The present invention improves the performance of each lead battery, and simultaneously to establish overall balance, by inspecting the degradation state of each lead battery and performing the most suitable treatment while maintaining a state in which lead batteries are energized. This performance improvement method is for stationary lead batteries configuring a battery assembly which connects the terminals of the multiple lead batteries by means of a conducting plate, and involves a step for performing checks for inspection operations of the lead batteries, a step for measuring the internal electrical conductivity of each lead battery with the lead batteries still being in a normal use state, and a step for adding an electrolyte and purified water according to the state of each of the lead battery.