Quoted from this ebay auction:

For sale 250 6"x6", 3 bus bar multi-crystal solar cells. These cells are true A Grade factory tested (not color matched), binned together and each cell is 4 watts. This kit is not a mix of high and low power cells like you will see on other sites. DON'T buy mixed power cells. The solar panel will be only as good as the lowest power cell. So adding in 1 cell that is 3 watts with 99 more cells that are 4 watt will only give you a 300W panel peak, not 397 watts.

Power Ppm (W) 4.0
Max. Power Current Ipm (A) 8.4
Short Circuit Current Isc (A) 8.1
Max Power Voltage Vpm (V) 0.510
Open Circuit Voltage Voc (V) 0.55

Is the paragraph true? Do the cells work like batteries, and their power output is dependent on their internal resistance? So by putting one cell with a higher IR in series with others of a lower IR it would limit the current?

How can the cell put out more current (8.4A) in any situation than it does when shorted (8.1A)? If this is true, it makes me think that the amp output is not dependent on internal resistance.

Ho do the cells behave in <100% sunlight? If there is cloud cover, does the amp output go down or does the voltage go down?

Will the voltage sag like a battery? For example, when they tested the short circuit current and read 8.1A, can I assume that the voltage output was definitely NOT still .51V?

 

I'm only going to answer this question: "<100% sunlight? If there is cloud cover, does the amp output go down or does the voltage go down?" since I believe it is the easiest.

Yes, if there is a cloud over, the amperage will go down, the voltage might not be affected as much. I worked on a project which involved solar panels and wind turbines as power sources, the most difficult thing to get was amperage. Our volt meter usually displayed a stable 22Volts whether there were clouds, or even inside a building. The problem was the amperage, to get a high amperage the sun had to be shining like we were in the desert, perfect almost no clouds condition.

 

C'mon I know someone knows this.

BTW I received a email notification that Dmahalko had replied but it's not here. Was it moderated or what?

 

Simply put, batteries are modeled as voltage sources and PV cells are modeled as current sources in parallel with a diode. (a non-ideal source that can be connected in series)

http://www.ni.com/white-paper/7230/en

http://www.solar.udel.edu/ELEG620/13_PV_modules.pdf

 

Basically, from what I understand (I had many questions about how photovoltaics should be modeled as well), the cells act like diodes except the current through them is "flipped" on the x-axis. So, the higher the voltage, the lower the possible current, and the voltage hardly changes the possible current at all until you get passed a sort of "bias" point (if you want to call it that). Hopefully, this image will help you get what I'm saying: http://4.bp.blogspot.com/-jbCwRIvMpXg/T04ukQwK2NI/AAAAAAAAANE/xxUNJleozUY/s1600/290201.jpg
The more sunlight that hits the cells, the higher the curve is essentially "pushed up". Now, to figure out the voltage and current output given a resistive load (like a heater or something), you'd use something called a "load-line" to basically plot the operating points of the cell and of the load and find where they intersect. http://en.wikipedia.org/wiki/Load_line_(electronics)

Hopefully that's all correct and informative enough for your needs. Any extra input from more advanced members would be greatly appreciated by everyone I'm sure.

 

I wrote a reply but then wasn't sure my reply was correct. It looks like what I had intended was correct.

Take a look at this:
http://www.solar-facts.com/panels/panel-diodes.php

Solar cells will resistively waste power if they are in the shade, and they don't block reverse flow if they aren't producing power. So at night the cells will actually consume stored power, if there is no blocking diode.

The blocking diode prevents discharging in the shade and at night. But you need a certain voltage rise to overcome the diode, about 1.5 volts, so about three 0.5v cells on a panel aren't productively producing power but are just there to overcome the diode.

And when cells are wired in series, if some are shaded, they resistively waste power produced by other cells. So for that you have bypass diodes around individual panels... and bypass diodes also need about three 0.5v cells to overcome.



Also the photovoltaic cell power output ratings usually assume you are at the equator in full sun at the solar equinox. There is a loss of power as you go towards the poles because sunlight passes through more of the atmosphere, so the sunlight is dimmer and the cell efficiency is lower. Clouds also reduce panel output or may halt power production completely.

This is called Solar Insolation.. here are some maps of how this works:
http://en.wikipedia.org/wiki/Insolation

This fluctuates throughout the year due to sun angle change but is normally strongest in deserts with few clouds, and near the equator.

 

...How do the cells behave in <100% sunlight? If there is cloud cover, does the amp output go down or does the voltage go down?

The best way to understand solar cells is that they are a constant current supply, and the current is set by the amount of sun. Also, there is a "zener diode effect" formed in the silicon of the cell.

If there is no load attached, the panel voltage will rise until the amount of current conducted by it's zener equals the current generated. This happens on most 12v panels at about 22v.

Into a short circuit (at 0v) there is no "zener effect" as the voltage is too low, so the current you read into a short circuit is the actual amount of current generated by the sun.

Anywhere between 0v and 22v will be a situation where some current comes out of the panel, and some is wasted in dissipation in the internal zener effect.

In fact, if you short out the panel it remains cooler than if you leave the panel open circuit where the panel is dissipating a lot more power internally.

Now to answer your question; if in less than peak sunlight the panel simply produces less current (and still the same voltage). In VERY low light the current is so low even the small zener leakage will drag the panel voltage down.

...
How can the cell put out more current (8.4A) in any situation than it does when shorted (8.1A)? ...

It can't. The max current that can be drawn from the cell occurs at 0v output, taking current from the cell when the cell is >0v means that some current is lost due to the internal zener effect.

At 12v about 15% to 20% of the current is lost to the zener effect so the panel output current is down to 80-85% of the short circuit current value.

By the time the panel is at 22v ALL the current is lost to the zener effect.

 

The best way to understand solar cells is that they are a constant current supply, and the current is set by the amount of sun. Also, there is a "zener diode effect" formed in the silicon of the cell.

If there is no load attached, the panel voltage will rise until the amount of current conducted by it's zener equals the current generated. This happens on most 12v panels at about 22v.

Into a short circuit (at 0v) there is no "zener effect" as the voltage is too low, so the current you read into a short circuit is the actual amount of current generated by the sun.

Anywhere between 0v and 22v will be a situation where some current comes out of the panel, and some is wasted in dissipation in the internal zener effect.

In fact, if you short out the panel it remains cooler than if you leave the panel open circuit where the panel is dissipating a lot more power internally.

Now to answer your question; if in less than peak sunlight the panel simply produces less current (and still the same voltage). In VERY low light the current is so low even the small zener leakage will drag the panel voltage down.



It can't. The max current that can be drawn from the cell occurs at 0v output, taking current from the cell when the cell is >0v means that some current is lost due to the internal zener effect.

At 12v about 15% to 20% of the current is lost to the zener effect so the panel output current is down to 80-85% of the short circuit current value.

By the time the panel is at 22v ALL the current is lost to the zener effect.

That's the best explaination I've ever read about this subject. Thanks a ton, sir. I'm gonna have to experiment with the mathematics a bit, see if I can find anything interesting, and to see if I can get a matching characteristic curve to what I see often online. Maybe find an easy way to find the maximum power voltage/current? Who knows.

 

Sure the max power is found at a higher voltage than the typical, so with a 12v solar cell is found at about 17v. Just adjust the load current, and measure both V and I. Then multiply to get the Power. It's easy enough to find a panel's MPP (max power point).

Thanks too for the nice words, it is appreciated.