Berniem could be a HDD issue but it could also be an issue of the battery itself. you did not tell us how old the battery is. I would suggest that you start with that. Follow this guide. Watch out for the battery connector on the logic board, it does have a bad tendency of breaking of the board. Check around the are for any signs of corrosion, and replace the battery. After that re-evaluate and let us know. It is of course also possible that it is your PMIC which controls the charge of the battery. Hope this helps, good luck.