The protection circuit is a known source of failure.
+1. I
used to think protected was better, because: no harm in having a protection circuit as long as it doesn't trip, right? (and certainly no harm if it
does trip!
). But as I learned more about Li-ion cells use & safety, protected cells are more of a gray area. Some pro's, some con's, even in terms of safety. Remember protection pcb's are produced cheaply. There's few components (= potential failure points) on there. But its construction or components
may be poor quality.
For protected cells, protection pcb can fail at any random time just sitting there because the protection pcb is ALWAYS powered. For naked cells, that possibility
does not apply. So in storage (>95% of the time in my case), cells are
by definition safer without a protection pcb. Of course during (dis)charge it's a different story.
If I manage to damage a wrapper, cell casing may contact flashlight's inner tube wall. In lights I have, that comes down to bypassing the tail switch, so that light goes on regardless of switch state. Not something I want, but not really bad either.
Protected cells I have, have protection pcb at the bottom (-) side. So a connection strip runs on the side that connects to + pole (top). If I were to damage that cell's wrapper near the strip, you have the chance of a short circuit between + and -. That
would be really bad. And if it were to happen with cell inside a light (say, when light is dropped), I wouldn't immediately see that damage occurring. Again this is a failure mode that
does not apply to naked cells.
Protected cells have their uses. For example it would protect against failure of a light's electronics. Perhaps a better question is whether you
need protected cells:
* Running cells in series, in a light that doesn't cut of when
individual cell's voltage gets too low?
* Using cells in lights where you have really low confidence in the light's electronics? (for example: cheapo light with poorly cooled LED or driver)
* Only button tops fit & the ones you can find are protected cells?
In those cases: go with protected cells. Otherwise: not much point for protection pcb's (if any), naked cells are fine.
