While I don't own a modus, in my experience with the rogue 1 and 2, li-ion doesn't work well. In the rogue 1, it would run slightly brighter for a few seconds, then just shut off. I don't know if the driver pooped out, or if the battery protection kicked in. Afterward the light works just fine with nimh and alkies, but I'd rather not risk burning out a nice light. I didn't dare try two 14500's in the rogue 2, but using one with a spacer seemed to work just fine, albeit impractical - a marginal gain in brightness for a major decrease in capacity.