Central locking with flat battery

[MD5]

My 08 SL500 hasn’t been used much since last year, so the convenience battery has drained a few times between uses, which means I have to unlock the car with the key in the door rather than with the fob, as the central locking is governed by the convenience battery. It starts fine as the starter battery is okay.

The odd thing is that the petrol filler cap won’t unlock from the door lock or the switches on the dash, only by the remote, which then depends on the battery being charged sufficiently. Is this a design fault or a fault on my car?

 

[MD5]

For anyone interested, the petrol filler flap lock is operated electrically, whereas the doors and boot work by vacuum. Manually locking or unlocking the door using a key blade won't alter the status of the petrol flap, which will only respond to the remote fob. Not good when your convenience battery is flat.