My Project- Building a 15 watt solar panel.
I have created a few groups of thin-film Copper Indium di Selenide (CIS) solar cells linked in series. The groups of solar cells will in-turn be linked in parallel to produce a desired 15 watt output. Each series group of solar cells should produce ~18 volts and ~.085 amps in direct sunlight. The cells are rated at 5V open circuit, and about 100mA short circuit.
My Problem-
I've noticed some of the series groups of cells seem to put out more voltage than other groups: 10.5 vs. 12.5 in lighting I'd describe as poor. The groups were tested under the same lighting- overcast, and through a window. I will be rechecking the voltage produced by each series groups this weekend- weather permitting. But am I seeing a potential problem? Is it normal to see a 2 volt difference between groups of cells I plan to tie together in parallel?

I would look for one cell that is backwards.

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Thanks for the reply- All the cells are lined up correctly. I mounted each series on a thin clear piece of plexiglas. I double checked the +/- from underneath and all looks to fine.
I never looked to see if the amps showed a similar discrepancy. The whole idea of quickly checking the voltage to make sure my soldering connections remained after I mounted the cells to the plexiglas. When I rechecked some of my previous work I notice the voltage discrepancy among some of the series.
I'm hoping this is normal condition for cells I'm using or that my readings are off- Hopefully with more posting and more testing this weekend I'll have a better idea of what's going on.

I always understood that thin films needed a "break-in" peroid before output stabilized. Just a thought.

Maybe some of your connections are better then others? Have you tested output under load. That maybe a better way to check?

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Thanks for the reply-


All the cells were set out in strong direct sunlight for over 3 hours (without a load) before I started using them in this project. I hoped that exposure would take the cells through their “break-in” period.
My worry is that some of my connections are not as good as they should be-


I just retested the six series groups I made @ 4:30- it was sunny and clear: four groups had voltages between 18.2 and 18.5. Two groups however did not make it to 18 volts- one series group was at 17.2 and the other at 17.5.
The reading seems to be more in line than my original test. I still have no idea if this is a good reason not to use the two 17 volt groups.

For thin-film, it takes 6 months for the cells to settle.

I would test all strings under a fixed resistive load. Then go from there.

__________________
My solar setup:
http://home.comcast.net/~n3qik
Updated 6-7-2009

Home Automation: http://68.81.42.56:5800/ Password = guest
Software/hardware 80% complete.

Very Good-
Do you (or anyone reading) know of any reading material or website where I could do some research on the CIS cells? I've never heard of the six month settling issue- I'm now guessing everything is probably fine, but I run some more tests and see what I can learn.

I know this from the Unisolar panels I have. They have that disclaimer on any of the power charts.

__________________
My solar setup:
http://home.comcast.net/~n3qik
Updated 6-7-2009

Home Automation: http://68.81.42.56:5800/ Password = guest
Software/hardware 80% complete.