Faulty bypass diode on solar panel?

[team karst]

Originally Posted by Nor

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

Yes, they will see a maximum Vrev of the rated Voc of ONE panel. You don't install these at the MPPT, you install these at the panel junction box, assuming only two conductors reach the MPPT.

[Nor]

Originally Posted by s/v Jedi

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.

I know it's not ideal, but why would adding the last panel give a worse result than without it? Isn't this the purpose of the bypass diodes? Would adding extra external diodes improve it?

I can disconnect and leave the 5th not connected, but as I have a 48V battery bank I don't think it's possible to charge them with a single 43V panel.

[Nor]

Originally Posted by team karst

Yes, they will see a maximum Vrev of the rated Voc of ONE panel. You don't install these at the MPPT, you install these at the panel junction box, assuming only two conductors reach the MPPT.

Thanks, I could give it a try, however the panels have 3 junction boxes, quite far from each other, one for input cable, one for output and one in the middle. I guess I could use split mc4 cables to get a bypass circuit and put the diode there.

[s/v Jedi]

I recommend to wire each panel with its own wiring to a junction box where series and/or parallel connections are made. This is also where additional bypass diodes can be placed.

Also, as in the box I recently built, on the left in the picture, I added surge suppression.

The box on the right is near the solar controllers, where wiring from the other box terminates, strings are put in parallel using breakers before connecting to the mppt controllers.

[team karst]

Its possible to buy DIN rail diode blocks. These are good for 5A. Maybe higher ratings are available.

https://www.directindustry.com/prod/...60-706605.html

[Nor]

How does the diodes work, for example if 10A is flowing through the first panels, and the last panel is partially shaded and can only do 5A. Will then some of the current from the first panels flow through the diodes, and then 5A flow via the 5th panel? Or is it all or nothing from the 5th panel?

[team karst]

Originally Posted by Nor

How does the diodes work, for example if 10A is flowing through the first panels, and the last panel is partially shaded and can only do 5A. Will then some of the current from the first panels flow through the diodes, and then 5A flow via the 5th panel? Or is it all or nothing from the 5th panel?

If the bypass diode becomes forward biased, then there will be only -.6V across the panel and no output power from that panel will exist, and the diode will conduct all of the good panels amperage.

But, lets say the shaded panel is capable of 4A, and the others 5A. As the shading increases, the voltage across the shaded panel will decrease. At some point, that voltage will reach -.6V, and the diode now passes the full 5Amp system current.

So, as the strong panels drive the weak panel (in reverse), the weak panel progressively provides less and less power to the system. At some point, the V across the panel is zero, no power is provided, and the diode will handle all the line current. If there is no diode, then the panel V will go significantly negative, and likely cause cell heating within. With enough negative voltage, the good panels will see a Voc situation (V total on the good panels is negated by the Vreverse on shaded panel) and provide no power either, since the 5A cannot be maintained into a virtual open circuit. Thus, the diode advantage.

In few words, the diode keeps the line current going, even if a shaded panel provides zero power.

[Nor]

Originally Posted by team karst

If the bypass diode becomes forward biased, then there will be only -.6V across the panel and no output power from that panel will exist, and the diode will conduct all of the good panels amperage.

But, lets say the shaded panel is capable of 4A, and the others 5A. As the shading increases, the voltage across the shaded panel will decrease. At some point, that voltage will reach -.6V, and the diode now passes the full 5Amp system current.

So, as the strong panels drive the weak panel (in reverse), the weak panel progressively provides less and less power to the system. At some point, the V across the panel is zero, no power is provided, and the diode will handle all the line current.

Thanks for explaining! In your example, when the shaded panels does 4A, is the whole array limited to 4A? And when it gets even more shaded the diode will kick in and then pass the 5A from the other panels?

When you say the voltage will reach -.6V, what do you mean this referring to? Is the voltage of the shaded panel -.6V? If the shaded panel is providing 1A and 30% of the voltage, will the diodes still be off?

And to conclude, when the diodes are active they are always "100% active" so the panel doesn't provide any current/votlage to the system, correct?

