iPhone 15 battery health drop to 89% with just 322 cycles

Batteries, like people are individuals. No two people age exactly the same and batteries don’t age the same either. The exact usage, chemical make, storage conditions, exposure to heat and cold and about a dozen other factors mean that every battery ages differently. Battery aging is not a linear (straight line) and degrades at different rates as the chemicals in the battery age and the other factors mentioned above. Apple expects your battery health to be at 80% after 24 months. If your battery health misses that expectation, contact Apple Support.

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Apple's literature says......

Batteries of iPhone 15 models are designed to retain 80 percent of their original capacity at 1000 complete charge cycles under ideal conditions.*

That * (asterisk) points to a lot of footnotes that describe conditions in a perfect world.

My world is a lot less than perfect. It sounds like yours may be as well.

In the past, Battery Health declined on average at about 1% for every 25 charge cycles.

To keep things simple, let's say that your phone has had 325 charge cycles.

325 divided by 25 = 13%.

So, in the past, I would expect the battery to be at about 87% at this time. At 89%, your battery is doing a bit better than average based on the past.

Battery Health is relative to when your iPhone was new, and has a limited lifespan — eventually their capacity and performance decline such that they need to be replaced.

Apple recommends you replace your battery after the capacity reaches below 80%, and it is normal for your battery health to drop.

iPhone Battery and Performance - Apple Support

All that you really need to pay attention to is replacing the battery when it is down to 80% Battery Health.

Don't keep worrying about your battery health, all lithium batteries will eventually go down.