Uninterruptible power supply

A power-converting apparatus, capable of holding an output AC voltage at a constant level when the variation in voltage input from an AC power-supply source remains within a predetermined range, comprises: a separating means (21) for electrically separating the AC power-supply source (1) from the input terminals of said power-converting apparatus; an AC/AC converting circuit (3) for rectifying AC current from said AC power-supply source (1), applying the rectified current to a capacitor for smoothening and converted the resulting DC current back into AC current; a battery (6);and a DC/DC converting circuit (5) adapted to charge said capacitor with a DC voltage delivered from said battery and to charge said battery with a DC voltage delivered from said capacitor bilaterally or unilaterally. Start controlling means are provided for controlling a starting of the operation of the power-converting apparatus. The start controlling means cause the capacitor (4) to be charged with a DC voltage supplied from the battery (6) via said DC/DC converting circuit (5) up to a voltage level substantially above the peak value of said AC power-supply source while said separating means (21) remains in an condition separating the AC power-supply source (1) from the input terminals of said power-converting apparatus. Subsequently, the separating means (21) is caused to connect the AC power-supply source (1) to the input terminals of said power-converting apparatus.