That is exactly the reason I decided to do this article, now anybody can give a good answer with just one link.
I tried to keep it simple, but I did get technical in the last part. I wanted to include some information about what unprotected cells was (I did have to open a few old cheap cells to find one with a PTC).
Great info. Very informative. Was just wondering if my batteries were protected or not, and Robo referred me to the thread. Turns out my batteries are protected.
It will make battery longer but the diameter will be almost the same or just slightly larger (~0.5mm or so) because when you add protection you have to use one more layer of wrapping foil because there is an additional “flat” wire that goes from protection PCB to opposite pole and it needs to be isolated.
Usual yes, due to the strip fro plus down to the protection board at the minus end.
Some just add a new layer of wrap, other removes the old wrap or buy unwrapped batteries to start with.
The length of a 18650 cells can be up to about 72mm, but usual is below 70mm
Accepting 19mm diameter and from 65 to 72mm in length will handle as good as all sizes 18650. There is also the issue about flat vs. button top, the extra length due to that is included in the 70/72mm, but remember that on a flattop the isolation can be a few tenths mm above the plus pole.
Want to see if its protected? Short it with 10 gauge or similar cable if it gets really hot or blows up its probably not protected. If it does nothing it probably has a protection circuit.