I think I just killed one of these with my Olight S20-L2. I had been using it on its first charge for so long I had forgotten than the S20's button LED was supposed to turn red to signal that the battery was close to empty, so I kept using it even after seeing the red light. What finally drew my attention was the fact that the flashlight ended up refusing to go into any of the higher modes beyond Moonlight and Low. At that point I figured it was time to charge, so I stuck the battery in my Chinese no-name single-unit 3.7 18650 charger, only to see the charger's LED stay green continuously, as if there was no need to charge the battery. Then I thought maybe it wasn't discharged enough
so I stuck it back in the flashlight and ran it down until the light turned completely off. Back to the charger, same story: green light, no charging whatsoever.
Then I came to the forum and found out about the protection circuit and how it needed to be reset etc., so I measured the open-circuit voltage (indeed 0 at this point) and then I connected the battery in parallel to a USB port's VCC and GND pins and to a small resistor I had around the house for a few seconds. Sure enough the protection circuit reset and the next voltage I read across the battery was approximately 3V, so it seemed all set for a recharge. Except it wasn't: even now if I stick it in the charger the LED will still stay green and there will be no charging going on no matter how long I leave it in there.
I'm looking at 2 possibilities now:
- The battery is fine but I need a better charger
- The battery is really damaged and will no longer charge with any charger, which means that Fenix' protection circuit sucks (didn't kick in when it should have).
Since the last (Ultrafire) 18650 I had went through multiple cycles with this charger and didn't have any problems, I have to assume the second variant is true and I'll just have to order a new (and different) battery.