Aura H2O2 Battery woes

I haven't been reading much for the last couple days. My eReader was at 18% so I powered it off for a couple of days and when I turned it back on, it was at 28%.
What?
And after 10 minutes of Unblocking It, it gave me a 10% warning. What the *Beeeeeep* is goin' on here?!
I'll try charging it to 100% and see what happens.