Battery Health Dropping Fast on iPhone 15 – Need Help

Battery Health decline is not exact science, nor it is it linear, meaning battery health may remain steady for a while, then drop a few points all at once. The reality is your battery health could be closer to 91% (like 90.9%), but it will be reported as 90%. After 300 cycles (actually in your case, 305), one would expect your battery health to be in the 93-94% range, but a point or two in either direction is not a cause for concern. If by some very odd chance your battery health dipped to 80% in the first 12 months, Apple would replace it under warranty, but you likely have more chance of winning the lottery than that happening.

So no, you should not be concerned, in my opinion. But there are things you should be doing to help your battery:

1. Never let the battery fully discharge. Doing so is harmful to Lion Batteries and can prematurely age them.
2. Avoid letting your battery go below 20% with any regularity.
3. DO, plug your phone in to charge when you go to bed at night, EVERY night, leaving it plugged in ALL night with Optimized Battery Charging enabled.
4. Read this too --> Batteries - Maximizing Performance - Apple
5. Stop worrying over the natural and inevitable chemical aging (which is decline) of your iPhone battery and instead, enjoy it and the day comes that its health dips to 80%, it will be time to pay Apple to replace the battery.

Apple considers the battery to be "normal" unless it drops below 80% in the first year of operation.......so there are no hard and fast rules to define "normal".

Apple does say that the battery in the iPhone 15 is designed to go up to 1,000 Charge Cycles under ideal conditions. That works out to about a 1% decline for every 50 Charge Cycles before the battery is down to 80%.

At 300 Charge Cycles, your battery would ideally be in the 93-94% range. But, the decline is not linear. Some months the battery might not drop at all and then drop 2-3% the following month.

90% is not really that far off. Unless the battery takes a sudden 2-3% drop in the next few weeks, I would not be concerned at this time.

It might be another discussion, but personally I consider Charge Limits to be more of a gimmick than anything else. I've always used Optimized Battery Charging and ignore the Charge Limit settings.

It sometimes happens if your battery is damaged. I had an old phone that lost battery very quickly. It shouldn’t happen to a phone that new though. Maybe you just need a new charger.