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Old 09-11-2016, 03:36 PM   #31
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After three years with my trailer and a single roof-mounted solar panel (90 watt), I am convinced (without any empirical data whatsoever) that sun angle is more critical than shade. My first year out I purposely camped in a canopied campground during the height of summer and I had no problem with recharging the next day. And since I am a winter camper, I added the additional 90-watt portable panel so I could point it at the sun during the day.
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Old 09-11-2016, 04:39 PM   #32
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The problem with partial shading is that a solar panel is a set of individual cells connected in series.

Each cell adds a "push" of voltage so the panel's voltage is the total of all the cells' contributions (about half a volt from each cell), just as the 12 volts from an Escape's dual battery setup is the total of 6 volts from each battery (and each 6V battery is in turn three 2-volt cells); if one of the three dozen or so cells of the panel isn't doing much that's not a big deal to voltage. The problem is that - again like dual batteries in series - exactly the same current passes through every cell in the series; if any one of them isn't getting enough light to pass much current, the whole panel's current is limited. If just one cell is completely shaded, the current output of the entire panel is hobbled by shading less than 4% of its area.

Imagine a bucket brigade, where each bucket-passing person represents one cell. If everyone in the line can handle a big bucket but just one person can only handle a teacup, the whole thing is limited to passing teacups.

(Better explanations from solar panel experts are welcome!)

An evenly shaded panel (even under a thin cover) still produces some power, and it seems like a partially shaded panel should do better, but the partial shading can dramatically limit useful output due to that current limitation.
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Old 09-11-2016, 05:00 PM   #33
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Looking for published sources to confirm or better explain the shading problem, I came across PVEducation. This site has excellent (in my opinion) explanations in a well-structured collection. The Shading section concludes:
Quote:
Shading of a Module

The cells in a module are all connected in series. Shading a single cell causes the current in the string of cells to fall to the level of the shaded cell.
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Old 09-11-2016, 06:45 PM   #34
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Another aspect. I can be in the shade from a tall tree and still get some power out of the panel, maybe 2 or 3 amps. Even under a thin trailer cover I'll see an .5 -1 amp. If I cover 1 cell with something like a piece of wood with the rest in the sun, I get nothing out of the panel. So it seems shade density has an effect too.

Although this has nothing to do with the OP.
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Old 09-11-2016, 06:49 PM   #35
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Quote:
Originally Posted by padlin View Post
If I cover 1 cell with something like a piece of wood with the rest in the sun, I get nothing out of the panel. So it seems shade density has an effect too.
I thought current panels all have bypass diodes to prevent that from happening.
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Old 09-11-2016, 07:18 PM   #36
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Quote:
Originally Posted by padlin View Post
Another aspect. I can be in the shade from a tall tree and still get some power out of the panel, maybe 2 or 3 amps. Even under a thin trailer cover I'll see an .5 -1 amp. If I cover 1 cell with something like a piece of wood with the rest in the sun, I get nothing out of the panel. So it seems shade density has an effect too.
That fits perfectly with the idea that the most-shaded cell limits the current.

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Originally Posted by padlin View Post
Although this has nothing to do with the OP.
The shading discussion came out of the observation that the batteries didn't seem to be charging as expected.

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Originally Posted by tdf-texas View Post
I thought current panels all have bypass diodes to prevent that from happening.
That makes sense, but I think bypass diodes are normally applied on a larger scale (that is, for a group of cells), not for each individual cell.
Bypass Diodes | PVEducation

For instance, the specs for the panels in the Samlex 100-watt and 150-watt kits say have three bypass diodes, so that would be one diode per group of 12 cells (since these are 36-cell panels as usual for nominal 12-volt operation).
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Old 09-11-2016, 11:18 PM   #37
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Quote:
Originally Posted by tdf-texas View Post
I thought current panels all have bypass diodes to prevent that from happening.
Virtually all panels have a blocking diode. Only a very few specialized panels have bypass diodes on each cell. These panels do exist but I don't have a reference off the top of my head.

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