LED's for dummies question

Okay, I'm tired of using search and not finding the answer to my question. I understand that Li rechargeable batteries are apparently particularly vulnerable to overcharge/over discharge and a few other quirks. I further understand that protected batteries do a good job at mitigating most of these risks. My question is what are the common denominators that apparently make some lights more susceptible to have a catastrophic type failure if theses occur....i.e. why is ABC light fine with unprotected rechargeables but "don't dare use" anything but protected in XYZ light. I figure there's a sticky on all this, but I couldn't find it. No problem, I've got a multitester and all my current rechargeables are the protected variety.

I am not sure exactly what you are looking for. Also, it is unlikely there would be a catastrophic failure with Protected cells. With out protected, yes there sure can be more easily. Most importantly stay away from cheap or china knock cells. Buy quality cells from a reputable dealer here on CPF.

Some of the higher output lights have such a high current draw they can trip the cells protection circuit. Also, there can be a length difference. Protected cells are longer and can make some lights not screw down properly. This could affect contacts and water tight seals. They can also get "crushed" by some lights and you can damage some lights trying to cram them in.
What lights & cells are you considering, if any?

Follow up question to the above: are you planning on using single cell or multicell lights?

Protected/unprotected isn't the issue, it's quality/cheap, or possibly attentive/inattentive; the most common kind of battery incidents are cheap primary 123 cells that randomly go off, so don't go cheap when you go primary. On the Li-ion side, most events are batteries not checked before charging and/or left on the charger. Use of a multimeter with Li-ions is not optional, and please be sure you remove batteries promptly after charging, set a timer reminder if necessary.

Some lights are made where a wide voltage range can be ok. Others require a specific narrow voltage range or their whimpy parts burn out.

Example would be a fully charged (non rested) cell of say 4.2 volts that runs down to say 2.8 volts would be ok with liights built for wide voltage ranges. The 4.2 may murder some whimpy lights and 2.8 would cause a slow burnout of a whimpy light.

Some have built in voltage protection so the unprotected cell will trip a circuit breaker built into the light at the high or low voltage limits.

Some lights like those high flow mega powerful ones will trip circuit protections in some batteries. The high flow unprotected would be best in that case.

On a related note, are LiFePo4's worth the extra cost to avoid all that above?