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.

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