iPhone 8 Battery Problems

My dad's iPhone is having a huge number of battery problems. They suddenly started yesterday with absolutely no warning, whereas the day before the iPhone was working just fine.

First, the battery will drain abnormally fast. With a full battery, we start watching a 5 minute video on YouTube, and midway through that video it will, suddenly and without warning, shut down with the battery at about 86%. Then, when we try to power it on, it will try to power on, then shut down in the middle of powering on, and then any further attempt to power it on will flash the "low battery" warning (but most attempts will result in nothing happening, as if the phone has been dead for at least a month). Then, when we plug in a charger, it immediately powers on, shows the battery to be at 11%, and then will proceed to charge to about 91% in about TEN MINUTES. After that, the battery takes about an hour to charge that remaining 9%. Aaaand, repeat.

These included screenshots show the battery as it charges from 51% to 98%. Pay attention to the time as well as the battery level.

The phone works just fine if it is constantly connected to the charger.

Some additional information is that the battery health is shown to be at 88% and the iPhone is running on iOS 12.3.1.

Why is this happening? Is this a battery issue or is this an electronics issue? How much would it cost to fix it? Please help us!

Thank you,

Damariobros