As for the possible shorted battery draining the (still) good one, couldn't you just put two diodes in-line so that it doesn't do that. Granted, you would have to disconnect the battery to charge it. OR,,, it just came to me, how about if you simply made a single "Y" connector wire setup that has a diode in each line between the "Y" and the battery and then, put another termial on each "Y" section between the diode and the battery. Those would be your charging jacks and the single line would then connect into your receiver, switch line or whatever. Maybe I'm thinking too much.