Defining the life of valve-regulated lead-acid batteries: a new approach to accelerated testing

Accelerated float life tests of lead-acid batteries are based on the assumption that positive grid corrosion is the dominant failure mode. While corrosion rates in valve-regulated lead-acid (VRLA) batteries have been assumed to be the same as in flooded designs, the float life of these systems is often less than predicted. Careful analysis of the material balances within a battery show that the corrosion current at the positive plate, which uses oxygen, must be balanced by hydrogen evolution at the negative plate. Therefore, the result of corrosion is the loss of water, which in a VRLA battery is irreplaceable. The corrosion rate can be determined from calculations based on this mass balance. The float current and gas evolution rates are measured and the gas composition is determined by a gas chromatographic method. Measurements on VRLA batteries of both the gelled and absorptive glass mat (AGM) types show that the corrosion current decreases with time, The activation energy for the corrosion process is also higher than predicted based on measurements in wet lead-acid batteries. Based on these results, a method of using high temperature tests to predict life at room temperature is proposed.