I tried replacing the stock cells with two 18650s. It almost worked. I got one 14 minute flight with it. When I tried some better 18650 cells, the battery control board(s) must have figured it out and bricked it (4 blinking red lights no matter what you do). I think the battery boards keep track if they get completely disconnected from power. Next time, I'll try maintaining power when I do the switch. Another tack would be to wait until hibernation to make the switch. I looked for some lipos to use but couldn't find a good size/weight/power. When my next stock battery wears out I might try again. I've got the 3d printed battery box worked out.