Why is my iPhone 15's battery health degrading rapidly?

You must be enjoying your phone a LOT as that's a lot of full cycle charges in just 6 months. There are only 180 days in 6 months, so you're doing over a full charge cycle on a daily basis. And every time you go through a full charge cycle, your battery health will decline. It can't be stopped, but there are things you can do to minimize decline:

1. Don't let the battery ever fully deplete as it can cause premature aging.
2. Avoid letting the battery go below 20% if at all possible.
3. Do - Plug your phone in to charge when you go to bed at night and leave it plugged in all night, EVERY night, with Optimized Battery Charging turned on.
4. Read this user tip from Lawrence Finch on when to charge your phone --> When to charge your iPhone or iPad - Apple Community
5. Read this about maximizing the performance of the battery in your iPhone --> Batteries - Maximizing Performance - Apple
6. When the health of your battery dips to 80% or less, which is absolutely will do if you own the phone long enough, it will then be time to pay Apple to replace the battery.
7. In the meantime, worrying over the natural decline in battery health is just not worth it. Batteries are consumable parts and the more you push the battery, the quicker its health will decline.

How many charge cycles are you at? Your iPhone 15 is expected to drop 1% per 50 charge cycles. Charge cycles are a more accurate measure of determining battery health decline then time. As someone who has used the iPhone more often and harder will see the battery health go down faster than someone who bought the iPhone around the same time and used it less. Also note that battery health decline is not linear. You can expect to lose a few more or less percent at a time.

Pratik_07 wrote:

I’m at 286 charge cycles

286/50 = 5.72. Apple rounds down any tenth of a percent battery health, so 6. That would be 94% battery health. But ultimately, the different is only marginal and it's very possible that your battery health will stay steady a few months now. I wouldn't worry and micromanage about it.