Where to start with solar.

[CazPaul]

We install a lot of solar and there is no hard and fast rule and lots depends on the individual job and what the customer wants. When we (Van Bitz) installed three 150w panels on my own motorhome they are connected to three Victron MPPT regulators, and the three regulators are connected to a Positive and Negative BUS bar and a 100 amp connection run directly to the batteries via the Victron battery computer.

The way I have built in back up in case of failure and I have minimised my chance of all three panels being in the shade, or, being able for one panel to influence the other two in any way.

Wow, that's an expensive but great way of doing it.

 

[HandyAndy]

I wouldn't advise panels in series on a motorhome roof. If the larger panel is in the shade it will seriously impact on the efficiency of other two.

Great on a house roof or a solar farm where the orientation and the calculated and there is no chance of shading.

If the panel has bypass diodes in the junction box of the shaded panel then I thought shading was no longer an issue?

 

[Em and Tim]

One thing I keep getting confused by, how many panels can you wire in parallel to a mppt control unit, in which case is it just a case of connecting them all into the same input port? Does this vary for different controllers (I know the total/sum of the voltage is model dependant).

 

[HandyAndy]

It's the same as batteries, at the end of the day it's just a power source (as long as the sun is shining lol)
Series, ups the voltage
Parallel ups the amperage.
You design it to fit the controller or have or you buy a controller that fits the panel layout that you can fit on the roof.
In your case and the Victron 100/50 means maximum voltage is 100 volts and maximum amps is 50 amps

 

[Em and Tim]

It's the same as batteries, at the end of the day it's just a power source (as long as the sun is shining lol)
Series, ups the voltage
Parallel ups the amperage.
You design it to fit the controller or have or you buy a controller that fits the panel layout that you can fit on the roof.
In your case and the Victron 100/50 means maximum voltage is 100 volts and maximum amps is 50 amps

So the mppt controller can still be as effective at doing whatever (dark mathematical magic) it does, with multiple panels wired to it, or is there a benefit to keeping it simple?

Also, does the voltage of all panels wired to it need to be the same?

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[eddie]

If the panel has bypass diodes in the junction box of the shaded panel then I thought shading was no longer an issue?

we don’t know that the OP has got. From doing our own tests, a speaking with our manufacturers, their is a potential advantage in wiring in parallel on smaller, mobile installations such as a Motorhome roof, but your right shading is less of an issue is the correct panels are used and set up correctly, but no problem if wired in parallel

 

[autorouter]

A solar panel are made from rows and columns of solar cells, wired in series and parallel to give the voltage and current rating of the panel. A popular arrangement is 9 x 4 = 36 cells.

If for example your 300W is 6 x 12 cells, and the 150W is 6 x 6 cells, then two 150W wired in series would be effectively the same as one 300W.

If that's not the case then, to be sure, you could have two controllers, one for the 300W, and one for two 150W wired in parallel. That way the shading of one panel will not affect the others, and you would have extra reliability in case one of them goes wrong.

 

[autorouter]

Also, does the voltage of all panels wired to it need to be the same?

Sorry I didn't actually answer your question. Panels in series are referred to as a 'string'. The voltage of each string needs to be the same if the strings are are wired in parallel.

If there is a small difference in the voltages, then I'm not sure if that would be a big problem. The MPPT controller would set the maximum power point, and it may be slightly different for the two strings, but in my opinion there wouldn't be much power loss.

 

[Em and Tim]

Once again, thanks.

I think we will go with just the 400w panel and a smaller mppt controller. If we decide to add more, it will be through an independent controller.

On a slightly different tack, is there any significance to the gap between the panel and the roof? I am planning to attach it using aluminium angle, so it can be tilted, but I figure that some ventilation may help it staying cooler. Is there any advice on this, or is it better to minimise the air gap for the sake of aerodynamics?

 

[Richard n Ann]

I put ours on channel and hinged it ?

20mm gap underneath for cooling is good.

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[DBK]

Once again, thanks.

I think we will go with just the 400w panel and a smaller mppt controller. If we decide to add more, it will be through an independent controller.

On a slightly different tack, is there any significance to the gap between the panel and the roof? I am planning to attach it using aluminium angle, so it can be tilted, but I figure that some ventilation may help it staying cooler. Is there any advice on this, or is it better to minimise the air gap for the sake of aerodynamics?

That sounds a sensible way forward.

Cooling will be a little better with an air gap but I doubt you would spot any difference in performance. This is because the panel will get hot when the sun is shining strongest which is generally when you will have a surplus of energy. It's when the sun is shining weakly at this time of year you need every watt but the panel isn't going to get hot.

A gap gives a bit of access for cleaning the roof below the panel but this won't be an issue if you make it tilting. I suspect any aerodynamic difference will be negligible.

 

[Em and Tim]

Once again, thanks, that makes sense.

