Battery capacity

To first identify whether your battery is ready for a change, you can check to see what the current health of your battery is by going to “Settings” > “Battery” > “Battery Health.” This is where you can find out your battery’s current maximum capacity. A small notice at the very top may appear if your iPhone’s battery is ready for a swap.

When its maximum capacity is close to 82-80%

(in Settings>Battery>Battery Health>Maximum Capacity )

Regards

Giulio