From the official:
About optimizing battery charging on iPhone
In iOS 13 and later, the iPhone will learn your daily charging habits, thus extending the battery life.
The life of a battery is related to its chemical life, not just the time after the battery is assembled. The chemical life of batteries is determined by many complicated factors, including temperature history and charging mode. All rechargeable batteries are consumables, and with the aging of battery chemicals, the performance of batteries will be reduced. With the aging of the chemical composition of lithium-ion battery, the amount of charge it can hold will decrease, which will lead to the shortening of battery life and the decline of peak performance. Understand the battery and performance of iPhone, and how to fully extend the battery life.
In iOS 13 and later versions, the function of "optimizing battery charging" is to reduce battery consumption and prolong battery life by reducing the time when iPhone is fully charged. When this feature is enabled, in some cases, the iPhone will delay charging by more than 80%. The iPhone will use the machine learning function on the device to learn your daily charging habits. Only when the iPhone judges that the charger will be connected for a long time will the "Optimize Battery Charging" be enabled. This algorithm is to ensure that your iPhone remains fully charged when the charging cable is unplugged.
After you set up the iPhone or update to iOS 13 or later, the optimized battery charging is turned on by default. To turn this feature off, go to settings > batteries > battery health > optimize battery charging.
When optimized battery charging is enabled, a notification will be displayed on the lock screen indicating when the iPhone will be fully charged. If you need to fully charge the iPhone as soon as possible, press and hold the notification, and then tap "Charge Now".
If optimize battery charging is not enabled
The optimized charging function is specially designed for places where you often stay, such as your home and office. This function is not suitable for occasions that often change habits, such as traveling. Therefore, some location settings must be made to enable "Optimize Battery Charging". Location information used by this feature will not be sent to Apple.
The following settings must be enabled:
Settings > Privacy > Location Services > Location Services.
Settings > Privacy > Location Services > System Services > System Customization.
Settings > Privacy > Location Services > System Services > Important Sites > Important Places.