Why is my iPhone 15 battery health draining fast?

Good questions. After 307 cycles a battery would normally be at 93%, so yours is a little low, but the gauge isn’t linear, because a battery is an analog device, not as precise as digital devices. Thus, at this point it doesn’t indicate a problem.

Actually, the best practice is to enable Optimized Battery Charging and turn off the 80% limit. Charging when it reaches 20% is a best practice, but keeping it above that is more for convenience, so it will be less likely shut down if you have a need for, say, a long phone call. It won’t hurt the battery however, unless it goes to zero and isn’t charged immediately.

If it goes below 80% within the 1 year warranty, or for as long as AppleCare+ is in effect if you purchased it, Apple will replace it at any Apple store for free while you wait usually. So check it just before the end of your warranty. If it is out of warranty the fee to replace it is reasonable.

Regarding your friend’s experience, a battery is a chemical device, and chemistry is generally pretty variable and uncertain, as well as being analog, not digital. Apple specs the battery capacity to remain above 80% for 1000 full charge cycles for your model, but that is a minimum requirement; there is no published maximum expected capacity. So sometimes batteries will perform much better than that minimum specification, and sometimes the change in maximum capacity won't be linear. There is no way to predict in advance what the real-life performance of any specific battery will be.

So my main advice is don’t worry or try to micromanage the battery; just enjoy using your iPhone.