I used to suffer from the random shutdowns on both Android and Windows whenever the battery got below 80%, and I think I solved it. If you take the hibook apart, in one of the corners there is a thin metal shield that appears to act as a heatsink for the processor. On my hibook, this shield was unclipped from the main board and not held down all the way. I clipped it back in and my hibook stopped shutting down randomly and I could use the entire battery.|
Well, until it popped off again. So I took it apart again, and this time I attached quite a few thermal conductive pads on top of the metal shielding so that when the rear case is put back on, it will constantly apply pressure to keep the shielding clipped in. Going on 2 days with that now and my tablet is still usable!
It would seem the shutdowns are a heat issue, though I have no idea what that has to do with the battery being around 60-70% and why it never shutdown while plugged in.