Maukka and manufacturer do not have problem using their Samsung flat top. However, my stock LiShen unprotected 21700 will not work with this light because of the size of anode connector is too big for the opening.
It consumes about 5mA of the battery with breathing light ON. You can further reduce the drain by disabling the breathing function for final retail version. (Note: unable to disable on prototype due to incomplete firmware).
All torches I have and I have several of them, have drainage below 0.1mA. All torches with an electronic side switch. Such a large consumption as FT02 has is a big drawback for me. It reminds me of Haikelite products, which also have such problems. I had HT07S HI and had 6mA.