Alternator or battery. Since you get higher voltage at idle I would imagine it is the battery. You can take it to Autozone (or the equivalent) and get it tested for free.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD