PURPOSE: To decide the lifetime of an operating battery power supply easily and appropriately.

CONSTITUTION: Voltages of a plurality of batteries 3-1-3-n constituting a battery power supply 3 being charged with output from a rectifier 2 are measured through a voltage measuring apparatus 4. An average value operating means 51 then operates the average voltage of the plurality of batteries thus measured and a differential voltage operating means 52 operates the differential voltage between the voltage of each battery and the average voltage. A comparing means 53 compares each differential voltage with a reference level and operates the number of batteries for which the differential voltage exceeded the reference level. A lifetime deciding means 54 makes a decision whether the number of batteries for which the differential voltage exceeded the reference level is higher than a preset value and then decides the lifetime of the battery power supply.