I did 2x16340 and 2x18350 runtimes in this review of the M2X-UT - and 2x18650 wouldn't look any different, except for the extended runtime.Could you post the M2X-UT runtime using the OEM battery extender (2x18650)? I have purchased 4 M2X-UT for my friends all with the battery extender. Just curious if the runtime curve is comparable to M3X-UT.
I know it's a bit hard to see in the graphs in this review, but output of the 1x and 2x 3.7V Li-ion sources is exactly the same: the light starts out ~950 estimated ANSI FL-1 lumens, starts a controlled (timed) drop in output starting 3 mins, leveling off at ~650 lumens after ~5.5 mins. It then stays regulated at this new lower level. The only different 2x18650 will make is extended runtime at this lower regulated level.
It's a different circuit form the M3XS-UT, but the M2X-UT handles all voltage sources exactly the same way.