Advanced Automotive Technology: Visions of a Super-Efficient Family Car

Figure 4-2 shows the relationship between battery weight and range

times the specific energy of the battery, battery weight gets impossibly

weight of the battery does not provide enough energy to increase

performance.

What does this figure say about the relationship between battery

As range approaches six

large, because the added

range while maintaining

weight and range for a

particular vehicle? If an EV were made by using a 1995 Taurus as a “glider,” with beefed-up

structure and suspension if necessary, obtaining a 90-mile range with an advanced semi-

bipolar lead acid battery^22 would require 1,600 lbs of battery, and the total weight of the

car would increase from the current 3,100 lbs to 5,240 lbs (in reality, useful range would be

only about 70 miles since lead acid batteries should be discharged only to 20 percent of

capacity).^23 In contrast, a nickel-metal hydride (Ni-MH) battery, with an SE of 72 Wh/kg, of the

same weight will provide a range of more than 150 miles. The weight of nickel-metal hydride

battery to provide a 100-mile range is 957 pounds, while the car weight falls to 3,305 lbs,

illustrating the importance of weight compounding effects in an EV.

The second constraint on the battery size is that it must be large enough to provide the peak-
power requirement of the motor, or else some peak-power device such as an ultracapacitor or
flywheel may be necessary. Using the same assumptions as before (about vehicle power
requirements and energy efficiency): to obtain a range of 100 miles, the specific power capability
of the battery divided by its specific energy must be at least 3.125 hr-1, or else the power
requirement becomes the limiting factor on battery size. If the range requirement is doubled to
200 miles, then the minimum ratio declines to 1.56 hr-1. For a 100-mile range, only the advanced
semi-bipolar lead-acid battery meets this requirement, with an SP/SE ratios of almost 5, while the
Ni-MH battery has a ratio of about 3. The existing “hot-battery” designs provide ratios of only
1.25, while more recent advanced designs provide ratios closer to 2. The important point of this
discussion is that doubling the specific energy (e.g., by substituting a battery with better
energy storage capability) does not automatically lead to half the battery size, if the
battery’s power capability is inadequate to provide “average performance.” Relaxing the
performance requirement reduces the required ratio, illustrating that hot batteries with good
specific energy but low specific power are best applied to commercial vehicles, where range is
more important than performance. One alternative is to include peak-power devices such as
ultracapacitors with these batteries to provide adequate peak power.

In evaluating the characteristics of EVs in each of the four market classes, OTA made several
assumptions about EV production. We assumed that each EV make/model could be manufactured
on a “conversion” assembly line to produce 2,000 vehicles per month (24,000 per year), implying
total EV sales (across all models and manufacturers) of at least several hundred thousand vehicles
per year. This assumption is required to establish economies of scale, and the assumption that
EVs will be based on “gliders” (conventional vehicles stripped of their drivetrain and modified as
necessary) is required to establish that the vehicle body technology will be similar to the

(^22) Assumed specific energy, SE,of 42 Wh/kg.
(^23) When battery weight equals body weight on the graph, the value of R/SE is 3.6. With an SE of 3.6, the semi-bipolar battery will obtain a range
of 150 km (42 x 3.6) or 90 miles when zero engine body weight (theoretical weight of the body with a weightless powertrain and secondary weight
reductionsaccounted for) equals battery weight- For a current (1995) mid-size ear like the Taurus, the zero engine body weight is about 730 kg or
1600 lbs. Methodology to use these values is described in appendix A