Charging nickel-cadmium cells

Fig. 8. Discharge capacity of small sealed nickel—cadmium cells where the hiitial charge is 0.1 C x 16 h at 20°C and the discharge is 1 C at temperatures of...

A number of manufacturers started commercial production of nickel—MH cells in 1991 (31—35). The initial products are "AA"-size, "Sub-C", and "C -size cells constmcted in a fashion similar to small sealed nickel —cadmium cells. Table 6 compares the Ovonics experimental cell and a similar sized nickel—cadmium cell. Ovonics also deUvered experimental electric vehicle cells, 22 A-h size, for testing. The charge—discharge of "AA" cells produced in Japan (Matsushita) are compared in Figure 22. 

Nickel-cadmium cells have a very low ac resistance of 1 mil. The charge state of the cells is of secondary importance. Nickel-cadmium cells must have sufficient current capacity and have current stability. They can be used directly as a dc decoupling device (Fig. 14-6) [6]. 

Oxygen evolution occurs on nickel oxide electrodes throughout charge, on overcharge, and on standby. It is the anodic process in the self-discharge reaction of the positive electrode in nickel-cadmium cells. Early work in the field has been reviewed [9], No significant new work has been reported in recent years. 

Fig. 6.S Charge retention in nickel-cadmium cells after prolonged periods of open circuit. (By courtesy of Chloride Alcad.)...

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. 

Nickel-melal hydride cells can be discharged at the 2 C rate (and in some cases at 4 C) and charged at 1 C. An AA-sized cell with a nominal capacity of over 1 Ah can thus be discharged at over 2 A and with a peak current of over 10 A. The energy density is highly dependent on rate, but for comparable conditions is 25% higher than an equivalent nickel-cadmium cell. Fig. 6.12 shows a comparison of the discharge characteristics of these two systems. 

The average cell voltage of 1.2 V is slightly lower than that of the Edison cell. Cadmium is preferred to iron in the nickel-aUcaline cell because cadmium hydroxide is more conductive than iron hydroxide. The absence of higher oxidation states for cadmium minimizes side reactions, which occur in the Edison cell. The nickel-cadmium cell can also be charged at a lower voltage since there is no overvoltage, as there is at the iron electrode. 

Because rapid oxygen recombination eliminates the pressure buildup normally associated with sealed nickel cadmium cells, high charging rates can be sustained even in the overcharge mode. Also, it is possible to use conventional nylon cell construction to produce prismatic sealed cells rather than the cylindrical design required for high pressure cells. The sealed... 

FIGURE 27.1 Constant-current charge voltage of vented sintered-plate nickel-cadmium cell, C/10 charge rate. 

The importance of the nickel cadmium cells lies in the fact that, by a special design, they can be made dry giving the portability and freedom of dry primary batteries and the economy of a storage battery. Toward the end of the charging process, both hydrogen and oxygen are liberated at the electrodes, the electrode reactions being... 

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