One possibility is that you might have an open circuited rectifier diode in the alternator. That reduces the amperage (not the voltage) to less than one third of the rated capacity. I've seen a 65 amp alternator with one bad diode drop to less than 9 amps max output- - - -and the regulated voltage stayed normal. The car would run normally in the daytime, but it would drain the battery as soon as the headlights were turned on. The reduced capacity alternator couldn't meet the extra current demand of the headlights. Testing amperage output requires loading the battery down with a carbon pile load tester with the engine at a fast idle while monitoring the current output of the alternator. It can't be done with just a voltmeter.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!