Battery Backup for radio presets

Is there a way to put a battery backup for just my radio? I'ld like to be able to prevent my radio station presets from getting deleted when either the trucks main battery dies or is disconnected. I want something which I can hookup and leave it. Basically this "backup battery" would ONLY provide backup power to the radio when the trucks battery becomes too low or is disconnected. When the trucks battery is connected and at proper voltage then this backup battery would be "turned off" so its not constantly being drained OR charged by the trucks battery. Anyone have a schematic for a circuit like this?

Thanks,

Scott

of course it can be done, you would probably want to avoid relays because of the high power wasted. Here is a little circuit that should work as long as the draw from the headunit is less that 600mA when in backup mode (if its that high you have a problem) I didnt really want to put a diode inline like that but it was the simpiest design i could come up with, give it a try, pretty much any PNP transistor will work so you could use a common type like a 2n3906, all the parts can be bought at the local radio shack...

i was thinking.. i think this would be a much better circuit, added a relay so diode doesnt waste power when vehicle is on...

Thanks for the circuits. This looks exactly what I want (my aftermarket radio does not save the presets when power is removed). There is one more thing i've got to do. In my last truck I had an aftermarket radio (different truck and radio). I noticed that I could go from the key being in the run postion (ie truck running and music playing) to the fully back (acc position) quick enough and not have the radio turn off. This truck I can't do that. I'm guessing perhaps I could put a capasitor (not sure of the size) on the 12v switched wire so that it would give me a split second to go from "run" to "acc" before the radio looses power. Is a cap all I need for this? Any ideas what type and size? Thanks again for all the help so far.