EFFECTS OF GRID ALLOY ON THE PROPERTIES OF POSITIVE-PLATE CORROSION LAYERS IN LEAD-ACID-BATTERIES - IMPLICATIONS FOR PREMATURE CAPACITY LOSS UNDER REPETITIVE DEEP-DISCHARGE CYCLING SERVICE

Premature capacity loss (PCL) has been demonstrated consistently by th e deep-discharge cycling of three-plate lead/acid cells configured wit h an excess of electrolyte. A capacity loss of approximately 2% per cy cle was observed with cells based on tin-free, lead-calcium positive g rids, under both constant-current and constant-voltage charging. The c urrent that flows during constant-voltage charging decreases markedly within the first few cycles. This coincides with the establishment of an appreciable corrosion layer on the grid, and also with the onset of severe capacity loss. Significantly, there is no corresponding build- up of lead sulfate within the porous mass. With constant-current charg ing, a change in the overcharge factor, from 1.1 to 1.2, approximately doubles the rate of capacity loss. The corrosion products in lead-cal cium plates exhibit a bi-layered structure: an outer corrosion layer o f PbO2 and an inner layer in which the composition approaches PbO. The se materials are prone to fracture and separation, especially between the two layers. Cells based on lead-antimony positive grids also suffe r PCL, but the rate of capacity loss (approximately 1% per cycle) is l ess than that observed for the lead-calcium analogues. The current und er constant-voltage charging of lead-antimony cells also decreases dur ing the first few charge/discharge cycles, yet, this effect is over-sh adowed by an increase in current due to the effects of antimony migrat ion. Increasing the level of overcharge under constant-current chargin g produces only a slight reduction in cycle life. In regions close to the grid, PbO-like material is much less abundant than in lead-calcium plates. The corrosion products are composed mainly of PbO2, and are m ore coherent under stress. Resistance at the grid/porous material inte rface, measured in situ, increases greatly during discharge for plates based on lead-calcium grids, but much less for the corresponding lead -antimony plates. This tends to emphasize the importance of the barrie r-layer model of capacity loss, which is supported further by the obse rvation of low-oxidation-state lead compounds in lead-calcium samples, but not in lead-antimony samples. The barrier-layer model cannot, how ever, provide a complete explanation for PCL: the corrosion layers of lead-antimony plates remain apparently conductive and coherent through out cycle life, yet, these plates suffer appreciable capacity loss. At least part of the decrease in plate performance may, therefore be due to changes in the conductivity/integrity/activity of the bulk porous mass.