I had this same issue and apparently all connections are snap on and the charge port connection was not properly seated and would cause random restarts and charge issues. Would state fully charged and then power off and restart and show a different charge level… I opened it up and found that the connection was just laying there barely making contact and at times not making contact at all. I didnt need a new battery or Board. Works like new and my girlfriend says its the fastest tablet she has used. Hope this helps someone fix there tablet without spending more money.
The charging port connection is not properly connected. Look on YouTube for video on disassembly and push down the charge connector and clean battery connection and charge port with electronic cleaner and test it before putting the back case back on to make sure it works. That is what fixed mine. Others replace battery, and I don't think that's the problem. It's just a bad connection.