When it comes to batteries bigger is better. I searched for specifications on the Micro-Start XP 1. Here is a rough assessment of the XP1 battery vs an automotive battery. The XP 1 battery has a capacity of 44 Whr. An automotive battery is usually more than 50AHr capacity. At 12 volts this is 600Whr or more than 13 times the capacity of the XP 1 battery. I don't think the XP1 battery has enough energy to start a car with a dead battery. Has anyone actually done this? I think I would save my money.