How many affordable lithium polymer batteries would it take to safely jump-start a car?
Just an add on to the already excellent comments here, If you are going to use your cigarette lighter plug to "jump start" a vehicle with a low battery, Do not expect to plug your jumper pack in and turn the key. The wiring for the lighter plug is not designed to carry that many amps. The answer to this is to plug the starter into the lighter socket, then wait about 15 minutes for the battery to "charge" your car battery, Then UNPLUG your jumper battery and then attempt to start the vehicle.