Charging sealed nickel-metal hydride batteries

CHARGING SEALED NICKEL-METAL HYDRIDE BATTERIES ... 

Most of the operating characteristics of the sealed nickel-metal hydride battery on discharge are similar to those of the nickel-cadmium battery. The sealed nickel-metal hydride battery, however, does not have the very high rate capability of the nickel-cadmium battery. In addition, the behavior of the two systems on charge, particularly on fast charge, is different. The nickel-metal hydride battery is less tolerant of overcharge and requires control of the cutoff of the charge, which may not always be required for nickel-cadmium batteries. 

The most common charging method for the sealed nickel-metal hydride battery is a constant-current charge, but with the current limited to avoid an excessive rise of temperature or exceeding the rate of the oxygen-recombination reaction. 

Sealed nickel-metal hydride batteries can be charged by several methods, ranging from low rate to fast charging, provided the change is controlled to prevent damaging effects. 

Three-Step Charge Procedure. A three-step procedure provides a means of rapidly charging a sealed nickel-metal hydride battery to full charge without excessive overcharge or exposure to high temperatures. 

FIGURE 29.23 Characterization of rate of self-discharge for sealed nickel-metal hydride batteries. Self discharge rate vs state of charge and tetuterature (from reference 6). 

Most important, sealed nickel-metal hydride batteries, as most battery systems, should not be exposed to extreme temperatures for any extended period. They can he stored, in a charged or discharged condition, at moderate temperatures without detrimental effects. 

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. 

While the memory effect may result in reduced battery performance, the actual voltage depression and capacity loss are only a small fraction of the battery s capacity. Most users may never experience low performance due to this behavior of the sealed nickel-metal hydride cell. Often memory effect is used incorrectly to explain a low battery capacity that should be attributed to other problems, such as inadequate charging, overcharge, or exposure to high temperatures. 

FIGURE 29.14 Charge retention characteristics of sealed cylindrical nickel-metal hydride batteries at various temperatures (Courtesy of Sanyo Electrical Co. Ltd.)... 

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. 

FIGURE 29.17 Cell voltage vs. charge input for sealed cylindrical nickel-metal hydride batteries. (a) At various temperatures (charge rate 0.3C). (b) At various charge rates at 20 U. 

FIGURE 29.19 Charge efficiency vs. charge temperature at various charge rates for sealed cylindrical nickel-metal hydride batteries. Discharge at 0.2C rate to 1.0 V. 

FIGURE 29.22 Cycle life and capacity as a function of charge termination method for sealed cyUndrical nickel-metal hydride batteries. Charge 1C rest 0.5 h discharge, 1-0 V rest 2 h. (Courtesy of Duracell, Inc.)... 

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