High volts

if it's a stock alternator it's the voltage regulator that is allowing your battery to overcharge.

however, it's not uncommon for it to charge at 15v (it has to be higher than the battery or it would not charge the battery if that makes sense)

what kind of batteries are you using? are they brand new? you could have just been trying bad batteries.

if it's a denso type alternator, with built in voltage regulator, you dont need the voltage regulator. connect the voltage sense wire to where the battery positive wire connects into your harness.