Faulty bypass diode on solar panel?

[Nor]

I have 5 x 550W solar panels in series. Yesterday I noticed I was only getting 140W from them. 1 was totally in shade by the boom, while 4 of them was fully in the sun. I remove the shaded one from the circuit and immediately got 1600W+ from the 4 panels.

I tried to measure the diodes at night when it was dark, I got around 5kohm both ways, but this reading didn't seem stable, not sure if it was correct. There was a little bit light from lights from the beach 200meter away, but almost dark, can this small light affect this measurement?

The panels are 550W from Sunket, SKT550M10.
https://cdn.enfsolar.com/z/pp/8606035fcbe21f6c.pdf
Is it common that these diodes go?

I wonder if one thing would have caused it. With the 5 x 550W panels in series I should expect a max output of 2750W. But I've several times gotten higher max output, as reported by the Victron app (using Multi RS Solar MPPT). The max I've got is 3395W! Not sure how this is possible, but I wouldn't think the panels could output more than they support?

[steve77]

A simple question - did this problem just start, or has it been present since the panels were installed?

Solar panels are quite capable of outputting more power than on the nameplate, especially when new. However, you are giving an example of 23% above the nameplate Wattage, and I would call that very unusual.

To test the diode, disconnect the panel from its neighbors and throw a towel over it to block any light. You should measure essentially zero Ohms in one direction and infinity in the other. Anything other than this will indicate a bad diode.

Cheers!

Steve

[Avionics_Joe]

According to the PDF your solar panels do not have any diodes.

[Pete7]

Originally Posted by Avionics_Joe

According to the PDF your solar panels do not have any diodes.

Although the PDF doesn't state they have diodes, what makes you think they don't have them in the connection box on the back. Looking closely at the picture, I think I can see 3 strings so is there 3 diodes as most have?

Pete

[senormechanico]

Most common digital multimeters have a diode checker setting.

You need to use that to check a solid state item, as the voltage present in a normal resistance setting is not enough to avalanche any measuring current through a diode, transistor etc.
Try again with the diode setting. A good diode will read infinite in one direction and around .550 to .700 in the other.

[Uplander]

There are two matters raised.
1. Panels overachieving. This is regularly caused by the sun shining through a hole in the cloud. (Cloud Edge Effect). There is the direct sun plus the sun reflected from the clouds as well.

2. Panels underachieving. You have the panels set up in series. I understand the maximum output from a series is the production from the weakest link. Hence you got more power when you took the shaded panel out of the system.


Cheers

[Nor]

Originally Posted by senormechanico

Most common digital multimeters have a diode checker setting.

You need to use that to check a solid state item, as the voltage present in a normal resistance setting is not enough to avalanche any measuring current through a diode, transistor etc.
Try again with the diode setting. A good diode will read infinite in one direction and around .550 to .700 in the other.

Thanks, I'll check again tonight using the diode setting, I just read the multimeters manual and found how to use it. I was measuring resistance, and got around 5kOhm both ways.

[Nor]

Originally Posted by Uplander

There are two matters raised.
1. Panels overachieving. This is regularly caused by the sun shining through a hole in the cloud. (Cloud Edge Effect). There is the direct sun plus the sun reflected from the clouds as well.

2. Panels underachieving. You have the panels set up in series. I understand the maximum output from a series is the production from the weakest link. Hence you got more power when you took the shaded panel out of the system.


Cheers

It appears you're not aware of bypass diodes in solar panels?

[Nor]

Originally Posted by Pete7

Although the PDF doesn't state they have diodes, what makes you think they don't have the in the connection box on the back. Looking closely at the picture, I think I can see 3 strings so is there 3 diodes as most have?

Pete

There are 3 small boxes (the cables are coming out of two of them. I can try to open one and have a look. But will have to unmount the panels first to do this.

I thought that all panels have them, especially newer big panels like this?

But why would diodes fail, so quickly?

[s/v Jedi]

Originally Posted by Uplander

There are two matters raised.
1. Panels overachieving. This is regularly caused by the sun shining through a hole in the cloud. (Cloud Edge Effect). There is the direct sun plus the sun reflected from the clouds as well.

2. Panels underachieving. You have the panels set up in series. I understand the maximum output from a series is the production from the weakest link. Hence you got more power when you took the shaded panel out of the system.


Cheers

Incorrect information. Why post when you don’t really know?

[s/v Jedi]

Originally Posted by Nor

I have 5 x 550W solar panels in series. Yesterday I noticed I was only getting 140W from them. 1 was totally in shade by the boom, while 4 of them was fully in the sun. I remove the shaded one from the circuit and immediately got 1600W+ from the 4 panels.

