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Solar panel array


gary955

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I'm fitting solar panels to my boat, 3x 100w panels. It's not a narrowboat  installation, and with the panels on the wheelhouse roof and the battery's near the bilge of our little harbour tug, there's some fairly long cable runs involved. The panels are wired in series and together will produce a max of about 57v at 5.5amps into a MPPT controller and from there to the battery's at 24v and about 12.5amps. These are theoretical maximums which in reality will never be reached with flat mounted panels.

The cables from the panels to the controller are 4mm2 and to get the controller within a metre of the battery's the cable run will be in the region of 8m. The supplied cable from the controller to the battery's is 6mm2 of 1m length.

The alterative is to mount the controller in the wheelhouse to make the panel to controller cable run 5m. The neg from the controller to battery could be made off to a nearby neg busbar which returns to the battery's via a 35mm2 cable and the pos from the controller could be taken to the battery's via a new  10mm2 cable of 3m length.

Which installation do our electrical experts prefer

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12 hours ago, WotEver said:

The former :)

 

Why? I don’t see it mattering a jot from an electrical point of view, unless I’ve misunderstood the question  

On the other hand having the solar controller easily accessible in the wheelhouse where you can see it’s diplay and dab it’s buttons sounds eminently more sensible to me, than tucking it away with the batteries in bilges. 

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58 minutes ago, Mike the Boilerman said:

 

Why? I don’t see it mattering a jot from an electrical point of view, unless I’ve misunderstood the question  

On the other hand having the solar controller easily accessible in the wheelhouse where you can see it’s diplay and dab it’s buttons sounds eminently more sensible to me, than tucking it away with the batteries in bilges. 

Because if he mounted the controller in the 2nd position he would have to purchase some metres of fatter cable to connect it up, whereas he already has 1m of 6mm2 supplied with the unit. 

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Mike has a point in that the wheelhouse installation would be preferable from a convenience point of view but I ask the question because the electrical integrity is paramount. 

What I didn't mention is that I have a Victron battery monitor installed and having the negative from the solar controller on the wheelhouse busbar would avoid the need for another ring terminal to be stacked on the load side of the battery monitor shunt (there are already three) The 3m of 10mm2 wire would be a minor expense in the grand scheme of things.

I'm not an electrical engineer but I'm assuming that the dissimilar cabling from the controller to the battery's would not be an issue. ie 10mm2 dedicated positive and 35mm2 shared negative. is this correct?

It's always a little difficult to explain to explain these things in writing, is my post clear? 

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On 28/01/2018 at 10:37, gary955 said:

Mike has a point in that the wheelhouse installation would be preferable from a convenience point of view but I ask the question because the electrical integrity is paramount. 

 

What do you mean by 'electrical integrity'?

Putting the controller in the bilge will possibly squeeze an extra 0.001% of performance out of the system in marginal conditions (i.e. the days each side of the winter months when it won't work at all) 

If you just mean not bodging it, either method is fine. 

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As long as the cable sizes have been calculated to minimise voltdrop at expected maximum amps (plus some more so you can add panels if required later) then I am with Mike on this. The dryer the solar controller remains the better I like it.  However if the controller were to go into a dry engine room rather than a typical wet narrowboat engine bay then I do not see any issues with putting in closer to the batteries. I would suggest there is not much in it either way unless damp is an issue.

As I have done exactly what you suggest, use the domestic bus bar negative as the solar negative, I don't think you will have any problems.

Edited by Tony Brooks
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On 28/01/2018 at 11:28, Tony Brooks said:

I would suggest there is not much in it either way unless damp is an issue.

 

I still maintain configuring and/or interrogating your solar controller is a lot easier and far more likely to be done properly with it on the wall at face height in a warm dry and convenient room inside the boat than on your knees with a torch in an engine room or battery bay. Therefore, in the wheelhouse is a far better position in the OP's case.

Even when not configuring it or interrogating it for historical data, it is nice to walk past it many times a day and see the display telling you the charge current at that instant.

Edited by Mike the Boilerman
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7 minutes ago, Mike the Boilerman said:

 

I still maintain configuring and/or interrogating your solar controller is a lot easier and far more likely to be done properly with it on the wall at face height in a warm dry and convenient room inside the boat than on your knees with a torch in an engine room or battery bay. Therefore, in the wheelhouse is a far better position in the OP's case.

Even when not configuring it or interrogating it for historical data, it is nice to walk past it many times a day and see the display telling you the charge current at that instant.

You are right Mike mine is in the electric cupboard and has to be adjusted with a puter and dil switches, but I would rather have it where its dry than where its damp, bilges are not the place to store lecce kit you know it will go wong

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41 minutes ago, Tony Brooks said:

As long as the cable sizes have been calculated to minimise voltdrop at expected maximum amps (plus some more so you can add panels if required later)...

Agreed that this is key. 

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What max current output is the controller?

