my iphone 11 battery health just dropped to 99% in a month

Here's something I found on another page that may help. Not sure what iOS you're running, but you probably want to make sure it's 14.5.

Recalibration of battery health reporting on iPhone 11, iPhone 11 Pro, and iPhone 11 Pro Max

iOS 14.5 and later includes an update where the battery health reporting system will recalibrate maximum battery capacity and peak performance capability on iPhone 11, iPhone 11 Pro, and iPhone 11 Pro Max to address inaccurate estimates of battery health reporting for some users.

Learn more about recalibration of battery health reporting in iOS 14.5.

Published Date: March 31, 2021"

That’s about right. Batteries are consumables; they lose a little capacity every time they are discharged, then recharged. On average this works out to about a 1% loss for every 25 “full charge cycles”. As one example, if you charge the phone overnight, every night (and that is what you should do; it is a best practice), it starts the day at 100%. If it drops to 20% by the end of the day before you charge it again overnight that counts as 0.8 full charge cycles (20% to 100%), or about 25 full charge cycles per month of use. For this example your battery capacity will lose about 1% per month. Of course, if the end-of-day level is higher than 20% the capacity loss will be a little less, and if it is lower than 20%, or you charge it during the day, the capacity loss will be higher.

Just curious, but how does the battery know if you're charging it during the day or night and why does it care? You say that if it is charged during the day, the capacity loss will be higher. If you charge it 4 times in a 24-hour period from 80% to 100%, then wouldn't the capacity loss be the same as charging it from 20% to 100% at night during the same 24-hour period based on your "full charge cycles"?

The phone doesn’t “care” when you charge it. All it knows is the number of full charge cycles (which it measures, and which it will report when asked when used with a Mac app such as Coconut Battery). Yes, charging it 4 times from 80% to 100% is the same full charge cycle use, but the key difference when you charge it overnight is that most of the energy use overnight (and the phone is always using energy) comes from the external power source rather than the battery, so it reduces the number of times the battery must be charged. If you charge it for short periods of time virtually all energy to run the phone comes from the battery, and none from an external source.

Enabling Optimized Battery Charging when charging overnight stops charging at 80% and switches entirely to external power. It then resumes charging so the phone reaches 100% around the time you start using the phone at the beginning of the day.This meets the principle that Lithium ion batteries should not be stored at 100% charge for long periods of time.

BTW, on other point regarding your original post: When the battery capacity shows 99% all it means is that it is below 99.5%, so it may only be reduced by 0.5%; the granularity of the measurement does not show partial percentages.