Why does my iPhone stop charging when it reaches 80%?
In iOS 13, by default, your iPhone will enable "Optimized Battery Charging." The new option will automatically stop your iPhone from charging when it hits 80 percent. At that point, the iPhone will act as if it at 100 percent, doing its best to conserve the percentage.
To fix this, you will need to turn this feature off. Click here for the guide on how to turn off "Optimized Battery Charging."