An 18650 batt fits once you rip the Ioniser board out (yes, that thing is a negative ion generator..).
BTW, while Boge claim the Revo2 has a 650 mAH batt, it is in fact only 500mAH.
If you could move the circuit board to the other side ... for example to the top of the box, or underneath the battery, and then an extension cord .... maybe you could put a smaller feeder bottle, just a thought. ... can not wait for mine.
and maybe even a VV board circuit would be nice.
