Date registered: May 2005
Vehicle: '07 GL320CDI
Mentioned: 0 Post(s)
Quoted: 2 Post(s)
If it's the original battery, it's old.
Starting always drains the battery a little and batteries in general lose voltage when cold. Yours would appear to be getting low and the start was enough to trigger the message.
It's highly unlikely it's the alternator and if you can run the car for a while and then restart it without getting the message again, it can be concluded the battery has been charged up by the alternator.