Nickel-cadmium cells performance

The sealed nickel-metal hydride cell (more consistently metal hydride-nickel oxide cell) has a similar chemistry to the longer-established hydro-gen-nickel oxide cell considered in Chapter 9. In most respects (including OCV and performance characteristics), it is very similar to the sealed nickel-cadmium cell, but with hydrogen absorbed in a metal alloy as the active negative material in place of cadmium. The replacement of cadmium not only increases the energy density, but also produces a more environmentally friendly power source with less severe disposal problems. The nickel-metal hydride cell, however, has lower rate capability, poorer charge retention and is less tolerant of overcharge than the nickel-cadmium cell. 

New, high-density nickel oxide electrode materials, coupled with nonwoven nickel-fiber current collectors, have significantly improved the performance of the nickel electrode even in the nickel-cadmium cells [5j. The alloys used to form hydride, which capture and release hydrogen in volumes up to nearly a thousand times their own, include rare-earth/nickel alloys (generally based on LaNis and called AB5)... 

In general the nickel-metal hydride cell performs in a similar manner to conventional nickel-cadmium cells. 

Despite these differences, fast charging (return to full capacity in 1 hour or less) of nickel-metal hydride cells with appropriate charge control and termination is fully feasible. In general, the same methods of charging used for nickel-cadmium cells work well for nickel-metal hydride although some of the control parameters may require adjustment for optimum performance and service life. 

The nickel-metal hydride cell is expected to have a service life approaching that of the nickel-cadmium cell. Effects of operating temperature on both discharge performance and charge acceptance are also similar between cells. 

Varta supply a sealed sintered electrode nickel-cadmium cell (RSH and RSX type) for very high rates of discharge. Figure 51.4 compares the performance characteristics of the RS type of cell and the RSH type. A by-product of the lower internal resistance... 

Nickel-cadmium batteries with thin sintered plates are used for on-board power supplies in aircraft, helicopters, tanks and military vehicles where their excellent low temperature, high rate performance is an important attribute. Modern 40 Ah cells designed for airborne use can deliver 20 kW of instantaneous power at 25°C and over 10 kW at —30DC. Again, the high cost of this system compared with that of lead-acid batteries has restricted its use. 

Mil-B-81757 (1984) Performance specifications, batteries and cells, storage, nickel cadmium. Aircraft General Specification, Crane Division, NSWC, 1 July 1984... 

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