Solar Panel Voc-MPPT?
Wondering if there is any relationship between measured Voc and MPPT performance with a 24v panel. My 12v panel has a Voc rating of 21.9 and Isc of 8.2, but I often see 8.2ish when Voc is only 20....
Forum Discussion
Voc and Isc are somewhat meaningless. at Voc power =zero, since panel current equal 0 with open circuit voltage, e.g. NO load. Isc is similar, at Isc, Panel output voltage is going to be very low so power is low that is with the panel output short circuited. Have never measured panel voltage at Isc, but I bet it is in the few volts range or less with a 24v panel, so power is very low.
The value in having a Voc and Isc spec it that it tells you what the maximum panel voltage is and max current is. As long as these numbers are below the max ratings for your controller you won't damage the controller.
the graphs from red 31 show exactly this.
the critical ones for determining max output power are Imp (mp=max power) and Vmp, which usually are fairly close to the Voc and Isc numbers. Note the "knee" in the curves from red 31. and the peak of the curves from red31 as well.
A well designed MPPT controller will constantly adjust the effective load to track Vmp and Imp for the panel under current conditions. Not sure exactly how, but I can imagine it dynamically varys the MPPT input load and watching output power and finding the peak. And the max power point for voltage and current is a function of temperature, sunlight etc. and depending on sunlight, temp etc. the Vmp and Imp numbers may not be the same as shown for the panel. Those number are for a specific set of conditions, usually panel temp of 25C and certain solar radiance level.
can you measure what Vmp and Imp are?? well, I think so. if you put a voltmeter at the input terminals for the controller and a current probe around one input conductor you will get Vmp and Imp (Voltage for max power and current for max power) for the amount of solar your panel is getting. Compare this power (Vin * Iin) and compare it to the Vout and Iout product from the trimetric. That will give you the efficiency of the converter at that set of input conditions.
then go back directly to the panel and measure the panel ouput voltage. You want that voltage to be as close to what you mesured at the controller input as possible to reduce power loss in the cabling.
PM if you need more info or have more ?
The value in having a Voc and Isc spec it that it tells you what the maximum panel voltage is and max current is. As long as these numbers are below the max ratings for your controller you won't damage the controller.
the graphs from red 31 show exactly this.
the critical ones for determining max output power are Imp (mp=max power) and Vmp, which usually are fairly close to the Voc and Isc numbers. Note the "knee" in the curves from red 31. and the peak of the curves from red31 as well.
A well designed MPPT controller will constantly adjust the effective load to track Vmp and Imp for the panel under current conditions. Not sure exactly how, but I can imagine it dynamically varys the MPPT input load and watching output power and finding the peak. And the max power point for voltage and current is a function of temperature, sunlight etc. and depending on sunlight, temp etc. the Vmp and Imp numbers may not be the same as shown for the panel. Those number are for a specific set of conditions, usually panel temp of 25C and certain solar radiance level.
can you measure what Vmp and Imp are?? well, I think so. if you put a voltmeter at the input terminals for the controller and a current probe around one input conductor you will get Vmp and Imp (Voltage for max power and current for max power) for the amount of solar your panel is getting. Compare this power (Vin * Iin) and compare it to the Vout and Iout product from the trimetric. That will give you the efficiency of the converter at that set of input conditions.
then go back directly to the panel and measure the panel ouput voltage. You want that voltage to be as close to what you mesured at the controller input as possible to reduce power loss in the cabling.
PM if you need more info or have more ?
