Why my iPhone 1 day beattry health drop 1 %

That's not really a big deal. There's no accounting for when they drop. Some batteries are actually less than 100% of the nominal battery capacity rating when new and then power management determines that rather quickly.

It's probably not worth thinking too hard about battery health. It's inevitably going to go down and there's really no accounting for how fast or slow it happens. Mostly we're dealing with averages, where will do better than average and some will do worse than average.

Apple considers a battery "depleted" at less than 80% although it's probably still usable. But that's also the standard for when Apple considers their devices eligible for an OEM battery replacement service.

Adhikarikiran Said:

"Why my ị phone battery health drop fast fast ?: battery also 100% charging than I used 1 hours 10 % please help"

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Why the Battery Health Decreased:

Unfortunately, there is no way to determine why, nor when the Battery Health decreased. Apple recommends you get a new battery, once the Battery Health reaches 80% or lower.