My issue is that my tablet might say that the battery is at 30%,I then rebooted it to update xposed framework. After it booted back up it said that the battery was at 6%. I rebooted again, then it said that the battery was at 8%. There have been many events like that. Would recalibrating the battery fix it. I have not yet flashed a custom ROM, I did however do a restore from a nandroid backup via twrp because I accidentally soft bricked my tablet while trying to flash a ROM. I had this issue with the battery even before I rooted my tablet.