not only that, probably it would be unstoppable!

i dont know about Merc diesels, but some diesels use a small solenoid in the diesel injection pump to stop the diesel flow when you put the key to "STOP". thus, if there is no 12v supply from a battery, there wont be a 12V supply on this solenoid when you try to stop the engine. so it will go on forever (or until you manually close the fuel flow).

if the solenoid is the other way round, i.e. 12V to open, 0v to close, the engine will stop if you disconnect the battery and alternator.