CA Traveler wrote:
In Post #3 and Post #9 ktmrfs observed that serial panels produce the current set by the highest output current while the second shaded panel has reduced voltage.
I expected the current to limited by the shaded panel. Ie At 1000W a 8A panel would produce 4A at 500W based on the typical IV curves.
So how did ktmrfs see 4.1A in direct sun and 4.1A with one panel in the shade and reduced voltage for the shaded panel?
CA Travel. Great questions.
In retrospect, I probably should have made some MPPT voltage measurements, it would have helped explain what is likely going on with series panels that are shaded. sorry for the oversight.
And at the same time had I done a parallel shaded output, it also in retrospect would have given better insight in how panels operate in series vs. parallel.
Here is what I believe is happening.
If it was a SINGLE panel, or two panels in parallel with one completely shaded, then yes, I would have expected panel output current to drop roughly in half, just as you outlined.
Effect would be the same, roughly 1/2 the current to the battery. That's what I've qualitatively observed in use with parallel panels.
I didn't directly measure the second shaded panel voltage, but the voltage set by the MPPT controller for max power dropped. I suspect it dropped from about 30V down to 15V or so into the MPPT controller. Since the panels are in series the current remained 4.1A, but the panel combined voltaged dropped probably to what a single panel would output so power dropped roughly in half. Output impedance of the shaded panel is still low enough to not impact the current output capability.
So series or parallel, shading 50% of the total panel area cuts charging current in half. In the parallel case, or a single panel case, current to the contoller will drop by 50%, but panel output voltage remains almost constant. in the series case panel ouput voltage drops by 50% but current remains almost constant and ends with a roughly 50% drop in charging current. In either case panel output power drops by about 50%.
Note, this is when a whole series panel is shaded, or how one of two parallel panels is completely shaded. how partial shading of a panel or panels effects output I didn't test.
Interesting note. If instead of shading a panel, you completely cover it with say a thick blanket, that is another story entirely, at least with the series case. in that case power and charging current drops to almost zero. Even in shade the panel will get some solar energy, enough to keep the panel output impedance "low enough" but not supply much current. When NO solar energy reaches the panel it's output impedance must go quite high. My first attempt at shading was t throw a big thick towel over the panel, and current went to less than 1A. removed the towel, rotated the panel 180 degrees so it was shaded, then current dropped by 50%. Picked the panel up and placed it in the shade between trailers, current stayed at the 50% level.
However, in a parallel panel case I suspect putting a towel over one panel will just drop the current in half. The covered panel has high output impedance but is in parallel rather than series and so it's effect is minimal.
But this kind of shading isn't real life either, so in a practical sense, it's a moot point.
