used iphone 8 plus charged battery drain
I love the new graphs that show you what happens in the battery for this over time.

Here’s what an iPad Pro that’s cranky after updating to iOS 14 and not recognizing the Magic Keyboard looks like sitting on a desk overnight, power to the keyboard hinge port and clearly not taking a charge. The gap before the brief charge was caused by a forced reset requested by Apple Support to triage and isolate what’s going on.
Here’s an iPhone getting much better charge results than the human did last night.

You can track things each day and get an excellent picture of what your device is doing before needing support to run a battery diagnostic in my experience.

https://support.apple.com/en-us/HT201264

Once you have some details and have read about the terms, maybe get Apple to see if you want a battery service?

iPad Pro 10.5&quot; down to 92% battery health after 6 months of use (update: 84% after 1.5 years)

I generally wait till it’s 45% to pay as I’m ok managing it and want to get the most life out of a safe but aging battery. You’ll want to be sure it’s not software or a busy app, though before any repair since a stuck app can drain a perfect battery rapidly.