iPhone 15 battery health dropped to 95% after one year of purchase

Because batteries are consumable products. The battery capacity of an iPhone battery will normally reduce at the rate of approximately 1% per month. So to lose 5% over 12 months, you are in good shape. Apple will only consider the battery bad if it goes below 80% in a year. You do not have a problem.

Battery Health / Maximum Capacity will decline about 1% for every 25 Charge Cycles.

157 Charge Cycles divided by 25 = 6.3%

So, your battery would be at 100% minus 6% = 93.7% or 94% rounded up.

If your battery is now at 95%, it is doing better than average. Lucky you.

Absolutely normal. How about a serving of data?

This is the battery history from my iPhone SE 2020 from new in Nov 2021:

(recorded using the Coconut Battery app)

Note that my battery started at 102% health. Health is an inexact and non-linear number not worth any loss of sleep.

Today, at just over three years old, my battery shows 86% and give plenty of runtime between charges.

It is not unusual for Battery Capacity to drop on average at about 1% a month. So, if you purchased your phone 12 months ago, the capacity would normally be in the 88-90% range at this time.

You are very lucky to have a great battery, so you should be very happy.

If you want a bit more accurate analysis, check on the number of Charge Cycles that your battery has completed, and post back with that number, so we can show you a different way to track the performance of your battery.