Was it chaging more then 13.50 volts when you first got the car?

Back to the basics.
As the other member suggested check the Battery voltage by it self after it has sat several hours.

Detach all of the Battery cables at both ends and clean the contact areas so they make good contact. Do the same with the Ground strap.

When I put in a Battery that was not made for the Mercedes I had to change to a new longer ground cable. Once the cable was changed I found I had gained a whole extra volt of changing. So it was likely that the cable had some internal corrosion going on.

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84 300D, 82 Volvo 244Gl Diesel