This question does come up a lot... Basically the general rule is, the 3.7V cells will work in single cell LED lights but in 2x123 lights, it may kill the circuitry or the LED or both. The exception would be the Bug Out Gear module (or any other similar module/head) which is indended for use with either 2 or 3 cell bodies. Since it can use the 9V it will receive from 3 primary 123 cells, it should be able to handle the ~8.4V it will intially receive from the 2 3.7V R123s.
My current favorite 2x123 light is the Bug Out Gear 3W module (in your host body of choice) for exactly that reason.
Mmmh...
I could have sworn I replied in this thread. :thinking: :shrug:
Oh well anyways, the regular R123s should work and yes, by work, I do mean it will regulate.
Of course, to be safe, you could contact Pelican and ask about the buck circuit datasheet.