Have no idea what is under your car hood battery wise, but if its around 40AH, it'll last a while. Charge a battery and then start the car. I don't think you'll have a prob. To even give you an WAG, we'd need to know what the auto battery specs are and what you're planning to charge, rates, how many batteries and how many times.

If the auto batt is in such bad shape that a single charge for an RC battery drains it and prevents you from starting, you need a new battery.