Charging at 15 Volts

Well, the thing is, 15 volts is too much.

What are you running for alternator / regulator?

The very first thing is to check if the regulator is operationally grounded, and if the sensing voltage IE the ignition voltage supplied to the regulator is "same as battery."

If the two above items are OK, test the battery, and if it checks out, replace the regulator with a good quality one.

SO once again..............

Turn the key to "run" engine off. Hook one probe of your meter to the regulator "IGN" terminal, and the other to battery POS post. You are hoping for a very LOW reading, and in no case, more than .2-.3V (3 TENTHS of one volt) ANY voltage in this path ADDS to regulator charging voltage setpoint

Check the ground. With engine running, and charging at an RPM to simulate "medium cruise," stick one probe on the battery NEG post, and the other directly onto the regulator mounting flange.

Once again, you want to see very low voltage, zero would be perfect.