After upgrading to iOS 18, iPhone 15 maximum battery health decreased to 99%

The Battery Health will always decrease and if your iPhone 15 is at 99%, then you are in good shape. On average you can expect the battery health to drop about 1% each month. This drop will not be linear, meaning some months it will not drop at all and other months will drop more.

Just use the Optimized Charging option and charge your iPhone every night.

If you bought your iPhone two months ago, it would be normal to expect that the Battery Health was now in the 98-99% range.

How many Charge Cycles has the battery completed?

Settings > Battery > Battery Health & Charging

If you are saying that the battery remained at 100% for several months before you updated, you have an amazing battery.

Battery Health normally declines an average of about 1% a month.

We can give you a more accurate answer though, if you can check the battery settings again to see how many charge cycles have occurred for the battery and post back with that info.

AshrafAlSayed wrote:

With my friend, the battery health is 100% and he had it at the same time but he didn't upgrade

It is guaranteed to drop and may even drop faster than yours. Having the phone the same time does not mean you have the same charge cycles or the phones were operating in the same conditions. Follow the advice to use the Optimized charging option and charge it every night. Enjoy your phone and quit worrying about your Battery Health.

The battery will decline on average at about 1% for every 25 charge cycles.

39 charge cycles divided by 25 = 1.56% Decline

So, it would be normal to expect that your battery was at 98-99% at this time.

At 99%, your battery is doing fine. There is nothing to worry about.

If you take the phone to an Apple Store and ask them to check and test the battery, they will tell you the same thing.

Enable Optimized Battery Charging

Connect the charger to the phone every night and leave it connected all night.....every night

Ignore the 80% charge limit (Because the battery will still charge to 100% several times each month)

Every battery is different.

My battery stayed at 100% for 4 months, then it dropped 4% the next month. Your friend's iPhone might do the same.

Please take your phone to the Apple Store to have it checked and tested. They will tell you that your battery is normal. There is nothing more that we can say or do to help you.

Please stop worrying about the battery and replace it when it is down to 80% in a few years.