If you manually apply "full field" 12 V across the alt field terminals, it should read <15 V at the battery. If over-charging the battery, it will start breaking the water into H2 & O2 but the voltage won't go up..
Bill that is only true on a battery that is "down." On a fully charged battery, and especially if you have a nice big alternator, such as 60A, you can EASILY drive the battery voltage right up there. I've done it many times