1972 Chevy Chevelle Checking alternator output

I have replaced the alternator on my Chevelle 2 times already and am still having issues with the battery going dead if I don't keep it charged. The new alternator has been bench tested at the place of purchase and I am told it is OK. I am trying to determine if it is actually putting out more than battery voltage. If I check the battery with a voltmeter it reads 12.6 volts. With the vehicle running I then check from the larger red wire on the back of the alternator to ground and it also reads the same 12.6 volts. Should this read the same or should I be expecting a 14+ voltage reading?