Started my 91 Bonneville SSE up for the first time in a couple months today, put a brand new battery in, all seemed fine, ran very nicely. Did about a 2 mile trip and the check battery light came on, and the voltage gauge was right around the 1/4 mark. When I got home I did the "turn stuff on and see what happens" test, turning on the radio did nothing, but turning on the headlights, the gauge just plummeted down to the 8 mark. Car was still running off of battery im assuming as it didnt die. Turned it off for a bit and came back out, it started up fine, but turning anything on inside resulted in the gauge once again plummeting down. I have access to a 94 Cutlass with a good alternator, and was wondering if it will work in my Bonneville? Or if this even sounds like an alternator issue...

Try these two tests.

https://www.gmforum.com/t279492/

https://www.gmforum.com/t279494/