Wire Gauge Required Question

tenbear wrote:
What happens when you have, lets say, 3 panels in series and one panel gets in the shade? Is the resultant current reduced to the current in the lowest current panel?

Well, after a quick experiment this morning I have a partial answer to the question.

I put my two panels facing morning sun and looked at panel output Voc (open circuit voltage) and Isc (short circuit current) with the panels in series.

With both panels in the sun here are the results.
Voc 43V
Isc 3.1A

Then I put one panel in the shade and faced it away from the sun.
Voc 41V
Isc 3.0A

Humm..... NOT what I expected, I expected the Voc to drop to 20ish volts.

So then I covered the shaded panel with a towel.
Voc 20V
Isc 3.0A

Exactly what I would expect to happen.

Now giving it some thought and my ancient knowledge of solar cells, the results so far make sense. Panels in shade do recieve solar radiation, so cell voltage goes up per the physics of the cell. However the output impedance of the shaded cells is going to be very high.

covering the panel with a towel eliminates virtually all solar radiation from impinging on the panel and panel Voc goes to zero.



Now, What I suspect but since my batteries are fully charged I couldn't verify, is that under Isc or Imp conditions with one panel in the shade the combined panel voltage will drop to about 20V. So max power goes down to a single panel. The high impedance of the shaded cell will drop it's output voltage to near zero with any noticeable current draw.

I was measuring panel voltage with NO load, NO current draw to the shaded panel is still a voltage source, just a very high output impedance voltage source. Attach a load to it and voltage from that panel will drop quickly.

next time my batteries are discharged enough to draw decent current I'll repeat and see if what I expect happens, with one panel in the shade output current will drop almost in half.

So, the panels act as two non ideal voltage sources in series. Each source has an output impedance that is determined by the solar radiation impinging on the panel. Since they are in series, when hooked to a load each panels output voltage will drop based on it's output impedance, but the total output current will not noticeably change. The total output voltage will change and hence the total output POWER will vary based on the combined panels output voltage and current.