iPhone 15 Pro battery health decline after iOS 18.1 update

Are you honestly suggesting in a year, you've only charged your iPhone 7 or 8 times? That seems hugely unlikely, unless you routinely let the phone run out of charge completely and don't charge it for extended periods of time.

The fact is iPhone Battery Health declines no matter what you do. On average, you can expect up to a 1% loss in health for every month you've owned the phone. The loss isn't linear as it can remain steady for some time, then dip a few point. When you updated to iOS 18.1 all that happened was the battery health % was recalculated. It didn't cause your health to decline.

If you have owned your phone for a year as your post suggests, then a loss of 8% in battery health over 12 months is actually QUITE GOOD and not cause for concern at all. As mentioned, Battery Health will continue to decline no matter what you do. When your battery health dips to 80%, which it will if you own your phone long enough, it will then be time to pay Apple to replace the battery. Until then, worrying over the natural decline in battery health is a wasted concern.