In the past I head read that Fenix 18650s were rewraps of Pansonic or Sanmsung cells.
I searched again today and can easily still find some sources online still stating this:
"The Fenix 18650 3400mAh (ARB-L2S) rechargeable batteries with electronics protection (protected) are based on Panasonic's top-of-the-line NCR18650B cells with improved LiNiCoO2 technology, which brings greater safety and longer life."
The Fenix 18650 3400mAh (ARB-L2S) rechargeable batteries with electronics protection (protected) are based on Panasonic's top-of-the-line NCR18650B cells with improved LiNiCoO2 technology, which brings greater safety and longer life.Li-ion…
www.outdoorweb.eu
Either way, I own over two dozens Fenix 18650s and only battery, after quite a bit of use, died. For the rest, they have all worked well and haven't had any problems with them.