Charging from a car battery is very doable, but it's has issues. I have charged off my Ford truck many times, but I have to leave the engine running. I drained the battery twice and couldn't get the truck started. I use jumper cables direct from the truck battery to my ICharger 306B. I think it would blow a fuse if I tried to plug into the 12 volt power outlet in the truck. I am able to get the 306B up to about 26 amps with the truck running, so I can charge almost as fast as I can at home. I don't know if a car alternator would be big enough to get the amps up. I don't charge at the field much, but it's nice when you need it.