Lead-Acid Battery: Positive Grid Design Principles

In order to insure that stand-by lead-acid batteries are always in a full state of charge, the batteries are "floated" at 2.17 V per cell which is 110 mV above the open circuit cell potential (O.C. = 2.060 V for 1.210 sp. gr. H 2 S0 4 ). Under these float conditions, P b 0 2 is the thermodynamically stable solid phase at the anodes. Consequently, the lead or lead alloy grid is oxidized to P b 0 2 . The initial layer of P b 0 2 formed on the lead anode is passivating and subsequent corrosion is usually a relatively slow process. The specific volume of P b 0 2 is 21 percent greater than that of lead. Therefore, the corrosion product (Pb0 2 ) induces stresses in the lead substrate from which it was formed. The induced stresses cause growth of the grid, which eventually results in failure of the battery. The effect of grid growth is illustrated in Fig. 1 which shows a severely corroded PbCa plate overlayed with a noncorroded grid to show original dimensions. This grid has grown to such an extent that cracking of the P b 0 2 pellets and loss of contact with the grid members 1279