Hmmm, my ADT system takes a single CR123 at each sensor, so an 18650 won't fit. A 16340 *will* fit, but I doubt very much those sensors can take 4.2V, and I'd rather not fry them. So far, the CR123s are lasting soooooo insanely long in each sensor, that I'm not sweating it at all -- in fact, I'm pretty sure it's *much* cheaper to run CR123s than 16340s (not to mention zillions of times easier, given no battery changes in something like 8 years), since I might go through multiple 16340s just based on time-based degradation alone.
Yes, they're single cell in the 4 sensors, but I have a smoke alarm, flame alarm (kitchen) and a motion detector which all take 2 cells, I believe.
My point is is while the cells do last long, I've had sensors go bad eating 2-3 in 3 months, before I figured out what was up and called the guy. Also, they need to be a solid ~3.2v otherwise they'll fault, or even false alarm. 3.0v is a knock on the door by the Po-Po, if you're away.
I know an 18650 won't fit and I would never try the rechargeable 123s in them. I was commenting on whether I should be using the 18650s in the 2 lights I currently have and saving the primaries for the alarm system and my P1D. I'm not made of money and I try and be frugal and look at things pragmatically.
Plus, if you think about it, there are a lot of batteries we need to keep on hand and I don't mean at 'hoarding' levels either. Things add up and I'd love to be able to go out and drop some coin on a 'gross' of Duracell Ultra 123s, but I can't afford to do that.
I haven't priced out a pair of AW 18650s yet, but I can get a Pila IBC for $57 shipped. Basically, that's 28 nice 123s and that's where the 'cost/benefit analysis' somes into play for me. I doubt I'll be cycling the AWs all that often, so lifespan shouldn't be a major concern, I wouldn't think?
I know it's somewhat 'anal', but hey...it's an anal hobby!
Chris