One bad diode will cut an alternator output to less than 1/3 of its rated capacity. I've seen a "65 amp" alternator produce less than 10 amps max while still producing normal regulated voltage (14.5+) It would keep the battery charged during daytime operation, but drain the battery with the headlights on. An electric fan will do the same thing- - - -draw more amperage than the alternator can produce, and grab the excess current from the battery. Voltage is only one leg of a 3-legged stool- - - -the others are amperage output and load. Have your spark chaser do a "full current" check on the alternator if he knows how to do that test.
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!