This has just happened to me. I was inside the car. As I pressed the start button the central locking activated but the engine didn’t turn over. I was locked in. There was not enough power to get the central locking to open! The car wouldn’t turn off.
fortunately I was able to open the driver window as there was enough power to do this. I then used the metal key to open the door.
if the battery had completely failed I would still be in my car!!!!!
scary