Patent ID: 6977448

Claim:
A backup power supply built in a device constituted by a power supply circuit for converting an alternating current (AC) received from a commercial AC power supply to a direct current (DC), and a load operated by said direct current (DC) generated by said power supply circuit, the backup power supply comprising: at least one AC-DC converter connectable to said commercial alternating current (AC); a load connected to a DC output side of said AC-DC converter; at least one two-way DC—DC converter in which one side of said two-way DC—DC converter is connected to said DC output side; a Ni-MH secondary battery or a Li ion secondary battery connected to another side of said two-way DC—DC converter; detecting means for detecting a service interruption of the AC power supply or a failure of said AC-DC converter; means operable during occurrence of a detected service interruption or failure, for discharging electric power from said secondary battery, to supply said load; charge state determining means for determining a state of charge of said secondary battery; battery current detecting means for detecting battery current of said secondary battery; and control means for altering and controlling a charging current which flows to said secondary battery, according to the state of charge determined by the charge state determining means, based on the battery current detected by said battery current detecting means.