Battery Technologies

The battery is the critical technology for electric vehicles, providing both energy and power storage. 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.

Numerous battery types are in various stages of development. Although there are multiple claims for the efficacy of each type, there is a large difference between the performance of small modules or even full battery packs under nondemanding laboratory tests, and performance in the challenging environment of actual vehicle service or tests designed to duplicate this situation. Although the U.S. Advanced Battery Consortium is sponsoring such tests, the key results are confidential, and much of the publicly available information comes from the battery manufacturers themselves, and may be unreliable. Nevertheless, it is quite clear that a number of the batteries in development will prove superior to the dominant conventional lead acid battery, 2 though at a higher purchase price. Promising candidates include advanced lead acid (e.g., woven-grid semi-bipolar and bipolar) with specific energy of 35 to 50 Wh/kg, specific power of 200 to 900 W/kg, 3 and claimed lifetimes of five years and longer; nickel metal hydride with 80 Wh/kg and 200 W/kg specific energy and power, and claimed very long lifetimes; lithium polymer, considered potentially to be an especially “EV friendly” battery (they are spillage proof and maintenance free), that claims specific energy and power of 200 or more Wh/kg and 100 or more W/kg; lithium-ion, which has demonstrated specific energy of 100 to 110 Wh/kg; and many others. The claimed values of battery lifetime in vehicle applications should be considered extremely uncertain. With the possible exception of some of the very near-term advanced lead acid batteries, each of the battery types has significant remaining challenges to commercialization—high costs, corrosion and thermal management problems, gas build-up during charging, and so forth. Further, the history of battery commercialization demonstrates that bringing a battery to market demands an extensive probationary period: once a battery has moved beyond the single cell stage, it will require a testing time of nearly a decade or more before it can be considered a proven production model.

Non-battery Energy Storage: Ultracapacitors and Flywheels

Ultracapacitors

Ultra-capacitors are devices that can directly store electrical charges—unlike batteries, which store electricity as chemical energy. A variety of ultracapacitor materials and designs are being investigated, but all share some basic characteristics-very high specific power, greater than 1 kW/kg, coupled with low specific energy. The U.S. Department of Energy mid-term goal is only 10 Wh/kg (compared to the U.S. Advanced Battery Consortium midterm battery goal of 100 Wh/kg). Other likely ultracapacitor characteristics are high storage efficiency and long life.

Ultracapacitors’ energy and power characteristics define their role. In electric vehicles, their high specific power can be used to absorb the strong power surges of regenerative braking, to provide high power for brief spurts of acceleration, and to smooth out any rapid changes in power demand from the battery in order to prolong its life. In hybrids, they theoretically could be used as the energy storage mechanism; however, their low specific energy limits their ability to provide a prolonged or repeatable power boost. Increasing ultracapacitors’ specific energy is a critical research goal.

Flywheels

A flywheel stores energy as the mechanical energy of a rapidly spinning mass, which rotates on virtually frictionless bearings in a near-vacuum environment to minimize losses. The flywheel itself can serve as the rotor of an electrical motor/generator, so it can turn its mechanical energy into electricity or vice versa, as needed. Like ultracapacitors, flywheels have very high specific power ratings and relatively low specific energy, though their energy storage capacity is likely to be higher than ultracapacitors. Consequently, they may be more practical than ultracapacitors for service as the energy storage mechanism in a hybrid. In fact, the manufacturer of the flywheel designed for Chrysler’s Patriot race car, admittedly a very expensive design, claims a specific energy of 73 Wh/kg, which would make the flywheel a very attractive hybrid storage device. Mass-market applications for flywheels depend on solving critical rotor manufacturing issues, and, even if these issues were successfully addressed, it is unclear whether mass-produced flywheels could approach the Patriot flywheel’s specific energy level.