The alternator light and and low voltage warning light mean that the alternator out put is below battery voltage. That information doesn't gel with the positive charge on the ammeter. I would be taking the alternator to a work shop for checking/overhaul.
The alternator may have a dropped winding on the stator, a bad diode or two, or worn brushes, also could be a failed regulator.
P.S. An alternator does not put a load on the engine or battery on start.