Look around and see if you can find a damaged battery back up (UPS) from a computer repair dude and beg for it. The batteries in the UPS are 6v or 12v, depending on the UPS. That is what I use. I have several of these battery sets I salvaged. You can make your own battery charger from a 12v wall supply. Basically, this is what you get from Cabelas or whoever sells the 12v sla and charger.