A red iPhone battery icon means that the battery life on the iPhone is low, about to run out, or completely drained. If the iPhone is still on, once you plug in the charger, it turns green to indicate that it is now charging.
If the iPhone was completely drained then the battery icon will remain red and the phone off until it receives enough power to turn on. It usually automatically starts once you connect the iPhone to a power source.
- Status icons and symbols on your iPhone | Apple Support