Charging at 15.1 volts. Anybody?

Hello, Ever since I bought this dart it has charged at 15.1 volts. It's a 74 Dart Swinger 318. I've given it a full tune-up Plugs, wires, Cap, rotor, new distributor, Coil, voltage Regulator, Alternator, all filters, fuel pump, new carburetor, etc. I haven't changed the battery, it seems fine. Checking the battery with the engine off it reeds 12.88 volts. With the engine running at about 750 rpm it reads 15.1 volts. Could it be a bad battery? Any suggestions? Thanks, Christopher