IOS 15.3.1 drain my iPhone 8 battery

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For info on Battery Health and Battery Life, see these Apple Support articles:

• About the battery usage on your iPhone, iPad, and iPod touch - Apple Support
• Check the iPhone battery health and usage - Apple Support
• iPhone Battery and Performance - Apple Support
• Batteries - Maximizing Performance - Apple
• Batteries - Why Lithium-ion? - Apple

Jack

Aburningstariv wrote: "...The 2nd iPhone 8 running on 15.3.1 will not even show a battery percentage on ANYTHING other than an OEM battery stripped from an original iPhone 8..."

Presumably the changes in iOS 15 are to avoid possible negative experiences for the user:

"Using nongenuine batteries might also result in unexpected behavior after installation, device software updates, or while charging. Using nongenuine batteries might also lead to safety issues." Source:

About genuine iPhone batteries - Apple Support

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I have 2 iPhone 8’s both 256gb unlocked. One is running on 14.7.1 and works FLAWLESSLY with any replacement battery. The 2nd iPhone 8 running on 15.3.1 will not even show a battery percentage on ANYTHING other than an OEM battery stripped from an original iPhone 8. Currently trying to restore a backup of 14.7.1

and for good luck

User tip 

The last update killed my battery! - Apple Community

tohy ~ Does your iPhone 8 still have its original battery?

Also, what's the Maximum Capacity % here?...

Settings > Battery > Battery Health

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You’re gonna have to get your battery serviced at a Genius Bar.

[Edited by Moderator]

If you swapped-in an OEM battery yourself then Apple will not do any work on it