I would recommend re-chargeable nickel-metal hydride batteries. Buy extra batteries so you can take freshly charged one to replace the run-down ones. They last about 2-4 months depending on the temperature.

I use them in my walkie-talkies, GPS, and Stealth cameras. I purchased 16 batteries & one charger for $90.00 at Radio Shack. It takes 3-4 hours to recahrge 4 of them.