Battery status display
For iPhone 6 and newer, iOS 11.3 and newer offer new features for battery status indication and replacement recommendations. You can find this under "Settings" > "Battery" > "Battery status".
If the iPhone battery is no longer quite up to it, you should check whether it still has enough charge capacity. For a worn battery should be replaced.
Basic battery power management
All iPhone models have basic battery power management to ensure that the battery and the system as a whole function as desired and that internal components are protected. This concerns the behavior at high or low temperatures as well as the internal voltage management. This form of battery power management is required for safety and proper operation and therefore cannot be disabled.
