Low output problem from solar panels

2 Posts
Apr 24, 2008 09:14 pm
Low output problem from solar panels

I hope someone can help me figure this one out! I have two Siemens SM55 panels mounted in parallel with a Morningstar ProStar controller. When tested separately the open circuit voltage and amperage of each panel is  the same. When I connect them to the controller one panel is putting out the expected amperage but the other is putting out little or none (depending on how much sun). I have replaced the wiring from the low output panel to the junction box where it connects to the "good" panel and checked  all connections.

Is it possible for the panel to test OK but still have  something wrong? Is there anyway to test the wiring within the panel (it has blocking diodes in both the positive and negative junction boxes on the panel?

Any other ideas? Thanks

Tom Schaefer
 
8 Posts
Apr 25, 2008 12:11 am
Re: Low output problem from solar panels

Hi Tom,

I had similar problems some time back. I couldn't get a straight answer from anyone, so I had to work it out from first principals - so don't take any of this as gospel!
DISCLAIMER - I am just a hobbyist, don't hold me reponsible for any damage or problems caused by following this.

Yes, it seems that a panel can be bad even though VOC (open circuit voltage) and ISC (short circit current) appear fine. The "VI" relationship for a solar panel is a curve on a 2-dimensional graph - try searching on your manufacturers website, and you should find such a graph. Your VOC and ISC will be two points on the graph. You will notice that the shape of the graph is such that, as I increases from 0 (at VOC), the voltage decreases quite slowly at first, and then rolls off quickly when I -> ISC.

The only way I found to test the panel properly was to do a load test. I got some resistors, an ammeter, and created a few test loads, measuring Voltage (across the resistor load) and current (through the resistor load). Plot them on your graph, and it will tell you if your panel is bad. *NOTE* that common resistors are rated for very low currents, so keep the resistances relatively high (I think the lowest R I used was 333 Ohm).

What I saw when trying this on my panel, was that as soon as the current rose above zero, the voltage plummeted, as I was only getting 12V @ 0.1A (where I should have got more like 18V).

Once again, I wouldn't swear to any of this, so take it all with a grain of salt!

Regards,
         Chris.
 
578 Posts
Apr 25, 2008 10:37 am
Re: Low output problem from solar panels

hey guys, this question comes up a lot.

i need a little more info, and i will follow up later.

resistors work, but, in my opinion, is not the best diy option, as it requires more work and sometimes soldering.

your siemens 55 are likely pretty old, and back in the day only had 10 year warranty.

is your prostar the one with the meter or not?  do you have access to a dc clamp on ammeter (fluke 337 or similar) or hardwire in-line ammeters for dc?

lets go from there.

james
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2 Posts
Apr 26, 2008 03:59 am
Re: Low output problem from solar panels

Thanks for the replies. Very appreciated. The panels are now 8 years old. There are a couple of "brown" cells. I have the ProStar with the meter and I do have an in line meter capable of testing DC amperage. Thanks again and let me know what I should be testing.

Tom Schaefer
 
578 Posts
May 1, 2008 09:32 am
Re: Low output problem from solar panels

test each one alone on the prostar.  if one works and the other shows nothing, then we know it is a module issue. 

often when modules fail, you get absolutely nothing for VOC or something less than one volt.  since you are getting voc and isc numbers that seem normal, the diodes may be a good place to check next.  if they are the kind that are screwed into the j-box, that makes it easy to remove. 


you could try the resistor test, but this loading should be essentially the same as using your charge controller meter and comparing one module to the next. 

perhaps some folks can contribute some more thoughts.

i have only seen 1 intact module fail with my own eyes.  i am sure it happens, but maybe some folks with more pv years under their belts have some opinions.

i think a company makes sm55 clones (same frame size) but i dont know if they do it anymore.  i could look into it if you wanted.

james
Alt-E staff

AltE
"Making Renewable Do-able"
http://www.altEstore.com/

Tel: 877.878.4060 x107  or +1.978.562.5858 x107
Fax: 877.242.6718  or +1.978.562.5854
 

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