Bingley... currently I use the 18650 only in a P6 size high intensity LED light, a Solar Force L2M...the M series is modular, and the tube body unscrews... the 18650 is 18mm x 65mm so is larger diameter than a AA, and comes off my charger at about 4.2v... they take a dedicated charger for the Li-ion chemistry.... basically an 18650 is a replacement for 2 x 123s which are rather expensive... there are several chemistries (denoted by the prefix INR, IMR etc) on the battery label... some of the chemistries are "safer" than others and I would recommend you do research on that specifically, and there are protected and unprotected cells available, that limit the amount of discharge to around 2.5v...

my particular light has a low voltage LED module (XP-G) specifically for 18650s ... (.8 to 4.2v with a low voltage shut off circuit)and works very well.. with the short tube extension from Solar Force that typically allows a 3rd 123, I can run this low power module with 2 x AA, although the AAs rattle around a little due to their smaller diameter...the 18650 did work with a higher rated XM-L2 module

I'm sure more seasoned users will provide you with better information