Another relatively cheap solution you can keep at home or in your glove compartment are these battery-powered chargers. The one shown here is the Rayovac Turbo Charge. It can be found for under $10, it takes two AA batteries, and it gives you a few charges. It even offers a little LED flashlight. I figure that even if the zombie apocalypse takes out the power grid, I could still loot batteries from the local drug store.