I did the same thing with my droid x phone. The charging was completely messed up after applying certain ROMs. Since my phone is VERY important to me, it was unacceptable. Basically, fully charged overnight with the power off deleted the battery file as they direct you in the calibration. I say this only to show that battery issues are not unique to cheap chinese tablets.
If you charge your tablet overnight with the power off, does the charge light extinguish?