Self‐Discharge Reactions In Lead‐Acid Batteries

Abstract

A theoretical and experimental analysis of the self‐discharge of lead‐acid batteries shows that seven different reactions contribute to the process. The rate of each has been determined. It is shown that positive plate self‐discharge is due primarily to a reaction between and grid metal. The rate of this reaction decreases with increasing acid concentration because of the passivating action of the layers formed. The passivating action is decreased by the presence of antimony in the grid, producing pores in the layers and providing further access of the electrolyte to the grid. At low acid gravities antimony compounds become almost insoluble and can act as passivators decreasing the reaction between grid metal and . Other self‐discharge reactions in positive plates are the decomposition of water by with evolution of oxygen and the oxidation of separator material in contact with . Oxidation of hydrogen, coming from the negative plates, occurs at immeasurably small rates.Self‐discharge of the negative plates is due to the reaction between and lead, producing and . This reaction is slow in absence of foreign substances because of the high hydrogen overvoltage on lead. However, contamination of the negatives with antimony decreases the hydrogen overvoltage and greatly accelerates the self‐discharge of negative plates. Furthermore, negative plates are self‐discharged by oxygen dissolved in the electrolyte. The rate of reduction of oxygen is so fast on a lead electrode in sulfuric acid that the reaction is diffusion controlled.