Really it's the battery power - using lithium ion batteries is the best idea I believe, because, for example, an 18650 lithium ion high drain battery I can vape on all day after I have charged it to 4.1 volts, at the end of the day it's at about 3.6 volts, and I replace it.....a smaller battery, say for example, an 18350 (as in one you can use on a vamo v5 and reduce it's over-all size by about 3 inches or so...), you could still vape all day, but you would have to charge the battery to 100% (4.2 volts) and discharge it almost completely, thereby reducing it's over-all life by about two thirds...or, you could simple carry another 18350 aroun d with you - I agree wuith whoever mentioned the unneccessarily large atty size - I am tired of rebuilding my pro tank 2 so I have a kanger sub tank mini coming - but I really wish that the sub tank nano was rebuildable, that would be my choice - who really needs more than 3 ml in their atty at a time? I vape quite a lot, my pro tank holds 2 and 1/2 ml, and I don't always go through all of that in a day - in fact, many times I am only vaping about 1 to 1 and 1/2 ml a day.....