15 Pro Max Battery health issue

Batteries degrade over time with usage due to chemical aging, that's just how they work.

You can expect about a 1% drop per month with average use on an iPhone. This is an approximation, that is why it may be plus or minus a few percent on a month to month basis.

Apple recommends replacing it when the battery health reaches 80%.

iPhone Battery Replacement.

iPhone Battery Replacement - Apple Support

About Lithium Ion Batteries

iPhone battery and performance - Apple Support (IN)

Here are some tips on maximizing battery life.

Batteries - Maximizing Performance - Apple

You can expect about a 1% drop per month with average use on an iPhone. This is an approximation, that is why it may be plus or minus a few percent on a month to month basis.

From Apple's Support document:

Batteries of iPhone 14 models and earlier are designed to retain 80 percent of their original capacity at 500 complete charge cycles under ideal conditions. Batteries of iPhone 15 models are designed to retain 80 percent of their original capacity at 1000 complete charge cycles under ideal conditions.*

For iPhone 14 models are earlier, that translates to drop of 1% for every 25 charge cycles, which is often stated as about a 1% drop per month on average.

But with iPhone 15 and higher that works out to a drop of 1% for 50 charge cycles, which would be about a 1% drop every 2 months.

That's why I asked the OP for the number of charge cycles that have taken place on his iPhone 15 Pro. He stated that the phone was new "last summer", but that's not really enough information to go on unless we can get a more accurate purchase date.

Once we have an accurate purchase date or information about the number of charge cycles, we can make a better determination about how well the OP's battery is performing. It's still tough to figure out what "ideal conditions" might be though.

How many battery charge cycles on your phone?