As said bad reg or no reg ground. Less likely would be a grounded "green" field wire or alt field is grounded on the green wire side (brush assy). ASAP! pull the connector off of the reg till you get it solved. Deepcycling the batt by letting it run down with no charging is FAR less damaging than driving it with it full fielded like it is right now. That'll cook the batt/damage wiring & terminals/stress bulbs & accessories that are on as they ain't designed for 15+ volts