No charge

The factory gauge shows neutral. As in no charge or discharge. Then when we go from flag on alternator to battery the gauge shows charging. We also tested the output at the battery as 12.8 and diminishes more when the headlights are put on, or a blinker. The charge falls away and headlights dim. We have replaced the voltage regulator as well. No new changes to the wiring. He's been driving the car for several months and it just began running down on charge, then developed the ignition starting issue on top of it.