Battery health issue on iPhone

Batteries degrade over time with usage due to chemical aging.

You can expect about a 1% drop per month with average use on an iPhone. This is an approximation, that is why it may be plus or minus a few percent on a month to month basis.

Apple recommends replacing it when it reaches 80%. To check your battery health, go to Settings > Battery > Battery Health.

iPhone Battery Replacement.

iPhone Battery Replacement - Apple Support

About Lithium Ion Batteries

iPhone battery and performance - Apple Support (IN)

Here are some tips on maximizing battery life.

Batteries - Maximizing Performance - Apple

The only issue you can have with your battery health is if it is below 80%, in which you would need a battery replacement. Otherwise, there is no issue and nothing to worry about.

And what might the issue be? (We are terrible at guesswork).

Would help if you could tell us what model of iPhone you have and how old the phone might be.