Auxiliary battery malfunction

  mercedes/e-class/2010-2016

I have a 2010 e63 amg.. About 1 month ago my car started having starting problems it wouldn't start so I jump started it. It starts for a day or a couple of hours then when I try to start it again it needs another jump start. This also cause the aux audio option to completely disappear from my audio options list and after 1 month i just got a message that my auxiliary batter malfunction. I took it to Mercedes service dept two weeks ago but they said the battery was fine and I brought it back home and then it wouldn't start again..... any suggestions.

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