Eneloops are the absolutely best rechargeable AA's and AAA's I've used so far. They take up to 6 hours to recharge, but can be recharged 2000 times. They also can keep up to 70% of their charge after 10 years of storage.
And yep, I have salvaged a lifetime supply of laptop 18650's. I use new and much safer 18650's for the ones I carry on my person daily. Safety overkill? Probably. But $30 for 6x Samsung's safer chemistry batteries will last me a couple years. Maybe longer now that I found new shrinkwrap for my batteries. Found out my makita batteries are 6x 18650's as well. So, another handy source. Work goes through Dewalt batteries all the time, so... probably could source a hundred per year between them and the work laptops.
Salvaged 18650s may or may not have an integrated protection circuit. Heck, some cheapo no-names brands of new 18650 batteries advertised as having integrated protection are, at best, very questionable. Hawk, stick with standard CR123 cells unless you are intimately familiar with lithium cell technology, 18650 cell systems in particular. Lithium fires are bad, m,kay...
Mmm. Depends on the chemistry of the battery. ICR, yes, you want a protection circuit. IMR, because of the chemistry, it discharges much low temp, so it's very difficult to get it to ignite. Laptop batteries will be ICR for the moment, because cheaper. And the protection circuit is outside of the battery.
INR is somewhere inbetween, and that's what I tend to use. Samsung 25R's also have their proprietary polymer additive which allegedly helps too. I concur that it's worth buying new rather than using salvage if you're keeping it on your person. They're so cheap, why not? If not kept on person, I wouldn't be overly worried about using ICR 18650's in flashlights. Especially if you pulled the battery between use.
I definitely would not tell someone to ignore 18650's and stick with CR123a's. Just tell them not to pinch pennies and buy a good quality better instead of using used batteries. "Buy an IMR or INR battery with a name branch like Panasonic, Sanyo, LG or Samsung." is more than enough knowledge to be safe.
https://batterybro.com/blogs/18650-wholesale-battery-reviews/18880255-battery-chemistry-finally-explainedMy source for 18650's:
https://rtdelectronix.com/