PURPOSE: To easily judge the charging period of a storage battery by calculating how many times the storage battery can be used more from its rest capacity and displaying the result.

CONSTITUTION: A rest capacity arithmetic circuit 4 receives a voltage, developed when a charging current charged in the storage battery 2 with a charging operation switch SWC closed flows to a resistance R1 for charging current measurement, as data on the quantity of charging electricity through an A/D converter 5. Then a voltage developed when the discharging current from the storage battery 2 flows to a resistance R2 for discharging current measurement is received as data on the quantity of discharging electricity and those data on the quantities of charging electricity and discharging electricity are processed according to a previously stored program to detect the rest capacity of the storage battery 2. Consequently, the charging period of the storage battery can easily be judged.