Back Up Battery Power Supply

I had done a 12V DC power supply. but as an additional, i wan to apply battery back-up for power failure.What can i do to add a back-up battery for the power supply??

im not too good with this but ill try to help. Not knowing your Chip doesnt help but..

i would probably do a A/D conversion to check power or possibly have a transistor switch power on a power outage type situation.

Not sure if any of that helps as i was trying to find this out myself .. i would love to know also. The above is theory.