I use the batteries from old UPS's, uninteruptable power supplies. I only charge them 2 times a year, using a motorcycle trickle charger overnight.
I use my old "D" cells from my Maglight flashlight to run the glo plug. If I don't let them freeze in the winter, I can get 2 years from them before I throw them out. Just put them in parallel, 3 or 4 old batteries will do the trick.