The 18650 is a single rechargeable lithium battery measuring 18mm diameter x 65.0mm length. The CR123A battery is a actually a combination of two naming conventions: It's a CR1 lithium battery that is 2/3 A size. An A battery is slightly longer and thicker than a AA battery , much like a AA battery is slightly longer and thicker than a AAA battery. CR123A primary batteries are 16mm diameter x 34mm length or 17mm diameter x 34.5mm length. So, a pair of CR123A batteries is 16 to 17 mm diameter with a combined length of 68 to 69 mm. That's the first part.
Only recently I heard about the 18650's. Would you recommend them? I heard that not all CR123's lights are compatible? How would I know?