Battery cycle time
The number of cycles of a lithium battery is about 500, and then the battery will begin to decline. When you see that the health of the battery has dropped to 80%, you can consider replacing it with a new one.
The battery health is degraded.
Generally speaking, the health of Apple's mobile phone battery will drop to 80% after being used for one and a half to two years. If the maximum battery capacity drops below 70%, the mobile phone will start the performance management function, which means that you need to change the battery.
Official website price
If you change the battery, the price in official website is more than 500, which is a bit expensive.
Third-party battery
Third-party batteries are also a good choice, but please note that there will be an infinite pop-up warning above the iOS 13 system. So, if you want to change the battery, you have to think about it!