PURPOSE: To prevent a self-discharge during a battery is left unused and to improve battery efficiency by changing an electrolyte in a reaction chamber by means of a pump driven automatically when the temperature inside the reaction chamber rises beyond a specified value during the battery is left unused.

CONSTITUTION: When a temperature inside a reaction chamber rises with the progress of self-discharge, the temperature detected by a temperature sensor 52 is compared with a preliminarily determined higher reference value, e.g. 40°C, with the use of a hysteresis comparator 60. If the detected temperature is lower than the higher reference value, a pump 48 is kept being stopped. But, if the detected temperature is higher than the higher reference value, an 'on'-signal is outputted to a pump driver 53 and the pump 48 is driven. By the arrangement, an electrolyte is circulated between the reaction chamber 44 and an electrolyte storage chamber 46, and the electrolyte inside the reaction chamber 44 is exchanged.