Blocking Diodes in Parallel Solar Panels?

[Training Wheels]

I'm getting ready to get new solar panels. I'm looking 8-10 50W panels, none of which have blocking diodes. My plan is for 2 MPPT controllers, one for each side of the boat. I know the controllers have blocking diodes, but is it necessary to have a blocking diode on each panel? If one panel is shaded, will it draw current from the other panels in the circuit?

Appreciate your help, as I'm easily confused!

[sailorchic34]

Originally Posted by Training Wheels

I'm getting ready to get new solar panels. I'm looking 8-10 50W panels, none of which have blocking diodes. My plan is for 2 MPPT controllers, one for each side of the boat. I know the controllers have blocking diodes, but is it necessary to have a blocking diode on each panel? If one panel is shaded, will it draw current from the other panels in the circuit?

Appreciate your help, as I'm easily confused!

If your panels are wired in parallel, then blocking diodes are not required. They are required for series wiring as is common on land. In Series one panel without blocking diodes would shut the whole string down. That is not an issue in Parallel.

Why only 50 watt panels. Seems you could get one or two 300 watt panels and call it good with MPPT.

[Training Wheels]

Thanks. I'm going with flexible, walk-on panels and that's what fits. Plus, nobody makes a larger panel that will work in my application.


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[TeddyDiver]

Originally Posted by sailorchic34

If your panels are wired in parallel, then blocking diodes are not required. They are required for series wiring as is common on land. In Series one panel without blocking diodes would shut the whole string down. That is not an issue in Parallel.

Why only 50 watt panels. Seems you could get one or two 300 watt panels and call it good with MPPT.

If I'm not mistaken it's allmost the opposite. In series you need bypass diodes, and with parallel blocking diodes..

[Thumbs Up]

Don't you mean "bypass" diodes?

[Training Wheels]

Unfortunately, after doing some Google searches, it seems half the sites say you must have diodes on every panel, and the other half say it's not necessary. As I said, I'm confused. 🙁


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[sailorchic34]

Originally Posted by TeddyDiver

If I'm not mistaken it's almost the opposite. In series you need bypass diodes, and with parallel blocking diodes..

The purpose of the blocking diode is to prevent the panel from drawing a small amount of power from the battery's at night. That's for serial and parallel. All Charge controllers now provide blocking, so a lack of blocking diodes is not a major issue.

During daytime a panel that is in parallel and partly shaded, will not output as many amps but it does not block other panels from charging the batteries. It's like multiple energy sources. Now if one panel, say had a dark towel over it it, might act as a sink. But it would be tiny. For the typical installation, a partly shaded or even fully shaded panel will still have positive voltage and not be a sink. It just will not have much, if any current flow.

[Thumbs Up]

Blocking diodes prevent discharging at night. Mppt controllers will have this feature built in. Bypass diodes are for shading issues when panels are in series and are also built into some panels to bypass a string of cells within that panel. If you plan to run any of your panels in series then you will want them.

[TeddyDiver]

Originally Posted by sailorchic34

The purpose of the blocking diode is to prevent the panel from drawing a small amount of power from the battery's at night. That's for serial and parallel. All Charge controllers now provide blocking, so a lack of blocking diodes is not a major issue.

During daytime a panel that is in parallel and partly shaded, will not output as many amps but it does not block other panels from charging the batteries. It's like multiple energy sources. Now if one panel, say had a dark towel over it it, might act as a sink. But it would be tiny. For the typical installation, a partly shaded or even fully shaded panel will still have positive voltage and not be a sink. It just will not have much, if any current flow.

Depends how much shading there is. If one panel in parallel is partially shade (remember in panels there's cells in series) the voltage is also lower and thus it will drain power from other panels. So I would install blocking diode in every panel in parallel installation.

BR Teddy

[sailorchic34]

Originally Posted by TeddyDiver

Depends how much shading there is. If one panel in parallel is partially shade (remember in panels there's cells in series) the voltage is also lower and thus it will drain power from other panels. So I would install blocking diode in every panel in parallel installation.

BR Teddy

Hum, I don't think it will drain power as there is not a negative bias on a partly shaded panel. Actually VOC does not drop all that much with partly shaded panels. Current drops tons with just a tiny shading, but voltage not so much.

But more importantly the cells in that panel are not reversed bias. So while the panel may not add anything to the charging circuit, it does not draw power from other panels either. For that to happen you would need heavy black plastic to cover the panel. Too much reflected light during daytime to cause reverse bias in the cells.

[TeddyDiver]

Maybe so but given the price of schottky diode it 's not much of an investment. But why you would have blocking diode if in series?

[Thumbs Up]

Blocking diode= prevent discharge at night (obsolete, already built into modern charge controllers). Bypass diode= allows current to bypass shaded panels (or strings of cells within panels)

[sailorchic34]

Originally Posted by TeddyDiver

Maybe so but given the price of schottky diode it 's not much of an investment. But why you would have blocking diode if in series?

Myself If I had a MPPT, not having the blocking diode would raise the voltage about 0.55V giving a tiny bit more watts at least in theory. Though not enough to write home about.

I agree that blocking diodes are nice to have, but with the modern charge controllers, the lack of blocking diodes generally does not cause issues. Really even without blocking diodes the reversed bias load at night is not that great, due to the high resistance in reverse bias.

[noelex 77]

Sailorchic is correct blocking diodes are not required and generally will (slightly) decrease the output.

There are a couple of exceptions:

1. If you are not using a solar controller, or you are using one of the very rare solar controllers that does not disconnect the batteries at night. Even in these cases the net loss of the diode might exceeded the gain.
2. If there is a risk the panels will not produce power for extended periods of time. The most common reasons for this is when the panels are covered by snow. Another scenario is when the solar panel is only temporary and is otherwise stored down below for extended periods. In these circumstances it is better to isolate or turn off the controller rather than just use a blocking diode, but the diode will help slightly if this is not done, or if there is no controller.

Blocking diodes will produce a net loss in most installations and are better avoided.

Solar panels do vary slightly if you have any doubts a simple test is to cover the panel and connect it to the battery without the controller. The discharge current can be measured. You will find in most cases it is tiny.

[TeddyDiver]

Originally Posted by sailorchic34

... They are required for series wiring as is common on land. In Series one panel without blocking diodes would shut the whole string down. That is not an issue in Parallel.

This? Surely you mean bypass, not blocking diodes?