The battery health

See the reply marked as "Apple Recommended" in this similar thread:--> https://discussions.apple.com/thread/251292813?answerId=252490154022#252490154022

Regards

Giulio

Abnormal battery drain is most likely caused by iOS bugs, poor reception, apps that go crazy and keep running in the background etc etc etc. and has only indirectly to do with battery health. However if you drain your device every single day and let it go down to 20% or below before recharging, already being down to 99% is possible. And even though I get the anxiety because 100% is way more satisfying to see, don't worry about it.

Just charge it as often and as soon as you can. Keep the percentage above 50%. Don't just let the battery run out if you could plug in your phone instead.

Your battery is one percent from perfect.

Charge the device when no in use. Day and night. You can not overcharge it.