U still want to remove the black rubber Plug and turn the little donut around and use that as a cap, I have fluxos 2.8-3.0 and I don't go higher than 4.5-4.7V because although warmer and better vapor at Higher Volts, It burns the crap out of the
juice and the coil won't last longer than a few days , even on thinner Juices, Gave up in the Fluxos, all I do now is Boge 3.0 on Tanks, Those I can Push to 6V no prob and last me a couple days a tank. after doing this I haven't gone back to fluxos, vision, no CE2's as well , less work , less cleaning , Less refilling. Just a suggestion being that u still have 4 left , I can recommend to keep them at 4.5 or so, Still better production than 3.7V.