I tried to measure the diodes at night when it was dark, I got around 5kohm both ways, but this reading didn't seem stable, not sure if it was correct. There was a little bit light from lights from the beach 200meter away, but almost dark, can this small light affect this measurement?

The panels are 550W from Sunket, SKT550M10.
https://cdn.enfsolar.com/z/pp/8606035fcbe21f6c.pdf
Is it common that these diodes go?

I wonder if one thing would have caused it. With the 5 x 550W panels in series I should expect a max output of 2750W. But I've several times gotten higher max output, as reported by the Victron app (using Multi RS Solar MPPT). The max I've got is 3395W! Not sure how this is possible, but I wouldn't think the panels could output more than they support?

So you have shade from the boom. This is about location of panels.

With 5 panels, I am guessing one is on the centerline and then you have two on each side of the boom? This is a tough situation.

Normally I recommend to have the panel on each side of the boom on separate mppt controllers. Or if multiple, put the panels on each side in series and on separate controllers, so you have a series string on starboard on a controller and a string on port side on another controller.

But what to do with the center panel…. 3rd controller?

Another option you have is to install additional diodes, one across each panel and see how that works. It’s cheap to try and maybe it’s good enough. Here are suitable diodes: https://www.amazon.com/gp/product/B0C7L3JGY4

[Nor]

Originally Posted by s/v Jedi

So you have shade from the boom. This is about location of panels.

With 5 panels, I am guessing one is on the centerline and then you have two on each side of the boom? This is a tough situation.

Normally I recommend to have the panel on each side of the boom on separate mppt controllers. Or if multiple, put the panels on each side in series and on separate controllers, so you have a series string on starboard on a controller and a string on port side on another controller.

But what to do with the center panel…. 3rd controller?

Another option you have is to install additional diodes, one across each panel and see how that works. It’s cheap to try and maybe it’s good enough. Here are suitable diodes: https://www.amazon.com/gp/product/B0C7L3JGY4

I have 4 panels on the stern arch and one on the cockpit roof next to the boom. All connected to a single Multi RS Solar.
Shouldn't the diodes in the 5th panel work to avoid this problem when this panel is shaded? Why would I need external diodes?

Are those 50V diodes enough when there are 5 panels in series?
Thanks!

[Nor]

UPDATE: I managed to measure correctly with the diode settings, and got 0.43V one way and OL the other, so the diodes seems to be working correctly!

So, then I'm puzzled. Why did I only get 140W from the 5 panels, when 4 was in full sunlight? And when getting 1600W after disconnecting the shaded panel ?

I've now connected the 5th panel back into the circuit, will see how it works the coming days.

Would adding an external bypass diode work better?

[team karst]

watch this one; although not on a boat, this guy does seem to get it correct.

https://www.youtube.com/watch?v=-x_6OLdAods

ie; use panel bypass diodes when panels are series connected . Use "blocking" or "isolation" diode when parallel connected with other panels.
Note this needs to be understood in the context of PANELS. The panel itself has likely more than one string, and so the panel will have already some diodes to manage just the internal panel isolation.

In summary, you don't want to apply voltage on a panel from an external source that is higher than what the panel is trying to produce. That is the panels in parallel issue with a shading problem.

Panels in series use the bypass diode to allow current to flow "around" it rather than trying to force current through the panel which is shaded. I don't know if an externally forced current causes damaging heating, but where there is voltage and collateral current, there will be heat. Limiting that voltage to 0.6V seems to be a good idea.

In my EL utility days, I spent some time in huge solar systems, at 75MW rated output. These designs use a lot of series panels with working voltages around 1500V. I don't believe the panels are anything special, other than specing a good amount of insulation to earth, from the power wires.

[s/v Jedi]

Originally Posted by Nor

I have 4 panels on the stern arch and one on the cockpit roof next to the boom. All connected to a single Multi RS Solar.
Shouldn't the diodes in the 5th panel work to avoid this problem when this panel is shaded? Why would I need external diodes?

Are those 50V diodes enough when there are 5 panels in series?
Thanks!

That setup isn’t recommended. The four panels on the arch are fine and can be in series on one controller, but the panel next to the boom is best served on a separate controller.

An array of panels need to form a large panel with all individual panels receiving the same sun, same angle and same non-shaded view of the sky. Now the mppt controller can find that point of maximum transfer.
When panels are not part of the same array then they get different points of maximum transfer and the array will underperform.

I think you removed that one panel that is on the roof to get good performance. Keep that apart on a small controller.