Salvo wrote:
You may have a measurement error. Where are you measuring Vmp? It should be directly at the panel, not at the controller.
In any event, the controller is starved for volts. This system should show better results if battery voltage is dropped to 12.5V.
ktmrfs wrote:
Post #5
Parallel connection with 90ft of cable
Vmpptin=14.60V (TX3DMM)
Impptin=8.3A (DM330)
MPPT power in=14.60*8.3=121W
Vbat= 13.50V (TX3DMM)
Ibat=9.0A
Power to battery= 121.5W
No signficant measurement error. Math and calculations are very close.
As noted in the writeup I'm NOT measuring voltage at the panel, I'm measuring the voltage and current AT the MPPT controller input (Vmpptin, Impptin) to determine available power to the controller. Yes, it is getting starved for voltage, the 90ft of approx #8 cable has a 1.3V drop across it, so parallel connected panel output voltage is 14.6+1.3=15.9V. That 90 ft of cable in parallel connection is about a 1A disadvantage in charging current compared to a series connected set of panels with 90ft of cable. Driven by system losses between the panel and controller that are noticeably higher for the parallel case than the series case with the same cable length.
Below spec'd peak Vmp & Imp, yes, but it's fall, low sun angle, high latitude.
And battery voltage during charging is determined by the battery state of charge, and charge current. In this case with batteries at near 80% SOC 9.0A battery input current, battery terminal voltage rises to 13.5V. Using a 12.5V voltage source, battery acceptance current would likely go down, not up.
Now if the batteries were near 50% SOC, I suspect I would see higher current, because I would also see a lower battery voltage, but the power (watts) to the battery wouldn't appreciably change, it would still come out near 120W.
As to lowering the voltage to say 12.5V, sorry, when charging batteries you can choose the charging current or the charging voltage you CANNOT set both of them. The battery has some internal impedance. Solve the loop equation, since you can't change battery impedance, if you SET the voltage, the current will be determined by the loop equation solution. If you SET the current, the voltage will be set by the loop equation. You CANNOT set both arbirarily. Setting the voltage to 12.5 V would LOWER the charging current, not raise it.
think about it. If I "lowered" the converter voltage to 12V I wouldn't get 10A going into the battery, The Battery would likely be supplying current TO the controller and probably blow the fuse inline with the converter!!
The controller is behaving during bulk charging mode as a constant current device.
