Why do battery health decreasing?

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You need to mention which part from the above article was not clear to you.

Please go through the following links.

• iPhone Battery and Performance - Apple Support (IN)

Protect & Prolong the life of your iPhone's Battery: Batteries - Maximising Performance - Apple (IN)

Try some of these options too.

1. Avoid Screen Mirroring
2. Put on Low Power Mode
3. Put Auto-Lock on (1 min):- with the Low power mode it will be 30 Sec
4. Disable Push notifications for apps you do not use often
5. Keep your iPhone Facedown to prevent notifications to light up the screen
6. Enable Auto-Brightness:- Settings > General > Accessibility > Display Accommodations > Auto-Brightness [Enable]
7. When possible, manually lower the brightness from Control Center
8. If you want to watch continuous Videos/Movies. Stream it on TV or a Monitor
9. Don't force close apps; Apps that are not on the frontend screen are paused by the system to save energy. Apps require more energy to re-launch. Apps that are open in the background use large memory is a myth and quitting them is counterproductive
10. Location services may please be allowed only while using the app. Avoid the "Always" option wherever available
11. Set your iPhone to fetch email & Calendar data from the internet less frequently
12. Turning off Background App Refresh 
13. Charge before it drops below 20-30%
14. Periodically Check the Battery Health (Settings > Battery > Battery Health)