A battery-capacity management device comprises a charged/discharged-capacity integrating unit that detects a voltage and a current of a driving battery and that calculates a discharged capacity of the driving battery. The device also comprises a battery temperature sensor that detects a temperature of the driving battery, a memory, and an available-capacity calculating unit. Map data that represents relationships among voltages, currents, and temperatures of the driving battery at states in which remaining capacity of the driving battery has reached a preset value can be stored in the memory. The available-capacity calculating unit calculates a renewed available capacity of the driving battery based on a discharge capacity discharged over a period between a time when the driving battery is charged and a time when the voltage value detected by the charged/discharged-capacity integrating unit reaches a voltage value corresponding to the current value and the battery temperature in the map data.