Great information, thanks.
One thing I've wondered about is the situation where both the flashlight and the battery have a low voltage cutoff. Many of my lights have that feature, and although I mostly have unprotected batteries, some have PCBs and are slightly longer (and sometimes don't fit in the lights). But I've sometimes wondered what happens then. For example, does one (presumably) kick in before the other, and then the other circuit just becomes a backup, etc.?
Well, lights are different with their voltage cutoff and warning settings. The ZebraLights traditionally have a 2.85v cutoff, which is above the minimums of 2.75v for the S. Korean cells and 2.50v for the Japanese cells. For other lights, I don't know? Most of these lights, even going back to my first li-ion--a ZL SC600 in 2012, have some sort of step down, where the highest mode won't fire off, so that's a clue that your battery is getting low.
I've read it posted here that some of the 18650 PCB have a cutoff as low as 2.40v, but that seems a bit low to me, thinking 2.8v-2.9v would be normal. I'm pretty sure that I've gone lower than I should have and tricked the PCB on ultra low modes, but I don't recall the details. Some have said that's impossible. Just be aware and get in the habit of checking things often.
I'm not a protected cell guy. I still have my first two li-ion--EagleTac 3100s, a couple of Redilast 17670s and a couple of AW 16340s and that's it. The AWs are ready for the recycle bin, the Redilast sit in a SF 6P collecting dust and the ETs have been in the fridge for the past 3-4 years, lol.
For me, I've just trained myself to often check my cells and if I notice that I can't access a higher mode, then I use it on the lower modes until I can get home and charge things up.
I've had little 16340s run down on me and as long as I'm charging them up that day, I'll be fine, minus a few cycles off the life, but no biggie.
I guess I'm saying that running naked cells, using good chargers, in lights that don't have a low voltage cutoff isn't the 'disaster in the making' that noobs should be cautious of. Being prudent is a good thing, but frankly, who knows about the quality of those cells and has anybody vetted them? Just keep on eye on things.
Small 'CR2 type' generic cells are in use by the millions of devices. Just go to any big restaurant and look at all of the 'buzzers' they have. Still, I try and buy quality cells where I can.
Chris