When is capacity loss in lead/acid batteries ‘premature’?

Elucidation of the principal mechanism that underlies premature capacity loss (PCL) in lead/acid positive plates has always been hampered by the notion that different forms of PCL are responsible for severe and mild instances of capacity loss. Recently, though, studies focused on the conductivity of the porous mass have provided a clear, universal explanation for all examples of PCL. The evidence required to link the differing views has come from charge/discharge cycling of specially designed plates in which expansion of positive material can be restricted in a controlled fashion. In particular, two findings have bridged the gap between failure at the interface (PCL-1) and failure in the bulk material (PCL-2): (i) plates subjected to extreme conditions of service can cycle at constant capacity for long periods, despite the presence of ‘barrier-layers’; (ii) loss of conductivity in the porous material close to the current-collector can explain severe and rapid capacity loss. On examination, the latter situation is characterized by a localization of lead sulfate in the region close to the current-collector, in line with previous reports of ‘preferential discharge’. The capacity loss for any plate/cell configuration can now be placed on a continuous scale — the rate of loss is determined by the degree to which the configuration, and conditions of service, are able to control the decrease in conductivity of positive material close to the current-collector. Development of positive plates for advanced lead/acid batteries must consider strategies for maintaining conductivity through management of the combined effects of expansion and redistribution of positive material.