The next thing that I am unsure of, is the performance if our current panel (100w 12v see picture), which has given us good power throughout the Scandinavian summer, but is now not giving us anything, even when tilted to the sun on a clear Catalonian day. How do I test it? I put a multimeter on it yesterday, and got a voltage of about 1.5v (sunny but not tilted, I measured it at the controller in the cupboard). I am wondering if it has a loose connection, or if it is a problem with the pwm control unit? I am going to move it for the new panel to take the optimum spot, so I guess I want to work out if its worth putting back on/what needs doing to fix it.

 

[Richard n Ann]

If it's got the sun on it, it should be working, doesn't matter how low the sun is, and if it's tilted up towards the sun it should be near to full amps.

 

[HandyAndy]

I would unscrew the solar panel connecting cables from the charge controller and measure voltage directly at the disconnected cables to ensure you had an accurate panel output reading.

 

[Em and Tim]

I would unscrew the solar panel connecting cables from the charge controller and measure voltage directly at the disconnected cables to ensure you had an accurate panel output reading.

Doing that I get a voltage starting at 10.5v and dropping as the multimeter completes the circuit. It is giving 0.016mA I think though I have just noticed a cloud over the sun, wait, I'll try again when it has passed........or have a cup of tea and return to find that single black cloud has become a verified bank of the buggers (dont you hate it when they do that) I'll put everything back together and return next time the suns out. Either way, there seems to be a big drop in voltage along those cables. What could be causing that?

Just to discuss/reinforce my understanding, if the panel voltage is below the battery voltage, it does not charge. In poor light conditions, a panel builds up energy slower, thus the voltage rises slower. A PWM controller basically stops seepage back to the panel at night, and limits the voltage to a charging voltage say 13.4v, but does nothing to the amps. A MPPT on the other hand would take the excess voltage and turns that into more amps. I understand they also work the other way, so in poor light when for example the panel is not up to a charging voltage, they can turn the say 4amps at 10v into (for ease of simple maths) 3 amps at13.33v. Is that right, or should the voltage of a panel be constant despite light conditions?

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[DBK]

Doing that I get a voltage starting at 10.5v and dropping as the multimeter completes the circuit. It is giving 0.016mA I think though I have just noticed a cloud over the sun, wait, I'll try again when it has passed........or have a cup of tea and return to find that single black cloud has become a verified bank of the buggers (dont you hate it when they do that) I'll put everything back together and return next time the suns out. Either way, there seems to be a big drop in voltage along those cables. What could be causing that?

Just to discuss/reinforce my understanding, if the panel voltage is below the battery voltage, it does not charge. In poor light conditions, a panel builds up energy slower, thus the voltage rises slower. A PWM controller basically stops seepage back to the panel at night, and limits the voltage to a charging voltage say 13.4v, but does nothing to the amps. A MPPT on the other hand would take the excess voltage and turns that into more amps. I understand they also work the other way, so in poor light when for example the panel is not up to a charging voltage, they can turn the say 4amps at 10v into (for ease of simple maths) 3 amps at13.33v. Is that right, or should the voltage of a panel be constant despite light conditions?

Have you disconnected the controller when checking the panel voltage? You need to do this in case the controller is shorting the panel due to a fault.

But it sounds like the panel has failed but to be sure wait for a bit of sun and check the voltage - it should be around 20+.

An MPPT controller adjusts the panel voltage to give maximum power, in the case of your 100W panel this is around 18 volts. If the panel is only producing say 10 volts the controller won't do anything. It will just sit twiddling it's thumbs until it see something above say 16 volts. The exact voltage will be in the controller specification.

 

[HandyAndy]

Doing that I get a voltage starting at 10.5v and dropping as the multimeter completes the circuit. It is giving 0.016mA I think though I have just noticed a cloud over the sun, wait, I'll try again when it has passed........or have a cup of tea and return to find that single black cloud has become a verified bank of the buggers (dont you hate it when they do that) I'll put everything back together and return next time the suns out. Either way, there seems to be a big drop in voltage along those cables. What could be causing that?

Just to discuss/reinforce my understanding, if the panel voltage is below the battery voltage, it does not charge. In poor light conditions, a panel builds up energy slower, thus the voltage rises slower. A PWM controller basically stops seepage back to the panel at night, and limits the voltage to a charging voltage say 13.4v, but does nothing to the amps. A MPPT on the other hand would take the excess voltage and turns that into more amps. I understand they also work the other way, so in poor light when for example the panel is not up to a charging voltage, they can turn the say 4amps at 10v into (for ease of simple maths) 3 amps at13.33v. Is that right, or should the voltage of a panel be constant despite light conditions?

That panel sounds goosed to me. my 100w fold out solar panel is producing a stable 17volts (using fluke multimeter) right now, disconnected from any controller and it's 15:30 here and dull miserable day.

