UNINTERRUPTIVE POWER SUPPLY

PROBLEM TO BE SOLVED: To provide a small-sized uninterruptive power supply at a low cost by restraining the increase of battery capacity in the case when a load is increased and the increase of battery capacity when backup time is extended. SOLUTION: In a battery for backup which is driven in the case of service interruption, an additional battery 12 is connected with 1-N pieces of main battery 7, the output of a power supply unit 4 is supplied to a charging circuit 6 at the time of power feeding, and charging is performed by connecting the batteries 7 and 12 in series through a charging circuit 6. A change-over switch 11 is installed in a power supply circuit constituted of the batteries 7 and 12 in the case of service interruption. An SW 1 of the change-over switch 11 is turned on by a switching signal of a control circuit 10 which is operated by a detection signal of a service interruption detecting signal circuit 3, and power is supplied from the batteries. An output voltage of the battery 7 is monitored by the control circuit 10. When the voltage becomes a set voltage, an SW 2 is turned on by a switching signal of the control circuit 10, and an output of the additional battery 12 is added. Thereby the backup time due to batteries is extended.