My iphone stops at 80%. I have iphone 8 plus. Please help

Well, this is the chapter provided in the support guide that ChrisJ4203 suggested:

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 you want to feel more comfortable, since your iPhone is still on warranty, have it checked at your most convenient Apple Store or Authorized Service Provider: Apple - Find Locations - Location Selector

Regards

Giulio

See this support document. There is a section on 80%. If your iPhone, iPad, or iPod touch won't charge - Apple Support

Read the maximum capacity percentage going to Settings>Battery>Battery Life (beta) and tapping on it.

If it is > 85%, don't worry about it.

Then, I suggest that you charge your iPhone overnight (keeping in mind that when the bateery reaches 100%, your iPhone automatically stops charging).

I would recommend that you DON'T let your battery go under 20%. There's no need to do this with modern Li-Ion batteries.

Regards

Sorry about that, but you should understand that it's difficult for us (we are just users like you) to diagnose your issue remotely.

Have you tried to contact Apple Support online? Contact - Official Apple Support

Regards

Giulio

Scroll down and you'll see "Talk to us" then click on the "Get Help" button and follow the instructions.

You can chat with them, or they could call you back by phone.

iPhone Battery & Power - Official Apple Support