Why my phone has a battery problem
Why my phone said that unable to determine if my iPhone battery is genuine,Apple part
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An explanation of that message is in the article 'About genuine iPhone batteries' --> About genuine iPhone batteries - Apple Support
"An Unknown Part message will appear if the battery installation is incomplete or if the battery:
• Was replaced with a nongenuine battery
• Was already used or installed in another iPhone
• Isn't functioning as expected"
Get Apple to examine your phone.