i was thinking about this too, i was actually just gonna get 2 extra batts and keep em charged at all times, thats 12 hrs for 2 sets, so thats anywhere from 8-12 acts
i personally would just buy an extra set, and charge them overnite w/ a gas p[owered compressor/charger/thing(thats what my buddies had at IT, i just didnt need to use it....
how long does it take the jb3 to fully charge the jb3 w/ 2 batts
i didnt note how long it took mine
or if they charge fast, just get a simple power inverter for the car, and charge them every chance you get
save them for the latenite sets, everyone tapes the others