Power supply apparatus

The power supply apparatus is directed to enable a battery to be properly controlled for charging/discharging. A detection unit (21) detects battery voltage V, battery current I and battery ambient temperature T. A battery capacity calculation device (23) and a discharging rate calculation unit (25) estimate battery capacity and discharging rate respectively. A discharge-allowable-duration calculation unit (26) calculates discharge-allowable-duration. Thisdischarge-allowable-duration is used for restriction the discharge-duration of the battery (6). The battery (6) is chargedwith the output of the generator. Charging current is restricted in accordance with upper-limit-charging-current that is calculated in an upper-limit-charging-current calculation unit (27).