DMAN

Another way to look at the problem might be counting the revollutions of the disc. You could set the CD player to play one song on repeate and find a way to count the revolutions on the disc with some sort of counter circuit. You might be able to use the sensors from a old floppy drive, or even tap into the sensors on the cd player itself. The experiment could then be the number of revolutions you could get out of different battery makes and types. cheers