I didn't know that these batteries had a flat voltage output. I had assumed they fluctuated like a AA battery and would drop. I don't know, maybe that's when they drop below 50% charge.

I think I could figure out how to make the correct voltage from AC. I'd never heard of 240 volts being 339, but that could be it's peak without taking the RMS average? Originally I was talking to my brother (an electrician) about a 110 volt charger, upping the voltage through a transformer donated from one of the old tube radios that I work on. I figured I'd watch the voltages over some time (if I ever got to it), then essentially would build a trickle charger for overnight.

Anyway, I was just wondering if anyone had built anything that could just work with the batteries we already have. If it's cheap enough, works without damaging the car, and can add another few tenths of a mpg I'd be interested.