Used solar panel testing cloudy day

solarpowernovice

I may be buying 6 solar panels today and it will be cloudy all day. Is there a way that I can test them to make sure they are good? I have a multi meter with a clamp on ammeter. The panels are supposedly 4 years old and only used for 3. My current solar panels are about 4 years old and are 270 watts, these are 235 watts.

What should I be looking for when I test them? Can I use these panels with my current panels on my midnite 150 charge controller? They will not be wired in series with my current panels and will not exceed the charge controllers limitations.

BB.

You can find 80% of the problem panels by:

• measure the open circuit voltage (Voc) in any direct sunlight. Compare all panels (same brand/model) together at same time/temperature/angle to sun--The ones that are "different" (usually lower voltage by 10% or more, are questionable). Voc is temperature sensitive and hot panels may have 10-20% drop from Voc-std on the back of panel label--That is OK and normal.
  
• measure the short circuit current (Isc) in full sunlight with panel pointing at sun. First (panel away from sun, or something dark over panel, connect the + and - panel leads together (short). Point at sun, use clamp meter to measure current. Note that Isc is proportional to amount of sun hitting the panel... Nominally if measured current is >0.5*Isc in full mid-day sun--probably OK (since you have multiple panels, quickly measure in panel the same way, quickly between all panels... If there is (one or more) different by more than ~!0%, probably something is wrong).
• Look at panels closely... You are looking for brown/black areas under panels, looking for water or corrosion, damaged face glass, damaged rear plastic (note that if panels are dirty--Brown spots on surface of glass from leaves/dirt can look like bad cells/wiring underneath--particularly where water pools near the frame).
• Look at all panels and find any "differences" in coloring/look of solar cells. Not all differences are "failures" or potential failures (some cells can reflect light differently as they age)--But it is usually not a good sign. Cracked cells are bad (there are some strange lines across cells that can look like cracks, but are not--Unknown if this is or will be a bad panel)
  
• Look inside j-box for corrosion/burned terminals (if that type of J-box). Solar panels are not really repairable (shorted by-pass diodes in junction boxes you can open can be replaced).
• Look through glass above J-Box--Sometimes an overheating J-Box can brown (light coffee color, or "rusty" haze) the area under the surface glass.
  

-Bill

solarpowernovice

Awesome! Just what I needed. Thank you

BB.

I should add--Look for delaminations (edge of panels, and delamination over cells). Another sign of impending failure.

-Bill