BFL13 wrote:
The amps showing at the controller depends on the MPPT's output watts. You divide the output watts by the battery voltage to get the amps. So yes, the SOC of the battery affects the amps you see.
EG output watts 200:
Battery v 13, amps 200/13 = 15.38
Battery v 14, amps 200/14 = 14.29
Not all those amps go to the battery. Any load gets first crack at the amps, with any left over going to the battery. (Same as with any battery charger when the battery is connected to loads while also being charged--as with an RV converter)
To get the amps to the battery, you need a second ammeter just for that, downstream from the controller. EG, a Trimetric monitor with the controller's output going through the Tri's shunt.
Of course there remains the usual tapering of the amps to the battery when they get nearer to being full. The SOC when the amps start to taper depends on the charging rate. Higher rate, lower SOC when amps taper. Solar usually is at a low rate so amps would not taper from that until SOC in the 90s percent range.
You also have the IV curve for your panel, where as battery voltage goes up amps start to taper a bit until they fall right off at the knee of the IV curve, but this will be a higher voltage than what you would set the controller to get to, except with MPPT controllers using Vmp before they go to PWM after the Bulk stage.
Thanks. I had a competent fellow install this stuff, then had to set the camper aside for 10 months. But I recall what you're saying. I've had math beyond calculus, so the workings of the MPPT and getting my voltage up to allow enough headroom for the MPPT to cook, is something I recall discussing before.
I had specified a pair of Morningstar controllers with internal displays, but after talking to my consultant, decided to go with a more simple controller that doesn't generate as much heat or have internal displays. I just put the batt meter on the Magnum 3012 and so I'm getting a read out at the battery of what's going in. I don't have a display of what's coming in on the controllers inside my fifth wheel, but I'm ok with that.
I was just confused on what the controllers where telling me but I think I got it now. Thanks.
