problem has been their relatively modest improvement in specific energy over advanced lead acid
batteries in comparison to their high cost. Modern Ni-Cd batteries have specific energy ratings up
to 55 Wh/kg, which is about 25 percent better than the Horizon lead acid battery. They cost at
least four times as much,^72 but these higher costs will be offset to an extent by Ni-Cd batteries’
longer cycle lives. High-energy versions of these batteries require maintenance and their capacity
changes with charge/discharge cycles. Sealed Ni-Cd batteries that are maintenance free have
significantly lower specific energy (35 to 40 Wh/kg), although there is ongoing research to avoid
this penalty. In addition, concerns about the toxicity of battery materials and the recyclability of
the battery has resulted in reduced expectations for this battery.
Nickel-iron batteries received considerable attention a few years ago, but interest has faded
recently. Their specific energy is about 50 Wh/kg, and their costs are similar to, or slightly lower
than, those for Ni-Cd batteries.^73 Although they have demonstrated good durability, they require a
sophisticated maintenance system that adds water to the batteries and prevents overheating during
charge. In addition, they cannot be sealed, as they produce hydrogen and oxygen during charging,
which must be vented and pose some safety problems. The formation of hydrogen and oxygen
also results in reduced battery charging efficiency, and these features account for the lack of
current interest in this battery.Nickel-metal hydride batteries have received much recent attention lately, and Ovonic and
SAFT are the leading developers of such batteries. The maintenance-free Ovonic batteries have
demonstrated specific energy values in excess of 80 Wh/kg at the module level and specific power
densities of over 200 W/kg.^74 However, automanufacturers have stated that these batteries have
high internal self discharge rates, especially at high ambient temperatures, with losses of 32
percent over 5 days at 40oC.^75 Automanufacturers have also noted that Ovonic batteries have
capacity limitations at low temperatures when discharged quickly, and they are worried about
hydrogen buildup during charging. Nevertheless, the Ovonic batteries’ demonstrated capabilities
and the potential to overcome these problems has led to optimism about their prospects for
commercialization. GM and Ovonics have entered into a joint venture to produce the battery, and
pilot production may occur in late-1996. It should be noted that a complete battery to power an
EV has only recently become available, and prototype testing will demonstrate the battery’s
durability in an EV environment.
Auto manufacturers do not believe that the Ovonic battery can be manufactured at low cost,
especially as other battery manufacturers developing nickel metal hydride batteries do not support
Ovonic’s cost claims. Ovonic has suggested that the batteries can be manufactured at $235/kWh
and perhaps below, whereas others expect costs to be twice as high (~$500/kWh) in volume
production.^76 It should also be noted that the batteries are not yet easily recyclable, as the
complex metal hydride used by Ovonic can only be regenerated today by an expensive process.