WHEN IS CAPACITY LOSS IN LEAD ACID BATTERIES PREMATURE/

Elucidation of the principal mechanism that underlies premature capaci ty 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 d esigned plates in which expansion of positive material can be restrict ed in a controlled fashion. In particular, two findings have bridged t he gap between failure at the interface (PCL-1) and failure in the bul k material (PCL-2): (i) plates subjected to extreme conditions of serv ice can cycle at constant capacity for long periods, despite the prese nce of 'barrier-layers'; (ii) loss of conductivity in the porous mater ial close to the current-collector can explain severe and rapid capaci ty loss. On examination, the latter situation is characterized by a lo calization of lead sulfate in the region close to the current-collecto r, in line with previous reports of 'preferential discharge'. The capa city loss for any plate/cell configuration can now be placed on a cont inuous scale - the rate of loss is determined by the degree to which t he configuration, and conditions of service, are able to control the d ecrease in conductivity of positive material close to the current-coll ector. Development of positive plates for advanced lead/acid batteries must consider strategies for maintaining conductivity through managem ent of the combined effects of expansion and redistribution of positiv e material.