NFARL – North Fulton Amateur Radio League has put together a kit and assembling instructions for a device called Power Gate. Unfortunately the kit is now sold out, but the assembling instructions with schematics and list of materials are still available.
The Power Gate allows you to run your station with full time, instantaneous power backup. A pair of Schottky diodes operate as an electronic gate to connect either of two power supplies to your rig. Usually one supply will be your normal AC connected station power supply and the other a backup battery. Switching is completely automatic in both directions; the source with the highest DC voltage will be connected automatically to your rig.