I charged my Kobo overnight and when I went to work this morning it was at 94%. It remained in my locker untouched until my lunch when it would not turn on.
I'm charging it again and the battery went from 8% to 0% in the blink of an eye while being plugged in.
The battery no longer retains a charge and I've only had it for 3 months.
Maybe your tablet's battery is not calibrated. Drain it first, then charge it while it's off. Once full, drain the battery again. Hopefully after that, your battery will properly hold a charge.