[team karst]

two terminal devices in series MUST have identical current flows. So, if your sunny panels want to deliver 5A with its associated Voltage, then the shaded panel, let's say without a bypass diode, will also "pass" 5A, but at a lower voltage, maybe even a negative voltage. It's hard to find V-I curves on panels with NEG impressed voltage, but it stands to reason that the non-diode equipped shaded panel will continue to resist current flow with ever increasing NEG voltage. That NEG voltage will diminish that set of panels, all of them. So, the bypass diode is important to keep the sunny panels producing power, by giving the non-productive panels a bypass mode.

-0.6 volts is the typical forward voltage drop of a silicon diode. I call it negative V, since it reduces overall V of the panel system by 0.6 V, WHEN there is shade on a panel. a tolerable number. If two panel become non-productive, there will be 2 x 0.6 V drop.

Of course, there is a corollary problem with parallel panels. Here you have all the panels fixed to one Voltage. That's rather a problem if there is a non-diode equipped panel that is not producing power.

[s/v Jedi]

Most decent solar panels are setup with multiple strings of cells which are connected in series and each have their own bypass diode. So when a panel is partially shaded, there is a good chance that only part of it is bypassed while another part of it still produces power.

I think most modern, high power panels have three such strings.

[Pete7]

Originally Posted by s/v Jedi

I recommend to wire each panel with its own wiring to a junction box where series and/or parallel connections are made. This is also where additional bypass diodes can be placed.

That's a neat way of wiring multiple panels up. Worth noting Jedi has used DC Din rail breakers, which are a little more difficult to find but worth the effort. We have one to isolate our panels if necessary.

Pete

[s/v Jedi]

Originally Posted by Pete7

That's a neat way of wiring multiple panels up. Worth noting Jedi has used DC Din rail breakers, which are a little more difficult to find but worth the effort. We have one to isolate our panels if necessary.

Pete

Yes, all the parts are readily available on Amazon and they are affordable.

The terminals in the left picture are also mounted on the DIN rail. You can see red jumpers across the center of them, which are the jumpers that connect panels in series. You can simply take a diode and crimp its leads together with the conductors from a panel in a ferrule and tick it in a screw terminal.

In between the screw terminals are surge protectors. Solar panels can be an ingress point for lightning related power surges.

[Icarus]

If the panels are connected in series and one is shaded..that will affect the overall output.
It's better to connect them parallel in case one or more are shaded.

[s/v Jedi]

Originally Posted by Icarus

If the panels are connected in series and one is shaded..that will affect the overall output.
It's better to connect them parallel in case one or more are shaded.

In parallel you lose the output of the whole panel while in series you only lose the one string that is shaded. Unshaded panels are not affected in either case.

That said, you simply should *never* build an array where such differences between panels exist. The array should form one large surface like a big panel.

Other setups need more controllers.

Cells in panels are connected in series to form strings. Those strings are connected in series to form panels and those panels are connected in series to form an array.

If you put a panel on each side of the boom then that isn’t an array and you need a dedicated controller for each panel.

[Nor]

I have now tried some days with only using the 4 panels, before adding the 5th panel back to the mix. I have not seen the low 160W output anymore. I'm not sure why I only got 160 W that day. I've seen a couple of other times the output go down to 250W for a few seconds before going back up to 1000W+. Maybe this is the Victron searching for optimal output voltage. Not sure.

Anyway, the clear conclusion is that I get more kwh when having all 5 panels connected. So even thought it's not ideal it works pretty good.

I normally get between 8 to 12kWh daily.

Originally Posted by s/v Jedi

That said, you simply should *never* build an array where such differences between panels exist. The array should form one large surface like a big panel.

Other setups need more controllers.

Cells in panels are connected in series to form strings. Those strings are connected in series to form panels and those panels are connected in series to form an array.

If you put a panel on each side of the boom then that isn’t an array and you need a dedicated controller for each panel.

I understand I could get an ever better output than I'm getting if I had more controllers. But it was easy and cheap to put the 5th panel next to the mast. Do you mean that I should rather have not put this panel there? Is there any other downside, will this panel take damaged from the more frequent shading and potential use of the diodes?
If no other downside and I'm getting more output than with 4 panels then I don't see why I shouldn't.