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Charge Equalization Unit for a NiCd Battery of Small Earth Observation Satellite EPS Simulation

In micro, mini and small satellites because of needs for higher bus voltage and small required battery capacities, batteries are nearly always used in series combinations of multiple cells. When a series string of cells is charged as a group, a single current is imposed on all the cells. However, if the voltages begin to differ, the result is a charge imbalance that can lead ultimately to battery failure. In any useful series battery charge process, some type of "charge balancing" or "equalization" must take place to restore balance or at least prevent it from growing. This paper reviews battery behavior and performance related to the equalization problem, in the context of Nickel Cadmium batteries. Equalization processes can extend the life of battery packs. Active equalization with a DC-DC converter speeds the process and supports exchange (sharing) with the weak cell(s) Passive equalization is too slow in most contexts for strings longer than about 12 cells especially in a space applications. A Matlab Simulink model is built for a charge equalization unit of a 22 cells NiCd battery stack. Model is tested with a resistive load as well as with real battery cells. Simulation results confirm the applicability of the equalization control technique.