs are clearly sufficient.

Next will be the plug-in hybrid. These models are a parallel hybrid (like all hybrids today) with an upsized battery pack and electric motor designed to take you 4-5 miles on batteries alone at speeds up to 55mph, turning into an normal hybrid after that. They’re just as equipped as any other vehicle but designed to get you to the neighborhood grocery store and other local errands exclusively on electricity. Lithium-ion would be better suited to keep a same sized pack as a Prius; three times the energy in the same weight and volume of a comparable NiMH pack. This is where newer, higher power batteries (above 3000W/kg) would be used. They have lower energy densities (85Wh/kg) than the type of li-ion batteries for E-REVs and BEVs, but a 2.5kWh pack could drive a 90kW/120HP electric motor and provide for 5 miles of driving. Cycle life will play a role if the vehicle is charged every day for short drives, and it could require a replacement if the battery is recharged more than once per day (at work, the store, etc).

NiMH batteries could also be considered for this application, however the batteries will be about three times larger and heavier than the comparable li-ion pack (30Wh/kg), and the top speed of the vehicle in electric-only mode may suffer slightly (850-1000W/kg). This might be an issue for compact sedans like the Prius in terms of weight distribution and fitting the pack in the vehicle.

E-REV, or extended-range electric vehicles, need the all-around best performing battery – a high specific power to drive 120-200kW motors, high specific energy to reduce the total weight of the pack since you’re still carrying around a gasoline generator, and a high cycle count because the vehicle will go through its full pack about once per day. In the long term, it might not be an issue – the expected 2020 battery technology would allow for light, cheap battery packs that can power a mid-sized SUV with little problems for only $5,000. However we have a long way to go between now and then, and that’s where the issues arise.

Because the number of cells is lower than a BEV, each cell has to produce more power, which when designing battery cells is usually a trade off against specific energy. What automakers would like would be a custom-designed battery with a particular W/kg for their particular sized battery pack. If the pack is going to weigh 100kg and needs 150kW, the pack needs a 1,500W/kg rating. As the pack size and specific energy change, it would be necessary to solve the equations for the specific power needed for the battery pack.

BEV, or battery electric vehicles, are large, most expensive packs. Because there are a large number of battery cells, the specific power can be fairly low, in exchange for a higher specific energy and energy density to decrease the size and weight of the pack as much as possible. Cycle count is usually less of an issue – if you have a 50kWh battery pack with an 85% depth of discharge, 200 miles per charge and a 500 cycle count, you’ll get around 120,000 miles out of the pack before it hits 80% of original capacity (80% is about 160 miles per charge instead of 200). Unless you drive 100+ miles per day cycle count is negligible.

Again, the future 2020 specifications seem to indicate that the trade-offs between specific power and specific energy will continue, and that a pack like the one for the Roadster would be cut in half both in volume and weight. Charging the entire thing quickly is still an issue however.

Hopefully you’ve got a better understanding as to why different types of electric vechiles will require differrent types of batteries. Different constraining requirements (size, power, energy) affect the performance characteristics needed of the individial battery cells.