You didn't mention what year your car is, but I would reasonably guess that your voltage regulator is starting to go out on you. I'm not sure but on newer MBs, it's internally mounted in your alternator...translation, you don't get to replace the regulator, but the more-expensive alternator it's embedded in!
I'm not sure if the problem will show up in a charging system test. Failure is abrupt in many cases. I've had vehicles that had a clean bill of health with the electrical test...only to have the charging system fail days later!
But as a backup measure, check your fuse compartment for bad fuses, and inspect the engine area for any loose connections...