yes, 15 is to much, normal range is between 13.8 and 14.2 volts. I had a similar issue with their alternator, turns out I had a weak ground and also the wire from the alternator has to go all the way to the battery. I had mine connected at the solenoid which was connected to the battery there. Why it made a difference? I don't know. But it had to run right to the battery.