1978 Buick Lesabre car runs off battery

Electrical problem
1978 Buick Lesabre V8 Two Wheel Drive Automatic

78 buick lasabre runs off battery. 2nd new battery and new alternator is rigged. Red wire from the top of my alternator has been cut, (there is a red and a black wire), and rewired to the alternator. It works, but what is the red wire they cut? It runs under the car and above the starter.