A fully-charged battery capacity detection method detects first and second no-load voltages (VOCV1, VOCV2) of a battery at first no-load timing and second no-load timing, respectively. Further, first and second remaining capacities (SOC1 [%], SOC2 [%]) of the battery are determined based on the first and second no-load voltages, respectively, when the first no-load voltage (VOCV1) falls within a predetermined voltage range. Moreover, a fully-charged capacity (Ahf) of the battery is calculated based on a remaining capacity variation rate (δS [%]) and a capacity variation value (δAh) of the battery. Also, the variation rate of the remaining capacity is calculated based on a difference between the first and second remaining capacities. The capacity variation value of the battery is calculated based on an integrated value of the charging current and discharge currents of the battery to be charged/discharged from the first no-load timing to the second no-load timing.