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How do you test a solar panel in winter?

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Posted by mike on February 1, 2008, 3:29 am
I found a solar panel locally at a betterthanaverage price.
It's winter, overcast, raining.

Short of taking along a 5KW light panel, are there any
tricks one can use to evaluate a used PV solar panel in winter?

For example, can I use a big photoflash and look at the output
transient on an oscilloscope and learn anything helpful?

It's not that I don't trust the seller...YES IT IS.... ;-)
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Posted by mike on February 1, 2008, 6:00 am
mike wrote:

I answered  my own question.

I have a small 6" square solar panel.
I'd plotted its output in direct sunlight.
Maximum power was with 220 ohm load.
Put out about a watt at 14.1V.
I lashed it up with a 220 ohm load and hooked
it up to the scope.
Flashed with my tiny digital camera flash.
Got over 20V out of it for 40 microseconds.
So, does look like there might be some usefulness
in a flash test.
Any obvious flaws in this test?
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Posted by spaco on February 1, 2008, 6:19 pm
 That's a good question.   I had similar issues when changing my test
setup's  wiring at night and wanting to make sure that it was going to
work properly the next morning.

I have read that solar panels that have individual cells do some funny
things if some of the cells in the panel are shaded.  Something about
the total panel output dropping to the output of the worst-case cell.
    I guess that some panel mfrs attempt to minimize this effect by
using  blocking diodes internal to each panel, but It would seem to me
that too many blocking diodes in the overall ckt would cost you in power
to the load.
  So, one might not notice this with a 6 inch sq. panel, but your flash
probably won't  illuminate a one meter sq or larger panel equally all over.

   A little off topic, but I have seen ads where "they" talk about using
Schottky diodes for this application.  I guess the assumption is that
they have lower forward voltages?  I can't find data to support this,
though, so there must be some other reason for using them?

Pete Stanaitis

mike wrote:

Posted by Jakthehammer on February 1, 2008, 6:30 pm

Schottky diodes:

A low forward voltage "drop".  What does it mean "Voltage Drop"?

"Meaning low resistance", it allows you to receive better power output.
Typical applications include discharge-protection for solar cells
connected to lead-acid batteries.

You can't test PV in winter, using incandescent light is not the same as

Posted by spaco on February 1, 2008, 10:58 pm
 Sorry; I guess I didn't ask the "forward voltage drop question" properly.

I know what forward voltage drop is.   The issue is this:

With silicon diodes, there is usually a drop of 0.6-0.7 volts and you
can get them withe pretty high current ratings and pretty high PRV's.
With germanium diodes, the drop is about 0.3 volts, which would be
better, but I don't think they can handle large currents nor do they
come with high PRVs.

The Schottky diodes with enough current carrying capacity that I have
found specs for have forward voltage drops in the same area as the
Silicon diodes do.
    So, if the panel mfr puts  a blocking diode in series with each
panel, and if there's a blocking diode at the input of the controller
(to keep from discharging the battery, etc.), then a 13 volt emf leaving
the solar cells is only about 11.8 by the time it gets to the regulator
ckts, where there will be at least one of two "diode drops" before the
    So, since I didn't see any difference in forward drops between
Silicon and Schottky diodes, I don't understand why Schottky diodes
would be preferred.
    Obviously, an MPPT controller could still charge a 12 volt battery
in this sitation, but you'd still be loosing voltage and therfore
wattage due to the upstream blocking diodes, I think.

Again: why bother with Schottky diodes?
I am not arguing with their use in this application, just looking for

Pete Stanaitis

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