/// Check the negative battery cable. YOu cannot see the joint inside the batttery cable end clamp. If this joint opens up due to reistance and heat the battery will not charge and the car may not run on the unfilted alternator output. Use both of your jumper cable clamps to bypass that cable. I have found that the insulator can fail at the alternator bolt so that the output is shorted to the alternator case by that insulator failure. This problem will not come and go once that insulator fails.

Failed ring diodes will lower the output of the alternator. Some test equipmant can detect missing pulses.

A sudden demand for current makes the alternator very hard to turn and if you have loose or worn belts the alternator may stop turning at that current demand. Test by attempting to turn the alternator by pushing on the alternator cooling fan fins on a stopped engine.