Solar cell output

[MikeK]

I picked up a small solar panel (80mm x 125mm) from Radio Shack and can't get it near the expected current output.  It's quoted as 250mA, but the best I can do in full sun is 150mA.

I noticed that the cell has 24 "sections", so I did some measurements this evening, under a desk lamp.  I found that at least 4 of the sections have almost no effect on the current output and a couple more have not much effect.

Is this normal, about what I can expect from a Chinese solar cell from Radio Shack?

[IanB]

I don't know--but what's the part number? Radio Shack has many small solar panels.

[MikeK]

It's this one: http://www.radioshack.com/product/index.jsp?productId=12628239

[amspire]

I picked up a small solar panel (80mm x 125mm) from Radio Shack and can't get it near the expected current output.  It's quoted as 250mA, but the best I can do in full sun is 150mA.

What part of the US are you in? Unless you are somewhere near the Mexican border, you never get a full strength Sun. More energy gets absorbed by the atmosphere before it reaches you. If you were living in New York, 150mA peak may be all you could expect from a 250mA panel.

[MikeK]

About 40 degrees N latitude.  I hadn't considered it, but that makes sense.  Thanks.

[IanB]

That panel has an area of about 17 sq in and a claimed maximum output of 1.5 W.

A typical solar panel has an output between 50 and 90 mW/sq in, with most being around 70 mW/sq in.

To get 1500 mW from a 17 sq in panel would require 1500/17 = 88 mW/sq in.

It's probable that panel is optimistically rated. Maybe the peak output is only achieved under direct overhead sun at the equator on a clear day.

[KTP]

As other have suggested, your insolation (this is the correct term) is probably not optimal to get the 250mA but those near dead cells worry me.  They could be actually stealing power.

On a side note...does anyone find it sad that RS condensed the entire electronics section to just 3 sliide out drawers?

[digsys]

I noticed that the cell has 24 "sections", so I did some measurements this evening, under a desk lamp.  I found that at least 4 of the sections have almost no effect on the current output and a couple more have not much effect.
Is this normal, about what I can expect from a Chinese solar cell from Radio Shack?

Nope, that doesn't sound good. Can you work out if ALL the cells are in series or - parallel strings of series?
Can you see if they use any protection diodes - these are used to short out any series cell(s) that have failed.
You're right, they should ALL behave the same, some tolerance expected.

[Jeff1946]

As long as the sun is reasonibly high in the sky, the output should be the same if you angle the panel 90 degrees to the sunlight.   After all the sun is about the same distance away (actually a little closer in the northern hemisphere in the winter).  For solar panel wavelengths there is little clear sky attentuation.

[amspire]

As long as the sun is reasonibly high in the sky, the output should be the same if you angle the panel 90 degrees to the sunlight.   After all the sun is about the same distance away (actually a little closer in the northern hemisphere in the winter).  For solar panel wavelengths there is little clear sky attentuation.

If you are north of the tropic of Cancer or south of the Tropic of Capricorn, the sunlight always has to pass through a longer path through the atmosphere then the peak sunlight  that you get between the two Tropic latitudes. The peak sunlight energy is less.

[Valen]

How exactly do you have the cell connected to your measurement device? The current it provides is also based on voltage, or the resistance of the load. Aside from the amount of light that hits it. You really ought to use 2 multimeters to measure current AND voltage together to be able to tell how much power it provides. And test it with different load resistances to figure out the I-V-curve. The shunt resistor in the multimeter you used might not get it at the optimal power generation point.

http://en.wikipedia.org/wiki/Current–voltage_characteristic#In_solar_cells

and

http://en.wikipedia.org/wiki/Maximum_power_point_tracking

[MikeK]

Am I going to lose anything if I get a piece of glass to cover it to protect against the elements?  The panel is epoxy coated which would likely scratch easily.

[digsys]

Am I going to lose anything if I get a piece of glass to cover it to protect against the elements?  The panel is epoxy coated which would likely scratch easily.

Unfortunately, even though most solar panels are bonded to glass, there can be many % loss due to refraction and
absorption in the glass. App 1-3% at high inclination and up to 8% at high angle. ie the "rays" bounce off.
They decided to use thick glass to protect from possible hail damage.
Now depending on the construction of the silicon, scratches may be NO ISSUE at all. Try this test -
Put the panel in the sun, connect to a load and measure the current (and voltage). Place your finger or a pencil
along ONE cell, then across TWO cells. If there is NO appreciable drop, then the cells are in a multi- series / parallel
arrangement and all you'll lose is the AREA of the scratch !! So no big deal.
IF you lose the entire cell, and possibly others in the series chain, then report back and we can rethink.
We pay up to $100,000 to have our Solar car laminated and micro-grooved for MAX collection.
Our panels are wafer thin and flexible.

[erupter]

http://aom.giss.nasa.gov/srlocat.html

This could help defining the correct output to expect, although I had a more specific link (tailored to solar panels installations) but can't find it at the moment.

Also this http://www.wunderground.com/calculators/solar.html
and this http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/redbook/atlas/