Causes for cell divergence in NiCd and NiH/sub 2/ batteries

Imbalance between the voltage or state-of-charge of individual cells in batteries can seriously degrade their performance. A brief review of the range of mechanisms that can contribute to such imbalance in Ni-Cd and Ni-H/sub 2/ batteries is provided, and a new process that can contribute to cell imbalance is presented. Detailed electrical, calorimetric and modeling data are presented to demonstrate that the nickel electrode in these battery cells can exist in two distinct states, which are differentiated by significant variations in charge voltage, charge efficiency and heat generation. The distinct states are attributed to surface area changes in the discharged nickel electrode active material arising from a combination of particle fracturing and ripening processes. Different combinations of these nickel electrode states can explain much of the history-dependent performance variability often observed for these battery types.