Disconnectubg battery

[markvt]

I'm curious to know what members have used to save memories when disconnecting/replacing their battery. Is there a commercial device that shops use? I've seen a $3 device that runs plugs into the cigarette lighter and runs on a D cell battery, but the ignition must be on for the lighter to work and it seems that the cars load of lights, clock etc. would drain the D cell in pretty short order.
Mark