Problems with battery

If your device stopped charging at 80 percent

Your iPhone might get slightly warmer while it charges. To extend the lifespan of your battery, if the battery gets too warm, software might limit charging above 80 percent. Your iPhone will charge again when the temperature drops. Try moving your iPhone and charger to a cooler location.

If your iPhone, iPad, or iPod touch won't charge - Apple Support

You didn't mention that in your original post. In that case, do as sergekills suggested, "If you still experience issues, schedule an appointment at a Genius Bar or the nearest Apple Authorized Service Provider. There might be a problem with the battery".

iOS stops charging if iPhone gets too warm during the charging process. Take off the case when you out your iPhone on a charger, use authentic cable and power adapter.

If you still experience issues, schedule an appointment at a Genius Bar or the nearest Apple Authorized Service Provider. There might be a problem with the battery.