Why my battery health down

Batteries degrade over time with usage due to chemical aging, that's just how they work.

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 Repair & 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

srikanta144 Said:

"Why my battery health down: My phone battery capacity down in 1 week 4 % [...]this is a normal"

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Why Battery Health Decreases:

Unfortunately, there is no way to determine why, nor when, the Battery Health decreased. But, it typically drops 1-4% right-out-of-the-box).