BATTERY WON'T CHARGE ABOVE 80%

How long do you let it charge? The phone will fast charge to 80%, then slow down charging, and may not appear to move for a while. It may take 1-2 hours after it reaches 80% to reach full charge. What are you using for a power source? And are you using an Apple or certified Made for iPhone cable?

If it reaches 100% eventually it is working correctly. If not make a genius bar appointment to have it checked out—>https://www.apple.com/retail/geniusbar/