Li-on 18650 charged that don´t work

[BIS]

I charge a Li-on 18650 battery with a trustfire charger, and the battery don´t work in the flasligth, I mesure 3,9 v. I use a torch ultrafire wf-016 ok

 

[TooManyGizmos]

Light must have a polarity protection ring.

Sounds like it's not making connection up inside the light at the head.

You may have to get another battery that has the positive button sticking out further.

I assume the batt. works in other lights ? just not this one .
.