I have I micro start xp-1 I carry around with me, and I know of several ppl that carry it around (got it as a safety gift from R.R. division). I have not started a car with it, but have heard of a few ppl that I work with that has started there vehicles with it, I have used it on my garden tractor once. I doubt it will start a dead battery, but weak battery should be fine, better then nothing.