Solar Panel Output.

[Franklin]

I thought I understood my solar panels pretty good - I've had them for 5 years now and watch the gauges they are wired into constantly (I work on the boat via telecommuting so I am pushing the limits of my panels to keep the batteries charged). They are 140 watt panels with a max output of 8.2... amps in perfect conditions, so for 3 panels I should get a max of 24 amps, which I have seen on many occasions as I have them wired through a good amp meter on the nav-station.

All that adds up just fine, but today, for a short few minutes I read 29 amps on my gauge. Did not think that was possible so either one of three things have happened: 1) my coleman 30 amp control which is supposed to be a PWM type (on/off) somehow reduced the voltage and increased the amps (acting like the more expensive type of controller) 2) the sun put out some extra power for a brief period of time - not sure that is possible solar panels can produce more than the max 3) my very trusty and accurate gauge is wearing out.

#3 seems more likely than #1 or #2 based on my knowledge but I hate to think my gauge is going and it went back to 24 amps after that brief couple of minutes at 29.

Thoughts?

 

[JohnShannon]

lots of varibles

http://www.canadiansolar.com/filead...n_Solar-Datasheet-MaxPower-CS6U-P-v5.52en.pdf

Under STC (standard test conditions) 1000 W/m^2 illumination and 25C cell temp
panel gives 315W

STC not likely, sunny day and cool to the touch panel.

Same panel under NOCT (normal operating cell temperature) gives 228W

So maybe it was really sunny with a cool breeze blowing.

A larger issue with that big a solar installation you should really be using a MPPT type controller Maine Sail has recommended some. That should boost your output by 15% easy

 

[Bill Roosa]

I'm thinking #3. Not much chance of a cell putting out more than its max current unless your controler is a bost-buck type. A MPP type will not produce more current than the max but it will throttle the voltage down (and increase the current) to make the product of volts and amps a max. That max is always less than max current and less than max voltage BTW

 

[JohnShannon]

MPPT referes to any number of algorithms in the controller that you place between the solar panel and the battery.
The controller is in round numbers 100% efficient (98%) so Power in (from the panel) = Power out (to the battery)

MPP on a typical panel for sailboats would be around 16-17V (depending mostly on temperature of the cells). Battery voltage will be 13 ish in bulk charge.

So a "buck" type regulator is a piece of power electronics that will be controlled by the MPPT algorithm and can step down the solar panel voltage to the battery voltage and increase the charge current by the ratio of Vpanel to Vbattery

 

[druid]

It's not unusual for a panel to exceed it's rating: the rating is kind of a "guaranteed" value - the panel is supposed to put out AT LEAST that amount. 18% over rated IS a bit high, though. Since your controller is PWM, maybe the ammeter is not reading it right?

I'd also say for that kind of power you should be putting the panels in series and using a MPPT controller.
druid

 

[WayneH]

IIRC, Solar panel output is usually measured at 17.2V for 12V panels. You need the higher voltage to feed the batteries. But Watts = Amps * Volts. So your 140W panel puts out 8.2A at 17.0V. Now switch things around so that Amps = Watts/Volts and in that case if your batteries are at 12.5V then your 140W panel will pump out 11.2Amps. Well, there are losses due to translation so maybe 10.5amps or three panels for 31.5amps.

My panels have produced 14.5amps even tho they are only rated to produce 10.2amps. Where I'm headed with this is short term production of more than rated capacity should not scare you. Your meter is still accurate in my opinion.

 

[Bill Roosa]

The math is right WayneH but the physics is incorrect. W=V*A but V and A are not constants and are related in the classic "knee" shaped graph. Max volts with an open (A=0) circuit and Max amps with a dead short (V=0) V and A are always in between those max values and the MPP is at the knee in the graph (aprox 0.76 Vmax) for any given solar intensity.
So if Amax is at V=0 your argument that Amax and Vmax at the same time is incorrect. See below

 

[Chief RA]

Sounds correct Bill. Chief

 

[Maine Sail]

Solar panels are a current limited source but they are also rated at 77°F and 1000 watts per meter². With a really cool day and better than ideal or better than STC test conditions, it is possible for the panels to approach the Isc or short circuit rating. It would be quite rare for panels to exceed the Isc rating seeing as this is the rating the NEC and NFPA use for sizing over current protection and what is defined as the maximum short circuit current the PV array is capable of under ideal conditions.

The Kyocera KD-140SX is rated for 7.91A at 77°F & 1000 watts per meter² of irradiance. Its short-circuit rating is 8.68A at 77°F and 1000 watts per meter². If we multiply 8.68 X 3 = 26.04. This is still a long way from 29A and it is unlikely a 140W panel can deliver 9.67A, without an MPPT controller, even if short circuited.

Can you snap a shot of your panels ratings tag?

Here is a chart that shows some potential scenarios for PWM vs. MPPT

 

[WayneH]

That's what I like about this site. You learn something new nearly every time you access it.

 

[jssailem]

Wayne

 

[LeeandRick]

Edge of cloud effect.

 

[Franklin]

Can you snap a shot of your panels ratings tag?

I am pretty sure you got it...I believe it is the Kyocera KD-140SX.

 

[LeeandRick]

I'm thinking #3. Not much chance of a cell putting out more than its max current unless your controler is a bost-buck type. A MPP type will not produce more current than the max but it will throttle the voltage down (and increase the current) to make the product of volts and amps a max. That max is always less than max current and less than max voltage BTW

Solar cells will put out past the name plate current given proper conditions.
http://www.pveducation.org/pvcdrom/solar-cell-operation/effect-of-light-intensity
This is why the rules require you to size conductors and some other components to 1.56 times the panel ratings. I was in the RV solar business for 15 years and replaced many a fuse and controller due to edge of cloud effect. To the OP, What were the weather conditions at the time of your current readings?

 

[Franklin]

To the OP, What were the weather conditions at the time of your current readings?

High noon, sunny and temps about 80 degrees.

 

[LeeandRick]

High noon, sunny and temps about 80 degrees.

Any cumulus clouds moving across the sun?

 

[Franklin]

Any cumulus clouds moving across the sun?

nope

 

[LeeandRick]

nope

Well forget my theory plus it was a little warm also. The Kyocera 140 watt panels are known to have outputs beyond name plate sometimes, but not more than 5%.