Why is my battery appeared UNKNOWN PART

There are only two reasons for this:

1) The battery was replaced with the wrong part

2) The battery is defective

Did you have the battery replaced recently? If yes, the wrong part was installed. Take the phone back to the shop and ask for a refund if they cannot install a genuine Apple battery.

The phone may overheat and Battery Health info will likely not be displayed. The message about an unknown part will continue to be displayed until the correct battery is installed on your phone.

If the battery was changed by a third-party repairer, it is unlikely be a genuine OEM battery. If a replacement battery is not of Apple-origin, the battery won’t be “recognised” as a genuine part - and you will see the “unknown battery” message. This alert cannot be suppressed.