im going on a loooong snowboarding road trip with my church and we are going on a charter bus, and i wont have access to a plug outlet or a cigarette lighter spot and i dont have a laptop, but i dont think my iphone will be able to stay charged long enough so i was looking for an alternative method, like some device that you put batteries in to and it takes the energy from the batteries and charges my phone with it, is there anything like that, that i will be able to buy at like a walmart or target or something?