Found this Memory Saver gadget.

Seems to offer the opportunity to save a lot of time when doing electrical work.

They Quote :-

"It is always safer to disconnect your battery when working on major components. But today's audio equipment needs a constant current to maintain your radio station presets and other settings--even your radio security system, car alarm memory and clock. Here is a simple solution. Our memory saver plugs into your lighter socket and has enough power to retain these important settings so that you do not have to reprogram all of your components. 9V Battery included. Comes with optional adapter for AA batteries. "

Might also work on some models that loose control settings when battery off for some time.

Memory Saver Link

Mike Stone