iPhone XR battery health dropped from 99% to 86% within a week

They discontinued the iPhone XR in Sept 2021, almost 5 years ago.

Did you just purchase this phone from someone? If so they boosted the battery reading which gave a false reading of 99%.

Unless the battery was just recently replaced, it would never have a reading of 99%. Even a reading of 86% would be high for a 5 year old phone but without knowing the details of your phone such as when and where you purchased it, it is hard to say what is really going on.

Ok, they used a shady method of hacking the phone called boosting. It makes the battery falsely report a higher health rating. Then when it's charged a few times or you update the operating system, the real rating is revealed.

You can take this issue up with the store but chances are they won't do anything. You will most likely need to replace the battery in a few months. Apple recommends replacing it when it reaches 80%.

There is no way to determine why, nor when, the Battery Health decreased.

Yes there is, they used an unscrupulous method of hacking called boosting. Many unknowing people have complained about this in these forums

"My Xr health battery dropped within a week from 99 to 86,what’s wrong?"

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Reason why Battery Heath Percentage Dropped:

There is no way to determine why, nor when, the Battery Health decreased.