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Old 10-11-2008, 10:05   #1
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REAL output of solar panels ...

I was a trifle suspicious of the output of my 2x solara 40 watt panels so we checked them out by covering/uncovering the panels on a bright sunny day in the med .the results were similar ..... / 0.33 of one amp ! each ..dissapointing, to say the least ! the main reason for their installation was to run the fridge ,on which they have a deficit of 0.4 amp . shouldn,t they be producing a total of 3.5 amps in perfect conditions ?
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Old 10-11-2008, 11:11   #2
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Gramos, if near the equator and tilted directly into the mid-day sun you should theoretically be getting about 3.3 amp/hr per panel (40 watts divided by 12 Volts). The practical best I have been able to achieve with my system (3 x 75 watt Kyoceras) in Canada is about 12 amp/hr, although the theoretical max. should be 18 amps. Since you are further south than I am, I would expect that you should be getting slightly better performance than I am (2/3rd's of the max. theoretical output). Are you sure that your meter isn't reading 3.3, rather than .33?

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Old 10-11-2008, 11:22   #3
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Are you getting 12 volts or a charging voltage of 14 volts?
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Old 10-11-2008, 11:24   #4
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By any chance were your batteries fully charged. Here in NC my 4x75w array puts out about 16 amps on a bright day against batteries discharged to about 12 volts, but when the batteries are fully charged output drops to the low single digits unless I put some load on the system. Even then they won't put out what they do when the batteries are discharged and can absorb the power.
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Old 10-11-2008, 11:32   #5
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Sounds like a problem. You may find this link helpful.
Solara AG Solar Panel Warranty Woes - Boat Design Forums

Most 40watt range models put out an unregulated voltage of around 17V so you should be seeing roughly 2.2amps unregulated on a panel hooked up to a somewhat discharged battery and pointing directly at a perpindicular sun. The first thing I would do is check out the individual panel wiring to the batteries to insure that indeed both panels are "dead". It seems odd that you would have an identical issue on both panels.
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Old 16-11-2008, 00:55   #6
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Are you getting 12 volts or a charging voltage of 14 volts?
the output of both panels ,in volts is always 17volts +.
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Old 16-11-2008, 00:59   #7
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By any chance were your batteries fully charged. Here in NC my 4x75w array puts out about 16 amps on a bright day against batteries discharged to about 12 volts, but when the batteries are fully charged output drops to the low single digits unless I put some load on the system. Even then they won't put out what they do when the batteries are discharged and can absorb the power.
good point , yes the batteries were fully charged when the test was done , but as i measured the output at the cable leading from each panel would this not read the battery state? just the panel output .
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Old 16-11-2008, 04:28   #8
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Interesting, two posters are using 75 watt panels and measuring similar outputs. I measured my 75 watt panel and saw exactly the same output as well, 3.8 amps(discharged batteries) and I'm at 28 degrees latitude.

I wonder how they arrive at output specs?

To the OP. If your batteries are charged, your charge contoller will limit current flow by design.
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Old 16-11-2008, 05:29   #9
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... I wonder how they arrive at output specs...
There are two general standards for specifying the output of Photo Voltaic (PV) modules (Solar Cells):

* STC: Factory Standard Test Conditions are 1,000 watts per square meter solar irradiance, 1.5 Air Mass and a 25 degrees C. cell temperature

*PTC: PV USA Test Conditions represent a more real life condition of 1,000 watts per square meter solar irradiance, 1.5 Air Mass, and 20 degrees C. ambient temperature at 10 meters above ground level and wind speed of 1 meter per second.

The PTC rating for a Photo Voltaic panel is ALWAYS less than STC rating.

Module operating temperature increases when placed in the sun. As the operating temperature increases, the power output decreases. The PTC ratings take this into consideration, to a limited degree, by calculating the PTC ratings based primarily on the specific module temperature characteristics. The PTC ratings are different for each module, and typically vary from approximately 75%-92% of the STC rating.

If a typical decrease in power output is about 12%, this decrease would result in an STC rated 100 Watt DC solar module being PTC rated at approximately 88 Watts DC.
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Old 16-11-2008, 09:26   #10
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Looking at Kyocera specs, they are rating the panels based on the maximum power transfer point.

From: http://www.kyocerasolar.com/pdf/spec...081508_web.pdf

Their 130 watt panel puts out 7.35 amps at 17.7 volts at the max power transfer point, which multiplied gives 130 watts. If you look at the graph they have, you will see that the max power transfer point is at the knee of the curve. The open circuit voltage (max voltage, no current out, DVM set as voltmeter across panel leads, no other connection) is 22.1 volts, and the short circuit current (put your current meter as a dead short on the panel) is 8.06 amps. It doesn't make sense to get power from these numbers by multiplying, as they are at opposite ends of the operating range. Both of these points result in no power transfer. 22 volts x 0 amps = 0 watts. 8.06 amps x 0 volts = 0 watts.

This site shows how to find the power output curve by using several resistors. What they don't make clear is that at the shorted value the DVM has to be set to amps and is placed as a dead short across the panel leads. Make sure your DVM is rated for the expected current before doing so.
Power Measurement of Solar Panels

I would think that if you disconnect your panel from the boat and read the open circuit voltage and short circuit current of your panel and they match the open and short specs given by the manufacturer, the panel would probably be functioning properly. If you still had doubts, buy a couple of power resisters and derive the actual power output as in the above link. Trying to test in circuit normally won't work as the battery/charger and state of charge of charge of the battery is messing you up.

For the OP, how are you making your measurements? The current in the wire from the panels to the battery has to be the same everywhere, unless there is another path like a dump circuit on your controller.

John
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Old 16-11-2008, 11:01   #11
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If a typical decrease in power output is about 12%, this decrease would result in an STC rated 100 Watt DC solar module being PTC rated at approximately 88 Watts DC.
That would certainly explain it as I did my checking back in June when it was close to 100 degrees F here.
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