this exact thing happened to me last night.
this morning and a new 130 amp alternator (151$ AutoZone) later, and I am back in business.
the Battery light came on (for the first time ever), after driving around and getting voltage spikes upwards of 16-17 volts!!! then, after pulling out of the gas station, nothing but battery voltage.
I have had an alternator go out before (two years ago). I was able to drive the car for 5+ miles before the battery was completely dead (new battery).
Hope this helps.
95 Red Cobra 357RWHP/386RWTQ TW heads, TwEECer, TFS R, 36 lbers/PRoM 77,70mm TB, 347, Al flywhl, 1 3/4 LTs, Dr. Gas X, 3" out the back phb/ta, griggs C/O MM C.C plates