Dallas Ward:
First, I think you are experiencing a voltage drop in ignition circuit that voltage regulator is reading, that caused alternator to charge battery at 15 volts. By that, the ING post on regulator is connected to ignition circuit that is showing low voltage for one or more reasons.
Is the fuel pump, electric choke, radiator fan, or other high use device feed from ignition circuit? If so, these are causing a voltage drop. Also corroded bulkhead connections, old worn switches with high internal resistance, and bad wire full of bad splices also can contribute to voltage drop, as well as corroded battery terminal, and grounding conductor to block connections.
Any and all additions to ignition circuit should be powered by relays, using the ignition circuit as the trigger, and battery or power post on alternator to feed the load. This will eliminate any parallel paths to ground, and greatly reduce current load on under sized conductors in ignition circuit, which will lessen voltage drop in that circuit.
Normal charging voltage should be 13.8 to 14.5 volts. Any higher and the battery will be over charging, and its electrolyte boiled dry over time
A battery killer.
Having battery in trunk can cause voltage drop as well just because it is so far away from alternator, and loads. Low voltage (12v) dose not travel distances well unless conductor size has been increased to compensate for losses. Conductor sizing can be calculated for any length, and load to lessen voltage drop.
What you need to do is clean up all ignition circuit connections, eliminate any heavy loads from ignition circuit utilizing relays. Check and clean all battery terminals, remove splices by running a new wire, and clean grounding conductor connections to block and chassis.