My 300E had the same problem. The alternator bench tested as being good, but the battery was not charging. When I replaced the alternator my problems were solved. It is probably an intermittent problem caused by worn brushes as 420SEL indicated. Since my wife drives the car and hauls the kids around in it, I went with a new alternator versus fixing the old one.
Experience is something you don't get until just after you need it.