Over the operational life of a battery, the voltage characteristics of its individual cells become mismatched due to differences in self-discharge rates, individual charge efficiency, active material retention, etc. As these cell differences increase, excessive overcharging, operating temperatures, and/or electrolyte consumption result. In a series of tests performed on 6-volt, lead-acid modules, the degree of cell mismatch and the response of the modules to various charge methods were examined. The results show that after about 150 deep-discharge cycles, each module contained at least one cell whose full-charge voltage level was reduced. Prior to any decline in module capacity, cell voltage differences of more than 0.4 V were measured during charge. It was also observed that the application of a fixed battery voltage for charge control eventually caused excessive overcharging, elevated temperatures, and/or an extreme overvoltage condition on individual cells. This paper describes the test procedures employed, presents the test data, discusses the results obtained and solutions being investigated.