Er, yeah, of course you're right. Adding that factor in will mean adding in even more fake, hand-wavy numbers, but doesn't really change the end result; whatev

Er, yeah, of course you're right. Adding that factor in will mean adding in even more fake, hand-wavy numbers, but doesn't really change the end result; whatever value you end up with, if you double the weight of the batteries, you will need to displace twice as much air (or move it twice as fast) to compensate, which means you'll need to use up twice as much energy, leaving you with the same flight time in the end.