I've had battery problems since I got this car, mostly because I needed a new alternator adjustment bolt to stop the belt from slipping and my '78 has an oddball bolt that took forever to find. I don't hear it slip very much these days, but lately the battery has been dead almost every time I go to start the car. Went to O'Reilly's and had them test the alternator while idling -- the results are it's only putting out 12V instead of 14 and is most likely on its way out.
Are there any other causes of a dead battery I should look into, and if not is it possible to fix the alternator instead of forking out a bunch of money on a new one?

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1985 190D 2.2L 717.411 5-speed
1983 300D Euro OM617.950