This circuit will provide electric power from a backup battery when AC power is cut, always providing a clean 5V power supply for sensitive equipment like microcontrollers and logic circuits.

The value of C1 is to be higher than 220 uF. the bigger the capacitance of this capacitor, the more power you can deliver to your circuits. The 9.6 NiMh Battery can be replaced with any equivalent battery whose voltage is more than 7.5V.

A charger can be constantly connected to the battery from the same 220V source as the rest of the circuit, but depending on the battery and charger type, this procedure can shorten its life.

Source: http://powersupplycircuit.blogspot.com/