You could load test your battery with a load tester. If it test good then I would look for a ground issue. I don't know where the ground straps are are on the MB, I've had other cars with similar issues. If your battery tests bad then it would point towards the alternator. Also look at the conections on your starter to see if there is any corrosion.

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1959 Gravely LI, 1963 Gravely L8, 1973 Gravely C12
1982 380SL
1978 450 SEL 6.9 euro restoration at 63% and climbing
1987 300 D
2005 CDI European Delivery
2006 CDI Handed down to daughter
2007 GL CDI. Wifes