Have iPhone and iPad battery status checked and changed.
Battery status display
For iPhone 6 and newer, iOS 11.3 and newer offer new features for battery status indication and replacement recommendations. This can be found under "Settings" > "Battery" > "Battery status".
If the iPhone battery doesn't really want to work anymore, you should check whether it still has enough charging 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.
