The tailcap popping off the Fenix lights is a design flaw. NiMH batteries occasionally vent hydrogen, but this doesn't always mean the battery is bad. Waterproof dive flashlights such as Princeton Tech, Streamlight, Pelican, and many other brands put platinum catalyst pellets in the lights to absorb the hydrogen so they don't build up pressure and explode. Fenix lights don't have these catalyst pellets, so they build up pressure because of the hydrogen can't escape. The rubber tailcap boot launching off does act like a pressure release to get rid of the hydrogen. The alternative is worse. Imagine leaving an L2D CE on turbo for a couple hours using NiMH batteries. The hydrogen would build up in the light and the heat built up in the body might ignite the hydrogen once it blasts its way out of the light and reacts with oxygen in the air. At least with the tailcap boot coming off, your whole light doesn't explode. Now we know why Fenix gives a spare tailcap boot with each light. If you're worried about the problem, you have four options. You can bring a spare boot or tailcap with you. You can bring a spare body and tailcap with you like the one that comes in the Christmas power pack. Think of it as a spare battery holder. You can open the light every once in a while to let out the gas before the problem gets too bad. The last option is to glue or epoxy a catalyst pellet inside the tailcap.
