Why did my iPhone 11's battery capacity drop from 85% to 76% in one month?

An iPhone’s battery health decline is not linear. Meaning there are months where you can lose more or less battery health percent than expected. It’s very possible that your iPhone’s battery health stayed steady for a while until suddenly dropping a few points to make up for that.

On the other hand, Apple recommends you replace your iPhone’s battery at 80% battery health. Now that you have passed the threshold, you can visit the Apple Store’s Genius Bar or an Apple Authorized Service Provider to schedule the battery replacement.

iPhone Battery Replacement - Apple Support

"Why did my iPhone 11's battery capacity drop from 85% to 76% in one month?: My iPhone 11 battery capacity reduced from 85 percent to 76 percent within one month. Why?"

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Why the Battery Health Decreased:

Unfortunately, there is no way to determine why (not when) the Battery Health decreased.

The bottom line.......your battery is failing, and will soon fail completely. It needs to be replaced at the Apple Store or an authorized Apple service shop.

About $90 (U.S.)

You can find a list of authorized shops in your area using this link:

Find Locations

If you have never replaced your iPhone 11 battery, it is way overdue. It would be very unlikely to have an iPhone 11 without needing a new battery. If the device was prevented from charging to 100% over several cycles, then calibration was never able to be completed, which is the reason you saw the sudden drop. Inaccurate readings is also a sign that your battery has failed. If you told me that your iPhone 11 after 5 years of use was showing 85%, then I would also suspect your battery was failing, or that it had seen very little use.