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Depth-of-discharge (DOD) is simply the percent of total battery capacity removed during a discharge period. Higher percentages imply shorter cycle life as shown in Fig. 11-11. Once we know the number of cycles and the average depth of discharge, we can determine the total capacity of the batteries.

Figure 11-11 illustrates the relationship between average depth-of-discharge (DOD) and cycle life for secondary batteries using nickel cadmium (NiCd) and nickel hydrogen (NiHj). Extensive data supports the predictions for both NiCd and NiH2.

The NiCd battery is still a common secondary energy storage system for many aerospace applications. NiCd technology has been space qualified, and we have extensive databases for nearly any mission. A 28 Vdc aerospace NiCd battery usually consists of 22-23 series-connected cells. NiCd battery cells for aerospace missions have typical capacities of 5 to 100 Amp-hr.

NiH2 technology has been the recently qualified energy storage system of choice for aerospace applications where higher specific energies and longer life are important The three space-qualified design configurations for NiH2 are individual pressure vessel, common pressure vessel, and single pressure vessel. The individual pressure vessel was the first NiHj technology used for aerospace application. Here, only a single electrochemical cell is contained within the pressure vessel. It has a working

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