How can I fix battery calibration on my iPhone?

How old is it? Do you know the number of cycles it's been through? If you have a Mac you can find this with coconutBattery.

The battery management system can only estimate the battery capacity. The model that it uses can be pretty good when a battery is new but it can break down as the battery gets older. Maybe something like a voltage drop and it realizes that there's less charge left than it previously calculated. If you see behavior like that it's often a sign that you might need a new battery.

There is one app that can be installed called Battery Life. It's in the App Store. It has a few ads but otherwise it's free. It doesn't report number of cycles, but it does have an estimate for how much of the original capacity is gone.

Carlos.Baltazar wrote:

I don’t remember exactly how old it is. But I will install battery life. Thank you!

I don't really like Battery Life, but it's the only official app from the App Store that seems to state anything about battery capacity/wear.

I also mentioned coconutBattery. I prefer it because it gives more information, but you need access to a Mac. Even a friend's Mac. Here's the link:

https://www.coconut-flavour.com/coconutbattery/

To recalibrate the battery indicator, let it run down till it switches OFF automatically.

Connect to wall power and charge it to full.