I have a 58 half ton with a 12 volt system.My amp gauge needle in on the center dot while running or on the road with high rpm. When I turn on the headlights it moves back just a bit toward the discharge.Should it not move toward the charging side while running hard. The ohm meter shows 13 volts at the back of the alternator and 12 volts from the battery while running. Is this a normal reading for the amp gauge.