Second battery that is paralleled with original while engine is running and isolated while engine is not running. Need to find 'SOMETHING' on the vehicle that has power supplied ONLY when engine is running. Use that power source to operate the isolation relay. You need to use a very low current relay to operate the isolation relay, which should be at least a 20 Amp rated 12 Vdc relay. The + terminal of the battery would be always connected to the cigarette lighter, and also connected, through the relay contacts, to the original car battery ONLY when the engine is running. The - terminal remains always connected directly to the chassis.
It would be best to have the second battery be comparable to, or even the same model as, the original car battery.
And to complicate things, I would add a switch to connect the second battery to the original, in case somebody left the lights on or whatever. 10 minutes of charging and 'crank 'er up'!
