the ICE vehicle is particularly fuel efficient. One such primary energy comparison between a
BMW El and VW Polo diesel,^19 which are comparable in size, is shown in figure 4-1. In this
comparison, the overall BMW El motor efficiency is very low, at 66 percent rather than 75 to 80
percent; if this were changed to 80 percent, then the EV would have the same primary energy
efficiency as the diesel car.
The BMW comparison also shows some real world effects of energy loss owing to battery
heating--the battery is a high-temperature Na-S battery--and includes accessory losses. Internal
self discharge or battery heating losses reduce efficiency in inverse proportion to miles driven per
day. Accessories such as the power steering and power brake consume a few hundred watts of
power typically, but the air conditioner, heater, and window defrosters are major drains on power.
Some EVs, such as the GM Impact, have replaced the conventional air-conditioner or heater with
a heat pump which increases accessory load to 3 kW.^20 A typical advanced EV will consume
about 12 to 15 kW at 60 mph (see table 4-6^21 ), so that accessory load represents a substantial
fraction of the total power demand of the vehicle. Thus, with these accessories on, highway
range can be reduced 20 to 25 percent; range in city driving can be reduced 50 percent.
Cold or hot temperatures also impact the battery storage capacity, so that the range reductions
owing to accessory power loss are only one part of the picture. In very cold weather, alkaline
batteries and lead-acid batteries have significantly lower energy storage capacities, as discussed
earlier. Peak power is also affected, so that both range and acceleration capability suffers. At
20 oF, the effect of accessory loads is also very high, as it is not unusual to need headlights, wipers,
defroster, and passenger heating in such situations. The combined effect of reduced battery
capacity and higher loads can reduce the range in city driving by as much as 80 percent. In hot
weather, the battery can be power limited owing to the difficulty of removing the heat created
when high power is demanded from the battery, and internal self discharge of batteries can also be
higher. Unfortunately, hard data on battery losses in hot weather is not available publicly.
The analysis of overall vehicle weight, and the tradeoffs among range, performance, and battery
weight are especially important for an electric vehicle. Generally, adding more battery weight
allows greater vehicle range and power. However, there is a limit to this relationship: as battery
weight increases, structural weight must also increase to carry the loads, and a larger--and
heavier--motor is required to maintain performance. This weight spiral effect leads to rapidly
declining benefits to each additional battery weight increment, and finally to zero benefit.
It is possible to examine these tradeoffs by using energy balance equations similar to those used
for ICE engines, coupled with some simplifying assumptions about motor output requirements for
normal performance requirements (50 kW/ton of vehicle weight to allow normal levels of
acceleration and hill climbing), and using a “best-in-class” specific traction energy measured in
kilowatt hours per ton-kilometer (kWh/ton-km), that is, assuming the vehicle being analyzed
attains the energy efficiency of the best available EVs with regenerative braking, which is about
0.1 kWh/ton-km.(^19) K. Scheurer et al., "The Electric Car: An Attractive Concept for City Traffic,” BMW Publications, 1993.
(^20) K. Scheurer, "The BMW E-1, A Purpose Designed EV,” paper presented at the 11th International EV Symposium, September 1992.
(^21) At 60 mph or 97 km/hr, an average fuel consumption of 0.15 kWh/km implies a power use of 97* 0.15 = 14.6 kW.