Those Powerfilm's are expensive once you get up into the higher wattage outputs...

Thinking about this more, if the OP is looking for something to use in case the car battery discharges, he is better off looking for a battery jump pack. These are reasonable in price and like the jump pack like I have, it is multi-functional as it has 12 volt accessory power and USB outlets. The caveat is that they are too big for a backpack if one were to head out on foot, but these jump packs really serves a better overall purpose not only in a vehicle but also at home.