accessory use saves as much as 9.5 percent of fuel consumption on the EPA test cycle. (Real
world fuel efficiency and range are considered following the discussion of the EV’s efficiency on
the EPA test) And although the EV may need some power for the brakes, this requirement is
probably small owing to the use of regenerative braking, as described below.Third, some of the energy lost during braking can be recovered by an EV, because the motor
can act as a generator when it absorbs power from the wheels. The energy can be stored in the
battery and later released to drive the motor. As noted earlier, the energy lost to the brakes in a
conventional car is about 35 percent of total tractive energy. For various reasons--transmission
and generator losses, battery charge/discharge loss, requirement for some conventional braking
capacity--the actual energy recovery is considerably less than this.^15 Actual systems in the Toyota
EV^16 and the Cocconi CRX,^17 which have the best regenerative braking efficiencies reported,
provide range increases of about 17 to 18 percent maximum. An 8 to 10 percent range extension
is more typical of current EVs, such as the BMW El.Fourth, the motor is quite efficient in converting electrical energy to shaft energy, with cycle
average efficiencies for good motors in the 75 to 80 percent range in the city cycle, as opposed to
gasoline engines, which have an efficiency of only 20 to 23 percent on the fuel economy test
cycle.There are several factors working in the opposite direction. Losses from the primary energy
source to energy delivered to the vehicle--critical for concerns about greenhouse gas production--
generally are much higher for EVs than for gasoline vehicles, because electricity generation
efficiency is quite often low (about 34 percent for a conventional coal-fired powerplant), and
electricity generation may add another 10 percent in losses. Additional losses occur at the battery
charger, in losses in discharging the battery, and in battery internal self discharge, wherein the
battery (or flywheel, or ultracapacitor) gradually suffers losses over time. Another important
factor is that EVs may be much heavier than an ICE-powered vehicle of similar performance (and
have lower range^18 ), because battery size is critical to range and power--the added weight then
creates higher rolling resistance and higher inertia losses (of which only a portion are regained
from the regenerative braking).Considering the fill range of energy losses, an EV may well be less efficient on a primary
energy basis than a conventional vehicle of equal size and acceleration performance, especially if(^15) For the motor to convert braking energy to electricity, transmission loss and motor loss in generator mode must first be considered. Typically,
transmissions for electric motors are simple drive gears, and can be 95 to 96 percent efficient. Motors operated in reverse generator mode typically
have cycle average efficiency in the 80 to 84 percent range. Hence, only 78 percent of the braking energy can be converted to electricity, which is
about 27.0 percent of traction energy. The storage and retrieval of electricity in a battery causes further loss, but this is very dependent on the battery
type, and its efficiency in terms of absorbing power pulses. This efficiency is only 80 percent or lower for lead acid and nickel-cadmium batteries, so
that regenerative braking recaptures only 0.82 x 0.95 x 0.80 x 0.35, or 21.8, percent of tractive energy. This assumes that all of the braking can be
done regeneratively but this is not true in practice, since the motor is connected to only two wheels, leaving the other two wheels to be braked
conventionally (proper handling during hard braking requires that all four wheels be braked for stability). (^16) K. Kanamaru, "Toyota EV-50: An Effort To Realize Practical EVs,” paper presented at the 12th International Electric Vehicle Symposium
,
December, 1994. 17
A. Burke, Institute of Transportation Studies, University of California at Davis, “Dynamometer and Road Testing of Advanced E1ectric
Vehicle,” 1995. 18
Matching the range of a similarly sized ICE vehicle may well be impossible for an EV, because the ability to increase battery size is limited by
the effect of the added weight on motor and structural weight. Consequently,“fair” comparisons of EVs and ICE vehicles may try to match
acceleration performance, especially at low speeds, but rarely try to match range.