Post #4
2)Can one predict accurately output power by measuring MPPT input voltage and panel output current?
3)In late fall at high latitudes how much power can I expect to get from the panel under optimal light?
I configured the panels in series with the shortest connection path to the controller possible and started taking measurements. With the panels in series I was able to measure panel output current with the DMM. In my case, there is not an easy way to measure the output voltage of the panel easily other than at the MPPT controller input. And its panel input power vs. output power we care about anyway. Vout (and battery voltage) measured with the DMM at the controller output terminals. With the configuration I have, the voltage drop between the controller out and battery terminals is only a few millivolts for the currents of interest. So rather than using the trimetric voltmeter with 0.1V resolution, I choose to use the Tek TX3 DMM with 1mv resolution. Battery current measured with the Trimetric with its 0.1A resolution.
So here are results for series connected panels:
Vmppt in= 30.74V (TX3DMM)
Impptin= 4.442A (TX3DMM)
Panel output power= 30.74V*4.442A= 136.5W
Vbat= 13.36 (TX3DMM)
Ibat= 10.3A (Trimetric)
Controller output power = 13.16*10.3= 135.5
Controller efficiency = 135.5/136.5=99% I don’t think SO! I suspect that the trimetric current resolution of 0.1A and trimetric accuracy is the issue. 10.2A for current yields 97% efficiency. So even 0.1A error is significant here.
And the 135W panel output power seems (at least to me) reasonable given the latitude and time of year. 135W is 83% of rated.
I repeated the measurements for parallel connected panels. In theory with a very short low resistance interconnect results should match very closely a series connection.
In the case of parallel panels I could not use the TX3DMM to measure current, but had to resort to the clamp on current probe, an unwelcome but unavoidable additional variable.
And here are the results for parallel connected panels.
Vmpptin= 15.43V (TX3DMM)
Impptin= 8.8A (DCM330)
Panel output power = 15.43*8.4= 135.8W
Vbat= 13.30V (TX3)
Ibat= 10.3 (Trimetric)
Controller output power = 13.30*10.3= 136.99W
Controller efficiency= 137/135.8 >100%
NOTE: I’m nailed by two low resolution current measurement tools, the DCM330 and the trimetric. So efficiency measurement is meaningless. However, output power and current are close enough to “identical” to the short series configuration that I’ll call them equal, which is what one would expect.
I’m not going to “dry lab” the numbers to try to correct the efficiency.
Conclusion: yes, by measuring controller input voltage and current you can get pretty close on predicting output power. Close enough for me anyway.
And looks like in the fall at 46N latitude, I’m getting about 135 Watts from a nominal rated 160W panel, or about 85%. Sounds pretty reasonable to me, but would be interested to know what others are getting under similar conditions.
