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

mid-1994. New technology competitors may emerge very quickly and new findings may render

existing “competitive” technologies poor prospects for the future.

Battery Technologies.

Requirements

A battery is a device that stores electricity in a chemical form that is released when an external

circuit is completed between the battery’s opposing terminals. The battery, which provides both

energy and power storage, is the critical technology for electric vehicles. Unfortunately, the weak

link of batteries has been their low energy storage capacity--on a weight basis, lower than gasoline

by a factor of 100 to 400. Power capacity may also be a problem, especially for some of the

higher temperature and higher energy batteries. In fact, power capacity is the more crucial factor

for hybrid vehicles, where the battery’s major function is to be a load leveler for the engine, not to

store energy. Aside from increasing energy and power storage, other key goals of battery R&D

are increasing longevity and efficiency and reducing costs.

Traditionally, the storage characteristics of conventional lead-acid batteries have been so poor

that electric vehicles (EVs) have been extremely heavy, with poor acceleration performance and

limited range. Battery technology research sponsored by the U.S. Advanced Battery Consortium

(ABC) has sought to develop new batteries with improved storage and other characteristics. The

performance characteristics of a battery relevant to use in vehicles can be defined by the following

parameters, for which ABC has set goals.^67

The specific energy is a measure of the total quantity of energy stored per unit of battery

weight. ABC has set a goal of 80 watt-hours/kilogram (with 100 Wh/kg desired) as a mid-term

goal and 200 Wh/kg as a long term goal for this parameter. In contrast, conventional lead acid

batteries have specific energy levels of 25 to 28 Wh/kg.

Specific power is a measure of how much power per unit weight the battery can deliver per
second to handle peak requirements for acceleration and grade climbing. ABC’s mid- and long-
term goals are 150 W/kg (200 W/kg desired) and 400 W/kg respectively for a 30-second pulse of
power. Conventional lead acid batteries can provide as much as 100 W/kg when fully charged, but
their peak power capability declines rapidly as they are discharged, and is about 60 W/kg at 80
percent depth-of-discharge (DoD). To some degree, specific power is a function of battery
design, and especially trades off with specific energy. Hence, batteries designed for high power
may differ from those designed for high energy.

The sustainability of peak power levels is an important issue for hybrid vehicles. The peak
power values quoted in this section are based on a 30-second pulse. Batteries may not be able to
sustain even half this peak level, if the duration is in the order of two to four minutes. However,
the capability of the battery to deliver high power is a function of its design as well as the battery

67 U.S. Advanced Consortium, “Update,” October 1994.