I am looking to replace the cell in a flashlight, which was supplied with a protected, flat top 18650 cell (now no longer working).
The flashlight has springs at both ends. The positive, in the tail, has concentric springs, a smaller one to provide an electric connection, and a bigger one whose purpose it to keep the cell in place, I assume? (sorry for the slightly out of focus picture!)
Usually, batteries don’t just ‘stop working’, so I’m wondering whether the protection circuit was triggered? If that is the case, you can reset it with the right kind of charger.
The flashlight is an inexpensive model that is kept around the house, I cannot say for sure that it wasn't dropped one time too many; after I picked it up and found it would no longer turn on, I placed the cell into my Zanflare C4, where it would read ~0.9V and only charge up to about 1V.
My DMM agreed, AFAIK Li Ion cells that read that low, and don't respond to charging, should be disposed of, so that's what I did.
The cell looked non-descript, with a plain blue wrapper and some manufacturer codes printed by a dot matrix. I looked up the codes but no luck.
Hence, I assumed it was a low quality product, which gave up its ghost one way or another.
