Check belt tensioner -- if it has failed, the alternator can slip and the battery will eventually go flat.
also, watch the alternator light at night -- if you get a dim glow fading in and out, the brushes on the voltage regulator are shot, and it need replacement. 5 min job with a screwdriver, just swap the regulator, or take the old one out and take it to an alternator shot and have new brushes installed. Bad brushes can be intermittant, especially working cold but not hot.
If the tensioner and brushes are OK, you probably have bad bearings, replace or rebuild alternator. With the belt off, the alternator should spin freely, no binding.
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!