Characterization of plates from a spiral wound HEV battery

Abstract

In support of its hybrid electric vehicle (HEV) battery program, Optima Batteries has developed a novel plate mapping technique to characterize plates under load. Voltages at discrete locations on single, unwound plates were measured versus a reference electrode by applying a current between a counter electrode and the plate. The technique was applied to 15 Ah valve regulated, thin plate spiral lead acid batteries that were hybrid cycled until they showed signs of "apparent capacity loss" (ACL). ACL is characterized by a decrease in capacity, power and efficiency that is reversed when the battery receives a conditioning charge. The ACL battery plate maps of both the positive and negative plates showed substantive voltage variations that were vertically oriented. The vertical bands were also present in control plates, although voltage variations were much less. This result was surprising, as it was expected that any voltage variations would be in horizontal bands. X-ray diffraction analysis of active material from the same ACL battery showed lead sulfate gradients on the negative plate, with the most sulfate being at the bottom of the plate. No lead sulfate gradients were observed on the positive plate. These findings suggest that the negative plate is insufficiently charged during hybrid operation, the overall result being ACL. The vertical banding under load seen on the plate maps point out the inhomogeneity of the electrodes.