Degradation in nickel-cadmium cells studied by impedance measurements

The impedance of 12 A h NiCd sealed cells was evaluated as a nondestructive, in situ means of determining cell lifetime, particularly with respect to the probability of premature failure. The NiCd cell impedance was measured (10 000 to 0.0004 Hz) as cells were subjected to charge/ discharge cycle testing under a variety of temperature and depth-of-discharge conditions. The results indicate that cell degradation is reflected in the low frequency impedance characteristics associated with diffusional processes. The slope ( W) of the low frequency portion of complex plane impedance plots obtained from totally discharged NiCd cells was found to increase with charge/discharge cycling. In addition, based on data for two cells, a high or rapidly increasing W value signals imminent cell failure by one mechanism. Degradation by another mechanism is apparently reflected in a fall-off of the imaginary impedance component at low frequencies. The frequency dependence of the absolute cell impedance at low frequencies (0.5 - 0.005 Hz) was found to vary monotonically with cell state-of-charge.