Ok to put dissimilar solar panels in parallel + Blocking Diodes

[uncle_bob]

Originally Posted by Zil

One thing not mentioned. With 3 or more panels in parallel there is a need for fuses. If there is some failure in one panel, the other panels can feed into the shorted panel. I don't expect a fire hazard, as 10awg can handle 60 or so amps, but this would knock out all charging from the array. With my 4 renogy parallel 100's, I have 15 amp fuse on each panel before the combining box. I would be interested in other thoughts. Bob?

Hi

Ummm ... errrr ....

10 gauge wire is rated to handle 30A. Three normal panels are not going to exceed that number.

If you want to go 100% with fuses:

Put one in each leg of each panel
Put one at the output of the combiner
Put one down at the input to the solar controller
Put one at the output of the solar controller
Put one right at the battery posts in the inverter cable
Put one right at the battery posts in the "other stuff"cable
Put one right at the battery posts in the link cable between the batteries
Put one at the output of the charger / converter
Put one at the input to the charger / converter
Put one at the source of the power for each load circuit

Yes that's a lot of fuses. To be fully effective, they need to be right at the source of the power. Many of them are now up on the roof and out in the rain. Since fuses fail randomly you will need a way to check that they are still ok on some sort of regular basis. You *will* be safer (but still not 100% safe, the actual 100% list is a bit longer still)

The big source of power is the batteries. The solar panels are complete wimps by comparison. If you want to add protection on a "most risk" basis, start with the fuses on the battery posts.

Bob

[daleyocum]

Originally Posted by Zil

One thing not mentioned. With 3 or more panels in parallel there is a need for fuses. If there is some failure in one panel, the other panels can feed into the shorted panel. I don't expect a fire hazard, as 10awg can handle 60 or so amps, but this would knock out all charging from the array. With my 4 renogy parallel 100's, I have 15 amp fuse on each panel before the combining box. I would be interested in other thoughts. Bob?

The folks at AM Solar, one of the top solar installers in the country, don’t put fuses on the panels. Their thinking is if a panel were to fail in such a strange way, you would notice it with the radically decreased power output and do something about it. There really isn’t a fire danger in that rare failure mode because the wires can handle it.

While not a danger, there is a good likelihood that one of those fuses will blow someday, or the holder will corrode, up on the hot roof causing one of the panels to go offline. That would be much harder to notice.

[Zil]

My combiner box is inside. It is a fuse panel from the solar to the controller. Here is an official instruction from Windy Nation. https://www.windynation.com/jzv/inf/...olar-pv-system
The wire I use is 10 awg rated safe to 60 amps. It all depends on the wire's insulation temperature rating as well as the copper. I use 105C degree.

Bob, listing all those fuses is not an answer. I expected better from you. Yes, I have battery terminal catastrophic fuses, only a fool would not.

[daleyocum]

Originally Posted by Zil

My combiner box is inside. It is a fuse panel from the solar to the controller. Here is an official instruction from Windy Nation. https://www.windynation.com/jzv/inf/...olar-pv-system
The wire I use is 10 awg rated safe to 60 amps. It all depends on the wire's insulation temperature rating as well as the copper. I use 105C degree.

Bob, listing all those fuses is not an answer. I expected better from you. Yes, I have battery terminal catastrophic fuses, only a fool would not.

Wow, that’s fancy. So with that system you had to bring each panel’s 10ga wire pair inside the trailer. Most folks combine on the roof and bring down a single 6ga pair. Besides the larger hole(s) in the roof that seems spiffy. I’ve just never seen it done that way.

[Jim J]

Here is an equivalent circuit for a photo cell.

If you put two such photocells in parallel and one is a 30 V open circuit and the other is an 18 volt open circuit, then the 30 volt cell will supply all the current until the voltage drop across the series resistance in both cells lowers the output voltage to 18 volts, AS the voltage continues to drop below that point, the 18 volt open circuit cell will begin to supply current to the load.

If the two solar panels are more closely matched, they will both supply current at about the same rate. If they are not closely matched, then one is just wasting space until a heavy load appears.

A blocking diode keeps you from wasting energy by sending current down the shunt resistor from your battery (or another solar cell in parallel).

[daleyocum]

Right. It’s just that as long as they are the same voltage output, it doesn’t matter if they are the same capacity. I ran across this video from alte that might be of interest. https://youtu.be/P3YtBrymJdA

In the real world I think subtle differences in the panels are less relevant than differences in shading and orientation.

[daleyocum]

It’s a tad off topic but here’s another video from altE, this time on shading https://youtu.be/ofo1HQyGG8s

[farafield]

Wow, I want you guys at my next dinner party! haha (actually I don't have many dinner parties and if I did, this is exactly what I would want to talk about!)

