Effect of calcium, tin and silver contents in the positive grids of automotive batteries with respect to the grid manufacturing process

Abstract

A study has been undertaken in order to compare the properties of grids produced by rolling expansion and by gravity casting in some specific test conditions. In both types of grids, the effects of calcium, tin and silver contents have been investigated. After bare grid study, a series of electrical tests has been carried out followed by thorough tear down analysis. The mechanical properties of rolled expanded grids have been found to be improved by an increase of tin content from 0.8% to 1.2%, silver and calcium content having no significant effect. The resistance to corrosion of bare grids at equilibrium potential is strongly dependent on the acid density. At low acid density corresponding to the most severe condition, grid corrosion is limited by a high tin content (1.2%) and by a low calcium content (0.04%). There is no effect of silver. In hard conditions of use such as storage and overcharge at high temperature, battery life is increased with a higher tin content for all technologies, and by a smaller extent by the addition of silver on gravity casted grids. A high tin content improves the battery rechargeability after a deep discharge. In conclusion, the addition of silver has only a poor impact on the behaviour of rolled expanded grids. On gravity casted grids, in the same conditions, the addition of silver associated with a low calcium content has been found to improve the battery life slightly. In high temperature conditions of overcharge and storage, the battery life obtained with rolled expanded grids is higher than that with gravity cast, whatever the alloy.