Originally Posted by AreCee
Let's see. The typical full size car battery has a cranking amp capaity of 400+.
A 4S lipo has 5 amps. It may last a couple of seconds but won't turn over the engine too much.
You're confusing current [instantaneous amperage] with capacity [amp hours].
Most Lead Acid car batteries are rated by their Cranking Amps [close to constant C ratings for LiPo] and CCA, Cold Cranking Amps [Burst C ratings for LiPo].
So yes, a 5000mAh LiPo [as low as a 20c constant rated battery will work on a passenger car, may want more for a larger engine/starter] will jump start a car, seen it happen, and as others in this thread have mentioned, it's better to use a 4s to protect the LiPo than a 3s.