This 9V battery backup circuit will work as a mini UPS. The circuit will instantly convert to battery power if the input voltage is absent due to which the operated device will not face any restart. The LM7809 voltage regulator IC is used as 12V to 9V converter. A 12V lead acid battery of any Amp Hour
(AH) will work here. The circuit will provide output current upto 1A. Use a good quality automatic 12V battery charger circuit or use a ready made automatic 12V battery charger with the circuit.
Note: Do not use less than 3A battery charger with the circuit, it will not charge battery properly because around 2A current will be required by the LM7809 IC.