A handheld battery case that takes rechargeable IMR18350 batteries instead of having to charge the whole unit up by itself (like a spinner)? That's genius!
Joye EGO-T 18650/18350 mods were quite popular here back in 2011-2012. Kamry did it with 14500 egos. I believe they are still on fasttech.
the ego form factor w/separate battery technology has been around for a long time.