Failure mechanisms in valve regulated lead/acid batteries for cyclic applications

Valve regulated lead/acid (VRLA) batteries are used in a variety of different applications, one of which is cycling. Cycle life testing of a batch of 40 Ah VRLA batteries showed a large variation in the cycles to failure ranging from 10 to 133 cycles. Further testing and the destructive examination of these batteries provided information on the likely causes of failure. Results from monitoring reduction in cell voltage during a final discharge/charge cycle, scanning electron microscopy (SEM), BET surface area analysis, X-ray diffraction, interfacial analysis and electron probe analysis, were used to identify the failure mechanisms occurring within the batch. Batteries that failed after a low number of cycles, 10 and 28, were believed to have done so due to sulphation of the positive plate. Thick corrosion layers were shown to be the cause of failure in the batteries that sustained high numbers of cycles, 92 and 133. Results suggested that batteries failing at intermediate numbers of cycles, 42, 49, 65 and 73, failed due to degradation of the cells simultaneously and a single failure mechanism could not be identified.