iPhone 15 Pro Max battery health rapidly declining after one year

Generally speaking, for "about" 50 cycle charges, your battery can lose "about" 1% in health. This is not exact as no two batteries will decline in the exact same manner. User charging habits and how the phone is used and for what, have a major impact on battery health. As you've been through 370 cycles, it would be more normal for your battery health to be in the 92% - 93% range. You're only 3% off from that, so not necessarily a cause for concern.

Nor is it not a defect in the battery, nor a warrantable item. The battery would be covered under warranty if its health had dipped below 80% in the first year.

Things you should do:

1. NEVER let the battery fully discharge - it ages a battery faster.
2. Avoid running the phone below 20% - for the same reason.
3. DO: Plug your phone in to charge when you go to bed and leave it plugged in to charge ALL night, EVERY night with Optimized Battery Charging turned on in Settings > Battery. Make sure Wifi is also on, so your phone will backup nightly.
4. When the health dips to 80% or just below, that's when you pay Apple to replace the battery and the health clock starts all over.
5. Stop worrying over the normal and unstoppable aging of a consumable battery.

You can have Apple run diagnostics on your phone if you want, but it won't likely reveal there is any issue with the battery.

I wrote this to help people better understand Battery Health Decline --> https://discussions.apple.com/docs/DOC-250012201

Is this rate of battery health degradation normal for an iPhone 15 Pro Max?

My opinion and experience.....Battery Health should be in the 95% range at this point.

Why did my battery health remain at 100% for a year and then suddenly start decreasing every week?

I don't know. Normally, Battery Health declines at about 1% for every 50 Charge Cycles. But, the decline is not linear. Some months, the battery might not decline at all, then drop 2% the next month.

Is a cycle count of 370 consistent with 89% battery health?

No, unless you are a heavy user.

Could this be a battery defect or a software-related issue?

Could be either, or both.

Can Apple run a diagnostic test to determine whether my battery is functioning properly?

Yes

If the battery is defective, am I eligible for a replacement?

Apple will replace the battery at no charge if you have AppleCare+ coverage. Otherwise, the cost of a replacement battery is about $100 (U.S.) since the normal one year AppleCare warranty has expired.

However, if the battery tests out OK when you take the iPhone in for service, Apple will not usually replace the battery unless it has declined under 80% Maximum Capacity.....even if you are willing to pay.