Brushes worn out on the alternator, my 300D did the same thing last month, although I managed to get to Autozone and buy another battery instead of getting towed home (didn't like the one that was in there, although I should have kept it).
you can either replace the voltage regulator or get new brushes installed in it by any automotive electrical shop. Voltage regulator is in the back of the alternator, held in by two screws. Just unscrew and pull it out. You will notice that the two carbon brushes are only 3-4 mm long. New regulator will have brushes 15 mm long or so!
Once you have the new brushes installed and regulator replaced, it will charge again.
You probably didn't notice that the alternator lamp was glowing dimly, just enough to notice it at night.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!