Seriously, thank you for this enlightening conversation. I learned a lot.

[Zil]

Daleyocum. I had a tin-box with the interior removed, so it was easier than installing on a complete AS. I didn't want wires trailed across the roof-top, so used sealed grommet-ty things to bring in the wires. Each set right under the individual panel. So I have two holes drilled through the metal for each panel. It does make the roof top look nicer than with cables strung around and the problem of holding them down. I was going to use MC4 fuse holders, but they would be hard to access on the roof, and are difficult to disconnect even with the special tool.

[daleyocum]

Originally Posted by Zil

Daleyocum. I had a tin-box with the interior removed, so it was easier than installing on a complete AS. I didn't want wires trailed across the roof-top, so used sealed grommet-ty things to bring in the wires. Each set right under the individual panel. So I have two holes drilled through the metal for each panel. It does make the roof top look nicer than with cables strung around and the problem of holding them down. I was going to use MC4 fuse holders, but they would be hard to access on the roof, and are difficult to disconnect even with the special tool.

That explains it. I’m sure that made for a much clear roof. Mine has wires EVRYWHERE. Only a geek would think it’s cool. Luckily my wife isn’t very tall!

[uncle_bob]

Originally Posted by Zil

My combiner box is inside. It is a fuse panel from the solar to the controller. Here is an official instruction from Windy Nation. https://www.windynation.com/jzv/inf/...olar-pv-system
The wire I use is 10 awg rated safe to 60 amps. It all depends on the wire's insulation temperature rating as well as the copper. I use 105C degree.

Bob, listing all those fuses is not an answer. I expected better from you. Yes, I have battery terminal catastrophic fuses, only a fool would not.

Hi

Then you missed the point.

There is *always* another fuse you can put in. Fuses are always "safer" right at the source. In just about every case, they are a lot less convenient right at the source.

At some point you need to calm down and accept a very low level of risk. Fuses are useless for protecting from lightning. That gets higher up the list than some added fuse risk wise pretty fast.

Bob

[uncle_bob]

Originally Posted by Jim J

Attachment 327494

Here is an equivalent circuit for a photo cell.

If you put two such photocells in parallel and one is a 30 V open circuit and the other is an 18 volt open circuit, then the 30 volt cell will supply all the current until the voltage drop across the series resistance in both cells lowers the output voltage to 18 volts, AS the voltage continues to drop below that point, the 18 volt open circuit cell will begin to supply current to the load.

If the two solar panels are more closely matched, they will both supply current at about the same rate. If they are not closely matched, then one is just wasting space until a heavy load appears.

A blocking diode keeps you from wasting energy by sending current down the shunt resistor from your battery (or another solar cell in parallel).

Hi

Based on looking at the V/I curves of normal solar panels and the same curves of that model ...... the model could use some work. That or the components in it have a whole bunch of qualifiers on them ....

Bob

[Zil]

Bob you are confusing me again. I wanted some feed back on fuses on solar panels. They are not needed if the wires are sized to handle the max amps from panels. But with 3 or more panels in parallel, most manufactures require fuses for each panel. With 3 or more panels in parallel.
Golden Rule; Fuses protect the wire, not the device.

[daleyocum]

Originally Posted by Zil

Bob you are confusing me again. I wanted some feed back on fuses on solar panels. They are not needed if the wires are sized to handle the max amps from panels. But with 3 or more panels in parallel, most manufactures require fuses for each panel. With 3 or more panels in parallel.
Golden Rule; Fuses protect the wire, not the device.

I'd never hear this as a requirement though I guess I can see that if you think there's a chance a panel could fail and become a dead short you could see how fusing it so that the wire going to it won't melt would be a good idea! With the 12ga wires on most panels and with high temperature insulation on them I personally feel fine about them handling 40 amps for as long as it would take for me to notice there's something amiss. This chart seems to verify that. 40 amps sounds like about 700watts of solar to me and that's being conservative. That being said, I've never heard of a panel shorting like that.

[uncle_bob]

Originally Posted by Zil

Bob you are confusing me again. I wanted some feed back on fuses on solar panels. They are not needed if the wires are sized to handle the max amps from panels. But with 3 or more panels in parallel, most manufactures require fuses for each panel. With 3 or more panels in parallel.
Golden Rule; Fuses protect the wire, not the device.

Hi


On the fuse thing.

If the wire is rated for 30 A and three panels in parallel put out 20A, I do not in any way see the point of what you are saying.

100W x 3 = 300 W

300W at 16 V is < 19A

If it's number 10AWG it is rated for at least 30A and quite possibly more current.

There still is no way for the combined panels to put out more current than the wire will handle. They will short circuit limit just a bit above 20A.

