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I thought the controller always showed the max potential harvest, when evidently, it is only displaying the usable amps based on the needs of the batteries"
Not if there is a load too. Then it (at the controller) will show the total demanded amps up to the limit of the output supply at the time.
I used to think my Trimetric showed "net amps" but was corrected on here by somebody who knows better. The Tri shows only the amps in or out of the battery. The math works as though it is net, but it isn't. You need two ammeters and then can derive it all.
How does the wiring for your "battery meter" (Magnum monitor) get to show the amps to the battery from the solar controller? AFAIK you need a shunt somewhere there. Interesting. I do wonder if you are seeing what you think you are seeing, but you know the set-up and I don't.
In the OP you mention that the amps are coming from the panels. Actually with MPPT controllers (which have buck converters) it is not really like that. You get input watts to the controller, then output watts, and only now can you take your amps from those output watts.
The input amps depends on the input watts divided by whatever voltage the MPPT controller is running itself at. Not much related to output amps because of the buck converter. With ordinary PWM controllers, there is more of a direct relationship between amps from the panel and controller output amps.
