PREMATURE CAPACITY-LOSS MECHANISMS IN LEAD-ACID-BATTERIES

The phenomenon known as 'premature capacity loss' (PCL) causes the early demise of lead/acid batteries based on a variety of grid alloys. It is also known to be a problem specific to the positive plate and is usually invoked by duties that involve repetitive deep-discharge cycling. In order to determine the cause(s) of the problem, an extensive study of the behaviour of cells based on a range of positive grid alloys is being conducted. Examples of PCL have been generated by subjecting three-plate cells to 100% depth-of-discharge, at I=C-8/8, with 110% overcharge. Cells based on antimony-free grids exhibit capacity loss at a rate of up to 5% of the initial capacity per cycle, with both constant-current and constant-voltage charging. With the latter charging method, most of the cells also develop extremely poor charge acceptance within 10 to 15 cycles. The performance of cells with high-antimony positive grids is significantly better, although substantial capacity loss is still observed. The latter cannot be explained by any of the classic failure modes for lead/acid batteries. Poor charge acceptance is not displayed by these cells. Plates show signs of physical degradation, but these represent a minor contribution to capacity loss. Phase composition of positive material does not vary with grid alloy and is typical of healthy plates. Investigations of corrosion-layer morphology have shown that Pb-Ca grids give rise to weak corrosion products that are prone to fracture and separation, while the corrosion layers on Pb-Sb plates are apparently more coherent and more strongly bonded to the underlying grid.