I check into this thread from time to time on the status of the 2x21700 adapter, but I am not displeased running my Wildcat XP-L and Hound Dog Super on 1x21700. The low is fine for indoor use, and I'm sure it will run forever. I use high every night for about 30 minutes to walk my dog, and only charge batteries every couple weeks. Obviously a bit less output than running on 2 cells, but still a ton of light. I go 2x18350 when I feel like maxing it out for a couple days, but the head does get warm. There is practically zero heat running on 1x21700, and I think the form factor is quite nice.