Re: batteries + flashlights = corrosion
As a mechanic by trade I have run into this issue with Fenix as well as tons of other name brand flashlights, electronic torque wrenches lighted inspection mirrors etc....
what I have found that works is put a dab of dialectric on both ends of batterys when new, and when you are done with tool (flashlight) unscrew the "waterproof" cap a couple of threads. I have found that this both allows gas to escape and breaks the contact with the battery so no battery drain or unintentionally turning on flashlight. I also store electronic tooling in cool spot in shop at home and work.
Hope this helps
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