Voltage readings across the battery, both Vdc and Vac, will give you a lot of information.
Check pre-start and post-start.

An area that caused my car not to charge, was corrosion on the surface of the alternator at the mounting location for the voltage regulator. The VR picks up its ground from the alternator case. A little cleaning and conductive grease keeps that problem at bay. (you only want a slight film of conductive grease, too much and it melts, causing shorts).

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )