How to improve iphone battery health

You cannot improve iPhone Battery Health, but you can help slow down the decline if you plug the phone into the power adapter and leave it connected all night, every night.

On average, Battery Health will decline about 1% a month, so after 12 months the Battery Health is going to be about 88-90%. Plan to replace the battery at about 24 months or 2 years.

Also make sure that Optimized Battery Charging is enabled on the phone. Settings > Battery > Battery Health