Once I was replacing the alternator and voltage regulator on my sister's old Datsun, and something didn't seem right.
I checked the charging voltage, and it was fine at about 14.5 volts, I happened to switch my multimeter to AC volts on a hunch, and measured like 100 VAC going to the battery.
Frickin Kragen parts, never again. I went through 3 alternators and regulators till I got parts that worked.
Check for AC leakage?