Bypass diodes

[Eddy]

Hi
Attached hopefully is a picture of junction box of a solar panel I recently purchased

The two diodes are they bypass diodes ? - I would have thought there should be a third connection at the common point of the diode. I removed the common terminal screw definitely no other wires underneath

The open circuit voltage of the panel is 21.7v, I shaded the bottom half with a sheet of cardboard and open circuit voltage dropped to 14.2v, shading one half of longer side of panel and the voltage dropped to 15.2v

The panel has 36 cells

DSC03448.JPG

[T1 Terry]

Hi
Attached hopefully is a picture of junction box of a solar panel I recently purchased

The two diodes are they bypass diodes ? - I would have thought there should be a third connection at the common point of the diode. I removed the common terminal screw definitely no other wires underneath

The open circuit voltage of the panel is 21.7v, I shaded the bottom half with a sheet of cardboard and open circuit voltage dropped to 14.2v, shading one half of longer side of panel and the voltage dropped to 15.2v

The panel has 36 cells

DSC03448.JPG

These are bypass diodes for series connected strings. They could also act as a bypass if part of the panel is shaded, long ways up either side on the very outer strings will generally see a 0.7v drop, shade up the centre of that effects 2 strings will see a voltage drop and virtually no current output. If you place a load equal to or greater than the panels advertised Vmp x 12v such as a 60w halogen bulb, you should see the effect it has on the output and that is a better indicator than open circuit voltage. If you have a clamp meter that reads DC current you can see just how much the shading effects output and just how little shade is needed to seriously reduce the panel output.

T1 Terry

[Eddy]

Thanks Terry - will have a play around with a globe and see what happens

[Eddy]

It was quite illuminating at times - did the halogen globe test with a clamp meter
No shading on panel and there was approx 5amps, one cell covered with a bit of rag brought it down to 2.5amps
Shading on the long side of the panel had the biggest effect - one line of cells dropped it to 1.6amp whilst two strips basically gave nothing
I guess thats why you guys are going the smaller wattage/physical size panels so can move further away from vents etc

[T1 Terry]

It was quite illuminating at times - did the halogen globe test with a clamp meter
No shading on panel and there was approx 5amps, one cell covered with a bit of rag brought it down to 2.5amps
Shading on the long side of the panel had the biggest effect - one line of cells dropped it to 1.6amp whilst two strips basically gave nothing
I guess thats why you guys are going the smaller wattage/physical size panels so can move further away from vents etc

That and fitting a lot more panels in the same area. When any 60w panel is shaded the loss is only 60W, up either side of the air con or large hatches has one side in shade in the morning, full sun around mid day and the other side is shade in the afternoon, so even though they don't have output for the full sun day they do add to the charging capacity for part of the day and all of them when it is overcast or raining. The exposed solar area is the critical part in overcast or rainy weather, you might only get 20% of the max output but it will still be all day and makes all the difference.

T1 Terry