Why is my battery health of my iPhone 13pro max drops to98 with less than one month

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.

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 Batteries - Maximizing Performance - Apple

eyerusalem ~ Because that number is just an algorithmic estimate of a dynamic, chemical process.

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Also, where did you get the phone? The iPhone 13 Pro Max was discontinued in September.