iPhone 14 Plus battery health decline in 15 days

Perfectly normal. Sometimes, a battery will go for 2-3 months and not decline at all, then drop 2-3% the next month. In other words, battery decline is not linear or constant. It will vary.

You also might not be aware that Apple uses special rules to calculate the battery percentage numbers. Any "tenth" of a percent is automatically rounded down to a whole number.

So, a battery might be at 92.9%, but Apple will display this as 92%.

It is very possible that your battery actually dropped from 88% to 86.9%, a drop of only 1.1%, yet this will look as if the battery dropped 2% when it didn't.

You don't need to worry about the battery and check it all the time. When the battery is down to 80%, it is time to replace the battery.

Battery Health decline is not linear. Meaning it can remain steady for a while, then drop a couple points. The more relevant question is how long have you owned your iPhone? On average, you can expect about a 1% drop in battery health per month of ownership (again, it's not exact and can as I said, remain steady for a while, then drop a point of more).

Battery Health decline cannot be stopped. Batteries are consumable parts, which lose their ability to hold their charge little by little with every battery charge.