battery health

The measure of battery health deterioration/drainage is subjective and can vary depending on individual usage patterns. There is no universal standard to determine whether the battery health deterioration/drainage is normal or abnormal because it heavily relies on how the battery is used and maintained.

Protect & Prolong the Life of your iPhone's Battery:

1. About the battery usage on your iPhone, iPad, and iPod touch ...
2. Batteries - Maximising Performance - Apple (IN)
3. iPhone Battery and Performance

"battery health: My phones battery health dropped by 3 % in a single month without using. I am using my phone daily 50 to 55 Minutes but still my battery health dropped by 3% and I charge my phone once in 3 days so how its dropped"

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Battery Health Decline:

All is fine. What is occurring is normal. Unfortunately, there is no way to determine why, not when, the battery decrease occurs. In fact, the Battery Heather percentage can even be seen to have increases, at times. The percentage is all just a guestimation, on Apple's part. Once the Battery Heath reaches 80% or lower, Applerecommends it is time for a new battery. So, you are fine, in the mean time.