Hey, don't need to feel bad despite the admonitions given, as VERY FEW people (including flashaholics) are aware of proper use and care of any type of battery chemistry. Many people buy extra battery pack(s)/cells because of anticipated obsolescence of custom made packs, or because at the time of purchase they anticipate longer practical single event usage time than what it turned out to be.
I can guarantee that every flashaholic has more Lithium Ion batteries than they need, and very few (if any who are being honest) take the time to drain them down to even 40-50%, let alone the ideal 30%, verifying with a DMM, keeping a light on with repeated DMM measurements just to drain to ideal level for long term storage. Then even fewer take the time to mark storage dates or when they put them in a freezer/refrigerator because when you need your light—having drained, frozen batteries just goes against human nature!
One easy habit is to mark the date of purchase with a sharpie when buying them. Li-Ion cells will still last a very long time even if not optimally cared for—just not as robustly as when new. I have some 25-30 year old Pila Li-Ion cells that still give a surprising performance in my custom halogen mag-lights. These old cells never even started with the current mAh storage capacities available as technology/improvements has changed. There's many sides of this issue. The important thing is to keep reading and learn as we all are doing. The battery section of this CPF Forum is awesome.
For Sale - Flashlight SureFire and some others
