I once had similar problems with my Steripen Adventurer. I used a set of off-brand CR123's on a short trip in the Rincon Mountains here in Arizona. They quit on the third liter of water I tried to treat.

Then, after reading about problems with the Adventurer on a forum, I started using Panasonic lithium-ion CR123's in it. Other than one problem arising from my not reading the directions, the Steripen has worked great since.

I used it on a through-hike of the JMT last year and only replaced the batteries, a scheduled replacement, once at Muir Trail Ranch. Even then, the batteries I had used from Yosemite were still working. I do carry Micropur tablets as backup though; enough to treat 3 liters per day for the length of the trip. They are really light.

It would be nice, however, if Steripen were to warn their customers about this problem.