We're happy to explain more about the iPhone's ability to tell you its related battery health.
You can easily check the Battery Health of your device by following Apple's detailed instructions below or simply going to Settings > Battery > Battery Health https://support.apple.com/en-us/HT208387
Once you reach the battery area of the menu, make sure to find the Maximum Capacity.
Within the Battery menu, there is a button that says "Battery Health"
If you click that, it will take you to a screen that shows the Maximum Capacity and gives a percentage.
The Maximum Capacity is the max amount of charge your device is eligible to receive.
If your battery health capacity goes below 80%, Apple recommends you change it.
If your battery health capacity goes below 79%, we would consider it to be in POOR condition, since this will affect the device's ability to perform normally throughout the day and it will display a message to any new user which would purchase your device.