Is bypass diodes useful in parallel panel setup?

sunrise

Solar Enthusiast
The NewPowa 75w high efficiency panels I am interested in might or might not come with bypass diodes, but after I read this excellent post

I think in my setup, 75wx3 in parallel, the bypass diodes are not useful anyway.

Most popular panels, like 100w Renogy has 2 bypass diodes in junction box, if a panel is partially and baded shaded, it would lose half of it's output voltage even with a bypass diode. which is ~18v/2 = 9v, this isn't high enough for MPPT SCC to charge my battery anyway.

Is this the correct understanding of bypass diode?

Thanks!
 

gnubie

Photon Sorcerer
You are on the right track. In an all parallel situation they don't achieve much.

The purpose of the diode is to allow current to flow around the shaded section of the panel but at the cost of reduced Vmp for that panel. In a single string series configuration this is a good thing as it allows the string to continue to produce power. In any parallel situation because the string (which might be a single panel) Vmp has been reduced by X volts the MPPT charger's MPPT tracking may result in little power coming from the affected string since it will be working on the wrong side of the IV curve. This is likely in a situation where you have 3 panels and just one is affected by shading since the power from the two panels at their Vmp will likely exceed any gains from pulling the entire array down to the lower Vmp of the shaded panel. Exactly where a MPPT controller will sit the array voltage will come down to the ratio of present power output of the individual strings at particular voltages and how well the tracker algo is done.

You don't have to worry about the shaded panel drawing current from the other two as it's Voc will still be higher than the other's Vmp which is where the controller will be holding the array. Even when the controller draws little current the Voc of the shaded panel will be comparable to the other's Voc (shading reduces current open circuit voltage largely stays the same, shading is not blacking out the panel with a sheet of plywood, cell temperature comes into play too).

This is all a bit of a moot point because, except in very low power panels, most panels have bypass diodes in the connector box on the back. Seems strange for someone to offer a 75W panel without bypass diodes but it takes all types to make the world go round.
 
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sunrise

Solar Enthusiast
Thank you @gnubie for answering my questions!

This is likely in a situation where you have 3 panels and just one is affected by shading since the power from the two panels at their Vmp will likely exceed any gains from pulling the entire array down to the lower Vmp of the shaded panel.

by this did you mean having a bypass diode is worse than not having it at all?

The reason why I think the 75w panel might not have diodes is that their >100w panels all have bypass diodes mentioned in their description, but not the 75w ones. I ask the company through varies channels (Amazon, their support ticket system etc) and the only response I got was ask me to read the description and datasheets which made no mentioning of diodes either way.
I think the diodes only cost about 15c, for them, i think maybe less than 5c. beats me if they indeed skip that for cost cutting.
 

gnubie

Photon Sorcerer
No, without the diodes the panel's power production will still fall, just without the diodes the Vmp will stay up towards the others. One way or the other the power coming from the affected panel will be greatly reduced.
 

sunrise

Solar Enthusiast
No, without the diodes the panel's power production will still fall, just without the diodes the Vmp will stay up towards the others. One way or the other the power coming from the affected panel will be greatly reduced.
With the diodes the much lower Vmp would drag down other 2 panels right? seems it's worse than not having diode so the other 2 panels would not be affected?
 

gnubie

Photon Sorcerer
That depends on the MPPT controller and the relationship between the power from the remaining strings and the shaded one. The shaded panel does not drag the others down itself. Regardless of what happens to the panels the MPPT controller will find the point at which it gets maximum wattage from the array.

If we have a situation where the maximum voltage (Voc) from the lower string falls below the Vmp of the higher strings then the lower will draw power from the higher when the MPPT is sitting at array Vmp but that's not what is going to happen with a simple case of shading.
 

rin67630

Solar Addict
The bypass diodes just make sense on series of multiple panels to avoid the shaded panel to turn negative and jeopardize the whole strain current.
But that is a scenario that could only happen with several panels in series and enough voltage difference between battery voltage and the total panel voltage. The diodes will bring nothing if the total panel voltage -1 panel does not at least exceed the battery voltage.

And with parallel strains, the chances that the strain with a shaded panel will keep feeding current are tiny anyway.
 
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RCinFLA

Photon Sorcerer
Bypass diodes would not do anything to prevent power dropout on a partially shaded parallel panel but that does not mean they are not necessary.

Bypass diode are for safety, not particularly for array shading power dropout. They limit the maximum reverse bias voltage on a shaded cell. There should be a maximum of about 20 series cells per bypass diode. That limits reverse bias to about 12v on a shaded cell.

Just about all PV cells have some amount of shunt resistance defects. Lower grade cells are worse (lower shunt resistance). During normal illuminated operation there is only a maximum of about 0.7v per cell. At this low voltage the shunt resistance doesn't have much effect, just a slight output current loss.

The shunt resistance can be due to a spot defect. When a high reverse voltage is put across the shunt spot defect due to partial panel shading it can get very hot. Hot enough to burn through the plastic backing panel seal, melt cell's interconnect solder joints, hot enough to crack the cell or even cover glass. Bypass diodes are to prevent these things.

Attached is an infrared heat picture of an individual cell with 20v reverse bias test. Although cell produced good illumination power output it was rejected due to excessive shunt defects.

IR image of PV Cell shunt defects.jpg

another pict with more common defect image

Hot spot PV cell.jpg
 
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