My batery level is low 78 capacity

Time to replace your battery.

Batteries degrade over time with usage, that's just how they work.

You can expect about a 1% drop per month with average use on an iPhone.

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

iPhone Battery Replacement.

https://support.apple.com/iphone/repair/service/battery-power

About Lithium Ion Batteries

iPhone Battery and Performance - Apple Support (IN)

Here are some tips on maximizing battery life.

Batteries - Maximizing Performance - Apple Batteries - Maximizing Performance - Apple

The battery needs to be replaced. Take the phone to an Apple Store or an authorized Apple Repair shop.