The 'external controller' I was asking about was a Link (not an external controller on the alternator). I was postulating that IF there was an problem, you would see an error code that would alert you.

My Heart 2500W unit (which I just replaced) used to have two (2) input circuits -- one for the inverter and one for the charger. You had to have both on. An older one, I recall only had one. You might check that from your manual. If you don't have one, you can (or used to be able to) download it from Xantex's website. If you have a "LinK" unit or any of their other equipment, DOWNLOAD those *.pdf's now, before they become obscure history.

My suggestion: Borrow a Honda or other portable generator and just try to power-up the charger. If the batteries are "down" you may exceed the capacity of the little portable.