Charging sealed nickel-cadmium batteries

High-rate charging Sealed nickel-cadmium batteries are capable of recharge at high rates within 1 h under controlled conditions. Many batteries can be charged in 3 to 5 h without special controls, and all can be recharged within 14 h. 

FIGURE 28.14 Resistance vs. temperature for fully charged sealed nickel-cadmium batteries, a—AA-size battery, b—sub-C-size battery. (Typical for sintered-plate electrode type batteries.)... 

FIGURE 2830 Methods for charging sealed nickel-cadmium batteries and charge control, (a) Semiconstant current —battery voltage —charge current t—time), (h) Timer control (t —timer start ... 

The situation is different for sealed nickel/cadmium batteries, due to the internal oxygen cycle. Figure 1.15 illustrates the heat evolution of a sealed nickel/ cadmium battery during constant-current charging with a charge factor of 1.4 (such an amount of overcharge is usual for conventional charging methods but can only be applied to comparably small batteries < 10 Ah). 

Figure 1.15 Charging of a sealed nickel/cadmium battery with constant current 0.2 C(A). During 7 hours 140% of the nominal capacity are recharged, which corresponds with a charge factor 1.4. For comparison, all values are converted to 100 Ah of nominal capacity. Actually, batteries of this type and for such a charging schedule are only available in sizes < 10 Ah.

The characteristics of nickel/metal hydride batteries are very similar to those of sealed nickel/cadmium batteries. The cell voltage differs by only 20 mV, and charging as well as discharging performance are so alike that both battery systems can be replaced by each other in all normal applications. The discharge curves in Fig. 1.38 confirm this. 

The active materials of the sealed nickel-cadmium battery are the same as for other types of nickel-cadmium batteries, namely, in the charged state, cadmium for the negative electrode, nickel oxyhydroxide for the positive, and a solution of potassium hydroxide for the electrolyte In the discharged state, nickel hydroxide is the active material of the positive electrode, and cadmium hydroxide that of the negative. 

FIGURE 28.7 Constant-current discharge curves for sealed nickel-cadmium batteries at 2(FC, charge O.IC, 16 h. Courtesy of Sanyo Energy Corp.)... 

FIGURE 28.21 Cycle life of sealed nickel-cadmium batteries at 20°C. Cycle conditions charge—O.IC X 11 h discharge—0.7C X 1 h. Capacity-measuring conditions charge— O.IC X 16 h discharge—0.2C, end voltage—1 V. 

This phenomenon varies with the design and formulation of the electrode and may not be evident with all sealed nickel-cadmium batteries. Modern nickel-cadmium batteries use electrode structures and formation processes that reduce the susceptibility to voltage depression, and most users may never experience low performance due to memory effect. However, the use of the term memory effect persists, since it is often used to explain low battery capacity that is attributable to other problems, such as ineffective charging, overcharge, battery aging, or exposure to high temperatures. 

FIGURE 28.24 Typical disdiarge voltage profile of sealed nickel-cadmium batteries after long-term overcharge (dotted line) vs. 16-h charge, both at C /lO rate. 

The voltage profile of a sealed nickel-cadmium battery during charge at the d 10 and C/3 rates is shown in Fig. 28.25. A sharp rise in voltage to a peak near the end of the charge is evident. 

The voltage profile of a sealed nickel-cadmium batteries is different from the one for a vented one, as illustrated in Fig. 28.26 The end-of-charge voltage for the sealed battery is lower. The negative plate does not reach as high a state of charge as it does in the vented construction because of the oxygen recombination reaction. 

Constant-potential charging is not recommended for sealed nickel-cadmium batteries as it can lead to thermal runaway. It can, however, be used if precautions are taken to limit the current toward the end of charge. 

FIGURE 28.25 Typical pressure, temperature, and voltage relationships of sealed nickel-cadmium battery during charge. 

FIGURE 28.27 Charge process of sealed nickel-cadmium batteries. (a) Charge efficiency at 20°C. Charge—O.IC x 16 h discharge—0.2°C end voltage—1 V. (b) Charge efficiency vs. 

An aqueous solution of potassium hydroxide is the major component of the electrolyte. A minimum amount of electrolyte is used in this sealed cell design, with most of the Uquid absorbed by the separator and the electrodes. This starved-electrolyte design, similar to the one in sealed nickel-cadmium batteries, facilitates the diffusion of oxygen to the negative electrode at the end of the charge for the oxygen-recombination reaction. This is essentially a dry-cell construction, and the cell is capable of operating in any position. 

The recharging characteristics of the nickel-metal hydride battery are generally similar to those of the sealed nickel-cadmium battery. There are some distinct differences, however, particularly on the requirements for charge control, as the metal hydride battery is more sensitive to overcharge. Caution should be exercised before using the same battery charger interchangeably for both types of batteries. 

In applications with permanent maintenance charge, the life of a battery is expressed as the multiple of Cs Ah of overcharge or as hours of operation, and the failure rate as failures per operating hour. As an example. Figure 4.5 shows the estimated cycling life at 20°C as a function of discharge for SAJT sealed nickel-cadmium batteries. 

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