First, let's figure out what a protected battery is. A protected battery is a Li-ion battery with a protection board. The protection board on the 18650 battery protects against overcharge over 4.2 V, discharge below 2, 75 and short circuit. The board is welded to the battery terminals with two metal strips. Like any electronic device, board, sometimes fails. Then, you can remove the board and get an unprotected Li-ion battery.
So, let's proceed to removing the board.
Remove the protective film.
Under the film, a board and a metal strip coming from it are visible.
The strip is spot welded to the contacts. It is necessary to tear off the strip and cut off part of the upper shell. By the way, the operation is reversible and you can make a protected one from an unprotected battery.
Or the whole shell.
Then, you just need to cover the battery with any dielectric film and that’s all, the battery works again (if the failure was in the board of course).
Now let's go back a bit. As you know, such batteries do not like overheating when soldering. But spot welding is not for everyone. In this case, when replacing the batteries with batteries, you can purchase protected batteries and remove the board leaving a metal strip. And to install the battery, soldering not to the battery contacts, but to a metal strip. Of course it will be more expensive, but there will be no risk of overheating the battery when soldering.
Prepared from materials from several sites.