Ways to recharge a car battery without plugging the car in, or using a gas motor?

I am working on a project to build an electromagnetic car engine, and I need a way to recharge a car battery without plugging it in, or using gas. I will consider even the most unlikely theories. Thank you.|||Any number of renewable technologies that are presently being sought out - geothermal, wind, solar, tidal, etc......|||Solar panels !|||They sell solar battery chargers that you plug into your cigarette lighter, but since you're open to the offbeat, get either a potato or a lemon and a piece of nickle and copper, jamb the pieces of metal on either side of it and attach wires to your battery. It's not much, but it does produce electricity.