Edit to add some pics for info

 

[Em and Tim]

Have you disconnected the controller when checking the panel voltage? You need to do this in case the controller is shorting the panel due to a fault.

But it sounds like the panel has failed but to be sure wait for a bit of sun and check the voltage - it should be around 20+.

Earlier, that was the panel connected to nothing other than the multimeter, giving 10.5v and dropping fast once the multimeter made the circuit. It had been in bright sun for a wee while, but a cloud arrived as I was doing it.
What may cause a panel to fail?

 

[DBK]

Earlier, that was the panel connected to nothing other than the multimeter, giving 10.5v and dropping fast once the multimeter made the circuit. It had been in bright sun for a wee while, but a cloud arrived as I was doing it.
What may cause a panel to fail?

I would guess a cell has failed. They are all wired in series so one with a fault will appear as at best a resistance. In your case several may have failed.

 

[Richard n Ann]

Earlier, that was the panel connected to nothing other than the multimeter, giving 10.5v and dropping fast once the multimeter made the circuit. It had been in bright sun for a wee while, but a cloud arrived as I was doing it.
What may cause a panel to fail?

The multimeter shouldn't affect the panel voltage at all, even the old analogue AVOs are 20k ohms per volt and the digital ones even higher. ?

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[Em and Tim]

I would guess a cell has failed. They are all wired in series so one with a fault will appear as at best a resistance. In your case several may have failed.

Have you disconnected the controller when checking the panel voltage? You need to do this in case the controller is shorting the panel due to a fault.

But it sounds like the panel has failed but to be sure wait for a bit of sun and check the voltage - it should be around 20V.

That panel sounds goosed to me. my 100w fold out solar panel is producing a stable 17volts (using fluke multimeter) right now, disconnected from any controller and it's 15:30 here and dull miserable day.

Sounds like it, it explains why we went from good to getting nothing in a short period of time.

 

[Em and Tim]

Its sunny so I'm back out on the roof, and today, when I have unscrewed the clips I must have gripped them differently ad both of the factory fitted clips have done this

Looking very corroded at the same point (please excuse the state of the roof under the panel, I need to do some serious cleaning before I glue the new panel up here,...... any advice on that? By which I guess I mean both cleaning the roof of years of grime and preparing a surface for sikaflexing aluminium to it). I'm now going to shorten the wires, re do the clips and see if that solves it!

 

[Em and Tim]

Interestingly about 8" back down the cable, there are still signs of corrosion, the first 2" were just dust.

 

[Em and Tim]

Having put the multimeter into the box, pre wires, I'm getting 20.8v and 5.2a. Result!

 

[Em and Tim]

Putting it through 12" of positive wire, negative still in the box, I'm getting 6v. I think I'll scrap the corroded wire and go straight into the box with the other wires (it'll be tight but ok) I've got new wires ordered with the new panel to relocate this one, so it will be a temporary fix.

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[Em and Tim]

And I think this sums up how corrodedbthevwires are:

 

[HandyAndy]

Nothing a soldering iron can't sort out. Good detective work there fella.

 

[Em and Tim]

A temporary fix.

And we have the pv light on on the pwm controller!

Now all I need to do is put it all back together again and clean up the mess on the roof. The full 3' of those wires was corroded beyond use.

 

[Em and Tim]

Oddly though, the voltage at the screws on the controller of the incoming wires is 12.48v (out to the battery is 12.40v), considering the other end of that 2.5m cable is 20v at the panels box, that seems ridiculous in terms of a voltage drop along a relatively short cable (it is 2x2.5mmsq). I expected that voltage to be the same (it a wee bit less) as the voltage at the panel junction box, or am I missing something here?

 

[Em and Tim]

Oddly though, the voltage at the screws on the controller of the incoming wires is 12.48v (out to the battery is 12.40v), considering the other end of that 2.5m cable is 20v at the panels box, that seems ridiculous in terms of a voltage drop along a relatively short cable (it is 2x2.5mmsq). I expected that voltage to be the same (it a wee bit less) as the voltage at the panel junction box, or am I missing something here?

To answer my own question. Please correct me if I'm wrong.

Now it's all hooked up, tye voltage is down to 12.6 in the roof, and 12.6 at the input to the controller, my understanding is now that the open circuit voltage (Voc) is that voltage when it's not connected. The maximum power voltage (Vmp) will be the biggest it can give to the controller and the controller sets the voltage at any point in time to just above that of the batteries, thus creating a charge. The MPPT, would do the same just with a few more amps as it does something a pwm doesn't to up the amps as it drops the volts.

Anyway, we have a working 100w panel and PWM controller again, which I will rewire when I move it to make space for the new panel. Thanks again for all your input, I wouldn't have had a clue without you all. You're stars!

Now, I could get the laptop out and get on with my tax return (which is what I was going to do this morning, but needed power), or could have a beer, hmmmmm, decisions!

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