iPhone 15 Pro Max battery health decline

That is normal.

Batteries are a consumable and your Battery Health is expected to drop 1-2% per month of use of your device.

If your battery drops below 80% within one year of purchase, Apple will replace it for free under warranty.

iPhone Battery and Performance - Apple Support

hamadd_ Said:

"iPhone 15 Pro Max battery health decline: I've used my brand new iPhone 15 Pro Max from 29th September onwards. I've never let the battery get below 15% and usually charge at 20% and only use the official 20W Apple charger but yesterday I noticed my battery health dropped to 99%. Has anyone else noticed a battery health drop and is there a way to reduce battery health degradation?"

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Troubleshooting iPhone Battery Heath Decline After iOS17 Update:

A. Downgrading the iOS:

Unfortunately, the iOS cannot be downgraded to a lower version. So, you only option, when it comes to restoring from an iPhone backup, is to restore from on that is of the same iOS version.

B. Report this to Apple...

Overeating iPhone and draining iPhone batteries are a well-known issues of the iOS17 update. Question is: "Just how many people have actually reported this to Apple?" So, do your Part: Provide Apple with feedback on this, letting them know about this issue.  Apple may not get back to you directly, but the more feedback they receive on this, the more they will know what is occurring, and what bug fixes to include in updates to come. Apple will know nothing of this, if you don't report it to them.

1. Go Here: Feedback - iPhone - Apple
2. Select: "Bug Report" for the "Feedback Type"
3. Comment: that this all began after installing an update. Include the URL of this thread
4. Proceed from there as necessary

Which, again, is completely normal and expected.

I’m at 25 cycles and at 99%. I have found varying declines in battery health with various users. Will use my phones as regular and see where it takes my phone in a year 😅