It takes 2 CR123 batteries. The trick to it is only put your batteries in it when you use it and it will go a lot longer. I've not pushed mine on that, I would keep a set charging and change them out every 2 days. I drink a and use lot of water. I do keep 4 extra batteries on me when I carry it, but I haven't had to go to a second set when traveling because I am constantly charging. Kind of like my Droid phone, I always keep charging all the time in case I don't have the opportunity to charge. The specs say my rechargeables can do 40 to 50 a charge, which means about 640 oz or 5 gallons on the low side. I was doing about 2 gallons a day for myself for 2 days and would change them out to always keep a full charge.
update:
I just thought of something, I may be wrong about the 10 gallons, they are saying 40 to 50 treatments, I use the 32 oz treatment which I believe is a double dose, in that case it would be 20 treatments =640 oz or 5 gallons which sounds more right. A 16 oz treatment would get 40 treatments=640 oz again at 5 gallons.That sounds more right.
Battery: Uses two CR123 batteries (included)
Battery life: Disposable -- 100 treatments (16oz./0.5L); Rechargeable -- 40-50 treatments (16oz./0.5L)
UV Lamp Life: 8,000 treatments
http://www.steripen.com/adventurer-opti