Voltage Depressions

Nickel-metal-hydride battery Cerium, lanthanum Minimum voltage depression... 

Figure 5.2 Memory effect and voltage depression effect frequently observed in Ni-MH batteries.

Actual determination of SOC in an Ni-MH battery is dependent on a chemical reaction between the SOC and OCV. This is true if one knows both the parameters of SOC and OCV. Determination of the critical value of OCV is again dependent on SOC, age of the battery, and the adverse effects of voltage depression. From comparing the OCV with the DOD curves for new and old batteries, one can determine the critical value of OCV. These curves indicate that as the battery ages, if the desired DOD is about 23%, the critical value of OCV increases. The longevity of low-power batteries is strictly dependent on the following conditions ... 

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. 

The extent of voltage depression and capacity loss depends on the depth of discharge and can be avoided or minimized by discharging the battery to an appropriate end voltage. The effect is most apparent when the discharge is terminated at the higher end voltages, such as... 

FIGURE 29.13 Voltage depression during cycling of sealed cylindrical nickel-metal hydride battery, 20°C. (Courtesy of Duracell, Inc.)... 

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. 

Voltage Depression An abnormal low voltage, below the expected value, during the discharge of a battery. 

The life curve (Figure 30.12) shows the typical performance of an E 12-type cylindrical mercuric oxide battery over the temperature range —23 to - -71°C for current drains encountered in many applications. Successful operation at temperatures above 120°C for short periods has been reported, but it is recommended that 70°C is not exceeded. Figure 30.13 indicates that voltage depression is slight at low temperatures when drains are 100 pA or less in the large cell types or when intermittent drains of this order are used with smaller cells. 

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