Hello,
If you want to devlop a battery backed system on own ,it can be done very easily through two diodes D1 and D2.
The diodes are connected as follows:
main supply ------- anode of diode D1------cathode of D1 to the device

Battery -----anode of diode D2-----cathode of D2 to the device(this cahode has to be placed after the cathode of D1).

The working is simple,
When your device is on normal supply ,the diode D1 is forward biased and conducts.
When the supply is Off ,the diode D2 gets forward biased and conducts ,diode D1 wont conduct now.

Dont forget to look at the current ratings of diode

Pravin