If a single panel shorts internally, the current from the other two will be less than 15A. Again well under the wire ratings.

Since people *do* go to silly small wire gauges, there are cases that might require fuses. Here on the forum everybody seems to love giant big chunks of wire ... that's a different case.

Bob

[Zil]

Another link about fusing solar panels, along with the one posted above.

https://www.civicsolar.com/support/i...-when-not-fuse
Quoted below; Now I have to leave and build something.

"It has an ISC rating of 8.87 Amps and a fuse rating of 15 Amps.
If a fault occurs in one string would there be a need for each string in the system to be fused in order to protect the circuit?
8.87A x 2 strings x 1.25 = 22.18A > 15A (FAIL)
So in this scenario we would be required to have fuses on the output of each string to protect for a fault if this ever were to occur in the system."



"As you can see in this example you could omit fuses on the output of each string, all thanks to the rating of the OCPD of the modules. I chose to show this example because it illustrates that careful evaluation of the modules in the system can help to not only determine when to fuse but also that in certain situations where you may think it is necessary it turns out that it may not. Considering these variables helps you to ensure that your circuits are indeed protected and in turn could save you from having to add fuses to the circuit outputs when it is redundant or unnecessary."

[Jim J]

Uncle Bob pointed out the first equivalent circuit I posted was wrong. Below is the commonly accepted equivalent circuit for a solar cell.

Many of these cells are connected in series string to achieve the desired open circuit voltage. Then strings are connected in parallel to achieve the desired output current.

If you match the panels you will get the most out of them. Start by matching the open circuit voltage as closely as you can.

[uncle_bob]

Originally Posted by Zil

Another link about fusing solar panels, along with the one posted above.

https://www.civicsolar.com/support/i...-when-not-fuse
Quoted below; Now I have to leave and build something.

"It has an ISC rating of 8.87 Amps and a fuse rating of 15 Amps.
If a fault occurs in one string would there be a need for each string in the system to be fused in order to protect the circuit?
8.87A x 2 strings x 1.25 = 22.18A > 15A (FAIL)
So in this scenario we would be required to have fuses on the output of each string to protect for a fault if this ever were to occur in the system."


"As you can see in this example you could omit fuses on the output of each string, all thanks to the rating of the OCPD of the modules. I chose to show this example because it illustrates that careful evaluation of the modules in the system can help to not only determine when to fuse but also that in certain situations where you may think it is necessary it turns out that it may not. Considering these variables helps you to ensure that your circuits are indeed protected and in turn could save you from having to add fuses to the circuit outputs when it is redundant or unnecessary."

Hi

..... but, each time I point out that the *only* way that can do any good at all is if the fuses are on the roof right at the panels ... you seem to push back. Why is that?

If the "fuse protects the wire" (as you have pointed out), the wire you are protecting is on the roof. The fuse needs to be there as well. It *must* be between the power source (the panel) and the wire. Otherwise you have just added a point of failure without any added safety.

Bob

[Al and Missy]

Originally Posted by Jim J

Attachment 327494

Here is an equivalent circuit for a photo cell.

If you put two such photocells in parallel and one is a 30 V open circuit and the other is an 18 volt open circuit, then the 30 volt cell will supply all the current until the voltage drop across the series resistance in both cells lowers the output voltage to 18 volts, AS the voltage continues to drop below that point, the 18 volt open circuit cell will begin to supply current to the load.

If the two solar panels are more closely matched, they will both supply current at about the same rate. If they are not closely matched, then one is just wasting space until a heavy load appears.

A blocking diode keeps you from wasting energy by sending current down the shunt resistor from your battery (or another solar cell in parallel).

Wikipedia has a different equivalent circuit.


https://en.wikipedia.org/wiki/Theory...nt_circuit.svg


While it is a "Norton Equivalent" and the circuit in the post is a "Thevenin Equivalent", the significant difference is the polarity of the PN junction in the solar cell. The wikipedia circuit shows that current from an adjacent panel will pass through the PN junction while the diagram in the post indicates that it will not. I'm not an expert in solar cell physics, so I don't know which is right, but in the absence of a reference for the diagram in the post I am inclined to go with the Wikipedia version.


Al

[DLDINS]

Hello all,

I am an electrical engineer with experience in solar technology. I could pontificate for a long time, but for this point of this thread (which initially asked about diodes) I would just like to mention that you can add a "ideal diode" circuit instead of a actual diode. It consists of a controller IC and power MOSFETs that then simulate the function of a diode. Since the FET has a very low "on resistance" you get the function of a diode with only 20mV to 50mV of forward voltage loss.

One such control IC is the LTC4359:
https://www.analog.com/media/en/tech...ts/ltc4359.pdf

There are families of this with different features, but I am just letting you know they exist.