Why my battery health day to day decreased?

How long have your owned your iPhone and what is the battery health percentage on your iPhone at present?

Battery Health & Performance 

Your battery health is probably fine and may be undergoing normal usage. Charge it whenever you want to.  Try and not let your battery charge get below 20%. Your iPhone cannot be overcharged because it has a safety circuit built into it. You should change out the battery once your Battery Health reaches below 80% which will take over 2+ years to reach under normal usage.  Until then enjoy your iPhone.

Here is some more reading on an iPhone battery and its performance ➔ iPhone Battery and Performance - Apple Support

Axel F.

Backup resolved Battery decrease day by day