My thoughts on battery protection...
They should ALWAYS be used if you are questioning its purpose.
the fact is, if you don't know 100% why it is there, you fall into the category of people that need it.
They should ALWAYS be used if you are stacking and are not properly educated in battery chemistry.
And NO these forums don't even come close to good sources of research.
I have seen way too many examples of people passing off a random post on these forums as facts when it comes to the batteries we use, and it scares the crap out of me.
We as PV users use lithium batteries, but are part of the plug and play society.
whether it is in a mod with protected circuitry or a mechanical mod with a protected battery. The majority of us need it.
i would guess at 99.9%
My biggest problem, we as PV users are using the lowest of the low in lithium chargers. The ones that are recommended on this forum are IN MY OPINION dangerous.
They pass for single cell applications, but for stacking... i dont think so... You will never be able to perfectly match cells in these chargers, and if they claim that they do, there is absolutely no way of knowing.
I would rather take my Hobby grade charger than the wall plugs that pass for chargers any day.
Quality hobby grade chargers aren't even that expensive....
the protected cells that we use claim to have overcharge protection BECAUSE the chargers themselves are the danger.
over discharge protection for me is just a precaution in maintaining cell life. if you over discharge, the battery becomes useless, and should be disposed of in a proper manner. THEY WILL NOT EXPLODE! The danger lies when you try to charge a dead battery with a cheap charger.
My charger sets off alarms if it notices that i am trying to charge a cell that is bellow a safe voltage.
But i have managed to bring a lithium cell back to life, that was just bellow 2.3V with 0% loss in capacity. Obviously i dont recommend doing this.
Lithium batteries become dangerous if they are overcharged, or when they are pushed beyond their capable current. This would be most common in a stacked configuration. where one cell is trying to charge the other within the mod. there are many variables why this could happen.
Best way to avoid this is to Build a cell pack(permanently solder them together PROPERLY) and balance charge them every few cycles, so that both cells are are not fighting with each other and work as one. (simplest way i could put it).
Take this rant for what it is... I'm not a professional, nor do i have a degree in this field.
I have however, been using Lithium cells for almost a decade and have acquired my knowledge though those that are qualified to teach it and through a lot of trial and error.
We must educate ourselves properly before more people give out more bad information...
one last thing i just came across...
That little flat wire that is on the side of a protected battery that is under the shielding, is NOT a fuse/thermal protection.

It is just lead so that the circuit protector can monitor the voltage of the cell... nothing else!
[/rant]