Quote Originally Posted by iAmKrzys View Post
Most SteriPen models require wider opening. SteriPen has one model that will fit narrow-mouth bottles but I think its batteries cannot be replaced (though I may be wrong on this.)
It's the opposite. The cheapest Steripen models, also the heaviest, that takes 4 AA's (disposable or rechargeable), can fit the narrowest opening which is a standard soda bottle size, perhaps smaller. The next category that takes a unusual battery size (CR-123 - disposable, or CR-123R rechargable) is smaller and lighter and can take a gatoraide bottle cap or larger. All the above is replaceable batteries.

The most expensive but lightest option is the non-user replaceable battery model (Freedom), which recharges via USB and needs a cap larger then a gatoraide bottle. Finding a bottle that complements the lowest weight (so keeps it the lowest in terms of a total water purification system including water weight), and leak proof but large enough to fit this Freedom model is a challenge.