A 12 volt draw on battery from a one wire alternator

Mechanic swapped my alternator to a one wire during a engine swap. Now the car is eating batteries left and right. Still charges at 14 but just checked draw on negative cables and had 12 volts. Disconnected alt. And draw went to 1.3. I'm new on working on cars so any help is appreciated and you may have to explain things. I'm not sure what some stuff is.