Best to follow what the manual for the equipment recommends, but a good rule of thumb for cable sizing is 'amps divided by three'.

So for a 30A one then 10mm2 would allow for some future expansion, likewise a 20A then use 6mm2 similarly. Volt drop will be fine for the controller up to 5m from batts on a 24V boat. For the panels just use standard 4mm2 solar cable unless 6mm2 is about the same price in which case go for that.

But the max distance (separation) you can reasonably go to keep the volt drop reasonable with 'amps divided by three' , depends on whether the system voltage 12v or 24V, and is whether the equipment is a charge source or not.

Good rule of thumb here is 5m distance on a 12V boat, halve it for charge sources, and double it for 24V. For greater distances just increase the cable size pro rata. So a charge source on a 24V well halve it then double it which brings you back to 5m :)

For a more comprehensive answer there's a few boatbuilding books around and probably a few guides out there on the web.

Edited by smileypete
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There isn’t a particular “right answer” because whatever you do, there will always be some voltage drop, it just depends on how much you want to accept. For a given amount of power, higher voltage means less current (and it is current that causes voltage drop) so in terms of minimising the copper used / getting the max benefit from the amount of copper used, it is better to make the high voltage low current section (controller to panels) long, and the low voltage high current section (controller to batteries) short. But there may well be other considerations that override that basic optimisation.

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My personal rule is to run the absolute largest wire / cable rated for the controller in question. This applies, especially for the run from the array to the controller, for reasons another member posted above - future upgrades. I see no reason to run smaller wires / cables that will "work for now" (rated for the current installation), when I already know I may "upgrade" them later.

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Thanks all for the reply's I think I am now able to make a decision.

Mike I agree with what you say about accessibility of the controller although ironically I've chosen the less accessible option which is within an electrical panel in the wheelhouse. If it turns out to be a right pain then I can mitigate it with a remote display available from the controller manufacturer. I'm hoping that having set up battery type, voltage and charging voltages on the controller initially, the battery monitor will keep me informed as to current in and out of the bank.

Smiley Pete, Nicknorman, Asia_off_grid I've worked out cable sizes from panel-controller and controller-battery's  to be appropriate for 1% voltage drop. Future expansion is limited by space available as I want the installation to be as discrete as possible to maintain the classic look of the boat. The panels are on the wheelhouse roof but I have to maintain access to the radar, navigation lights, GPS and VHF antenna's, perhaps I could squeeze one more panel on and I have included this in the calculations.

Tony. Good to know that you have successfully used a common neg return without affecting performance, I think this was my main concern. Logic suggests that a 35mm2 cable is going to be better than a 10mm2 cable but I had a lingering doubt, wondering if pos and neg wires should be matched pairs.

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No need for pos and neg to be matched. The voltage drop is created by both the pos and neg wires, so it would just effect the ratio of voltage drop  between pos and neg, which is not anything of significance.

Edited by nicknorman
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On the above post I forgot to mention to add a suitable fuse in the controller positive cable at the battery end, the Tracer manual should cover this, or I'd go a size up on what the controller output is or what the cable can handle.  So a 40A fuse for a 30A controller with 10mm2 cable. A midi fuse and holder will do and can take the ring terminals needed for 10mm2 cable.

Edited by smileypete
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On 1/28/2018 at 16:53, nicknorman said:

There isn’t a particular “right answer” because whatever you do, there will always be some voltage drop, it just depends on how much you want to accept. For a given amount of power, higher voltage means less current (and it is current that causes voltage drop) so in terms of minimising the copper used / getting the max benefit from the amount of copper used, it is better to make the high voltage low current section (controller to panels) long, and the low voltage high current section (controller to batteries) short. But there may well be other considerations that override that basic optimisation.

'Amps divided by three' gives a volt drop of 0.05V per metre, so half a volt for a round trip of 10 metres, or a quarter of a volt for a round trip of 5 metres.

Half a volt is about 4% volt drop on 12V which is acceptable for most things, a quarter of a volt isn't too bad for a charge source on 12V, in practice that'll fall to nothing as the batts get near to full charge anyway. :)

Sure there's probably somewhat better ways of working it out, though the cost/benefit doesn't hold up for the average narrowboater just looking to add a bit of solar...

Edited by smileypete
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It’s a good rule of thumb for working our voltage drop, what is harder is to know what voltage drop is acceptable. There is of course no “right answer”, it is a compromise and it depends where you want to strike the compromise. Plus, as you say, the voltage drop tends to near-zero as the batteries approach fully charged.

You could have slightly bigger solar panels, or use more copper, to get the same charge rate.

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1 hour ago, smileypete said:

Half a volt is about 4% volt drop on 12V which is acceptable for most things,

5 minutes ago, nicknorman said:

It’s a good rule of thumb for working our voltage drop, what is harder is to know what voltage drop is acceptable.

Typically, I try to stay at 3%, or lower.

 

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