Page 134 - Battery Reference Book

4/18 Nickel batteries
will require careful attention to selection of proper has to date not identified any significant concerns.
charge parameters. The occupational health and environmental disposal
concerns associated with the use of cadmium in
Despite these differences, fast charging (return to nickel-cadmium cells appears to have been eliminated
full capacity in 1 hour or less) of nickel-metal hydride with nickel-metal hydride cells. Testing of metal
cells with appropriate charge control and termination hydride cells indicates that their leachable cadmium
is fully feasible. In general, the same methods of char- content is below EPA's threshold limit value.
ging used for nickel-cadmium cells work well for Cadmium content of metal hydride cells is below the
nickel-metal hydride although some of the control European Community's limit of 0.025% by weight.
parameters may require adjustment for optimum per- No other environmental problems have been associated
formance and service life. with the metal hydride cell. However, the performance
The nickel-metal hydride cell is expected to have and environmental benefits of nickel-metal hydride
a service life approaching that of the nickel-cadmium cells come at a price. At this time, the price per
cell. Effects of operating temperature on both discharge cell for metal hydride cells is approximately double
performance and charge acceptance are also similar an equivalent high-performance nickel -cadmium cell.
between cells. This price premium will decline within the next couple
One difference in performance between the two cell of years as production volumes increase.
types occurs in self-discharge, i.e. loss of capacity
in storage. The nickel-metal hydride cell currently
self-discharges at a slightly higher rate (about 5% 4.6 Sodium-nickel chloride secondary
per day) than that seen with nickel-cadmium cells batteries
in storage. Development work is targeted at reducing
the self-discharge rate to about the level seen with These newly developed cells, discussed further in
nickel-cadmium cells. Based on normal usage profiles a later section, operate at high temperatures around
expected with portable computers, self-discharge is not 300°C. Their open circuit voltage is about 2.6 V in the
a major problem in this application. 200-300°C temperature range. The theoretical energy
Extensive testing of the environmental impact of density is about 750 W k/kgg' against practically real-
the materials used in nickel-metal hydride cells ized values of 100 Wag-' (160 Wh/dm-3).