New iPhone battery—bad in a week?

Installed a new battery in an iPhone SE 8 (iOS 13.3) days ago. Did the job myself and then calibrated it carefully (as spelled out here) and all was well—Battery Health Maximum Capacity was 100%, for a couple of days.

Since then, Maximum Capacity has fallen precipitously—to 91% now, 8 days later.

My usage is not unusual—I routinely turn off Bluetooth, optimize battery charging and run the phone in low power mode. And use it mainly to read mail and check Facebook (almost all reading, no video).

What would kill a battery so quickly? Wild theories welcome!

Assuming you did everything right and your iPhone is OK, is there any chance your replacement battery was simply a dud?

I guess it’s possible it was a dud, but in calibrating the battery, I drained it twice by playing video (with sound set to high volume) for close to nine hours when I first installed it. So it behaved pretty well for the first few days.

And that would suggest I did everything right, right?

How would one tell if the battery were bad? Before I complain to the vendor, any other possible explanations?

This is my bet too. I’d call the vendor without wasting time on trying to troubleshoot further.

If it started malfunctioning immediately after you got it – ie exactly what happened. Call the vendor.

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I believe the days of battery calibration are long past since the introduction of lithium-ion batteries. Personally, I would not calibrate lithium-ion batteries. Apple certainly does not recommend it any longer although there was a time that Apple did. Here is an article in part outlining why it is not a good idea:

Why you should not calibrate