Well you can pretty much forget using a car as a way to recharge batteries. I always figured it would be less than ideal. This week I was away from home and had the alternator quit on my car. While I was changing the low battery with my wife's car while I worked on the alternator change. The healthy sized group 48 size battery, roughly around 60ah was at 11.8v, it bulk charged for about 1.5 hours and absorb charged for at least a half hour and still wasn't all the way fully charged. The other car was only able to produce 25 amps with everything off at idle. Roughly 2 hours of charging used most of a gallon of gas and only produced around a half kwh. So relying on a car for battery charging looks like at best that its a bad plan.