Battery health decreasing rapidly in my iPhone 15

Time is not an accurate factor in determining battery health decline, rather charge cycles are. So go to Settings > Battery > Battery Health and Charging. How many charge cycles does it say?

You can expect your iPhone 15 to drop about 1% per 50 charge cycles on average. But that number will vary based on a number of factors such as conditions, usage and can be more or less percent.

In 2023, when I bought my iPhone 13, I purchased a charger adapter from a third-party store and experienced the same issue you’re having.

Later, I realized the adapter was fake, and it significantly affected the battery health of my iPhone.

(Also, using the phone while it’s charging can damage the battery health as well.)

Conclusion: The adapter or charging cable is likely fake!

I-Am-Smart wrote:

Other factors like how much/long you charge your phone can affects it’s efficiency. Try only charging your phone to 90% maximum and not let it go below 20%

With all the charging habits, there is not one that makes a big difference compared to the rest. Charge Limits will still occasionally charge to 100%. And over a few years, you may see a 1 or 2% increase compared to the rest, but it really isn't drastic.

Most senior community members recommend Optimized Battery Charging. You charge your iPhone all night, every night and it waits to charge past 100% before you need to use it. Ensuring you have a full charge when you wake up.

Other factors like how much/long you charge your phone can affects it’s efficiency. Try only charging your phone to 90% maximum and not let it go below 20%

iPhone battery and performance - Apple Support

in settings, check battery health and find out how many cycles the phone has done. If it’s more than 400 it’s normal for it to be ~85%

Sounds perfectly normal to me for an iPhone 15