MPPT solar controllers - Take 2

[Tim Good]

Ok so following on from my last thread: http://www.ybw.com/forums/showthread.php?448805-MPPT-Solar-Controllers&highlight=

So we figured that there are advantages to running two panels off the arches in series to increase the voltage to 24V but keep ampage the same. We then use a MPPT controller and we're all happy. Often recommended is the Victron 70 series that come in a 70/15 (<200W) or 75/50 (<700W).

So i contacted Victron to clarify some points. I might add that the Email support from Victron would bloody awful with one word responses that didn't really address my questions. Anyway many in the previous thread a few people said "make sure you get a big enough controller to upgrade in future". I asked two questions to Victron then:

Q1. Can I get the 75/50 and add more panels to it in different locations on the boat, maybe different powers.
A: No , is not done , you cannot connect different panels to one input , you need 2 mppt controllers.

Q2. Ok can I have two MPPT controllers then for the same bank of batteries?
A: No.

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So clearly on a boat solar panels will be on different planes and likely of different powers. They said I can't have two MPPT controllers and nor can I input panels of different power ratings into a single controller. I seem to be in a bit of a loop hole. Is this advice correct?

What I am basically trying to see is if I can wire in two sets of panels in series to same batteries. I.e. does this work:

 

[Trundlebug]

Interesting.

My view is that because the panels are in series you'll be limited to max 50W output from the system.
OK it will maintain 50W through a far wider range of conditions and light intensities than maybe the case otherwise, but nonetheless you'll only achieve 50W because that's the rating of the smallest panel and they're all wired in series.

I think it would be better to have the 2x120w panels in series through one controller, and the 2x50w panels in series through another controller.

Yes people will tell you the 2 controllers will fight each other, but it still might be worth a try to see what happens in practice. It wouldn't harm anything.

Alternatively split your battery bank so some panels charge some batteries and others charge others - but that's not ideal either. The whole point is that to get capacity you need to group them all together, so splitting them is not desirable at all.

You're right, what we need is a MPPT controller with multiple inputs and one or two outputs, configurable. Amazing that none are available yet

 

[William_H]

Just from theory I would agree with Trundlebug above. However if you wired the panels with 2x120w in series to the MPPT controller then wire the other 2x 50w in series and connect in parallel to the controller what you might describe as a 24v solar system actually 40 v max no load voltage then it should work fine. The panels should contribute as much as possible into the MPPT input.
The MPPT should be able to decide the optimum voltage to load the panels in parallel.
The internal resistance of the panels will ensure that each gives what it can. However if one set of panels gets more sun than the other then the MPPT will have to determine a best power point from the average of the 2 sets. No great problem.
Your controller will have to be able to handle a max 8.5 amps at max 40 volts. (340watts)
If all panels are in series you will be limited to max 2.5 amps at 80volts (200w)
good luck olewill

 

[Tim Good]

Just from theory I would agree with Trundlebug above. However if you wired the panels with 2x120w in series to the MPPT controller then wire the other 2x 50w in series and connect in parallel to the controller what you might describe as a 24v solar system actually 40 v max no load voltage then it should work fine. The panels should contribute as much as possible into the MPPT input.

Your controller will have to be able to handle a max 8.5 amps at max 40 volts. (340watts)
l

Ok so we're talking this. If the panels are rated exactly the same voltage then might that cause issues?

 

[Trundlebug]

Yes I think that looks better than the original set up proposed with them all in series.
As long as the panels have internal diodes to prevent back current this should work quite well I think.
Good idea William.

I think it's better that the panels are rated the same voltage, otherwise only the higher voltage panels would ever contribute anything. At least this way there's a fighting chance of getting a reasonable output.
It's certainly worth a try and it won't do anything any harm.

I'm sure the purists will come along and poo poo it, but the reality is that a compromise is necessary, and this looks quite a good one. Let us know how you get on with it and how it performs.

 

[Pär1]

H.
Interesting tread I will follow. One detail, You should load the bank better If You take + in one end an - on the other, move - to the left battery.

 

[macd]

H.
Interesting tread I will follow. One detail, You should load the bank better If You take + in one end an - on the other, move - to the left battery.

+1, unless, of course, the cabling in the diagram is just a rough schematic. If it's the literal truth, it's a poor way to wire them.
This is how it should be done: http://www.smartgauge.co.uk/batt_con.html
(The link may be tired: it's been posted many times before )

 

[Tim Good]

+1, unless, of course, the cabling in the diagram is just a rough schematic. If it's the literal truth, it's a poor way to wire them.
This is how it should be done: http://www.smartgauge.co.uk/batt_con.html
(The link may be tired: it's been posted many times before )

Correct. It was a random pic I pinched off google to help illustrate

 

[William_H]

I don't see a great problem with panels of different voltage. ie max open circuit voltage one (pair) might be 18v one 20v. (36 or 40) The voltage becomes the same as they are wired in parallel and the actual supply voltage depends on the current drawn so whilst each pair might not run at exactly max power point the collective system will. good luck olewill

 

[ianj99]

The Victron MPPTs have a switch mode converter apparently, so you are not limited to the power of any one panel but at the higher voltage.
Thus four 12v 50w panels in series is 200watts, the controller converting the 48v at 4amps into 12v at 16 amps.
(rough and ready numbers).
I'd be interested to see a reply from Victron confirming the above which is what I am repeating from a earlier thread....

 

[Windway]

You might like to consider the Blue Sky Solar Boost MPPT controller. http://www.blueskyenergyinc.com/products/details/solar-boost-3000i. I fitted one of these to my boat and purchased 4x50w polycrystaline panels from Germany at £50 each and rigged them up on the lifelines either side of the cockpit, so I could raise one pair of panels (100w) depending on tack. In Sardinia I was getting 7.5 amps at midday. On very cloudy days this would drop to 4.7 amps. I used this set up to keep my three 125AH batteries in good condition for 9 months, without the need to run the engine, or seek shore power. I can put you in touch with the German supplier if need be and I bought the Blue Sky from Blue Marine Store in Palma, Majorca. I bought online and found them to be an excellent supplier. I have no connection with either Blue Sky or Blue Marine Store, but this set up worked very well with no problems whatsoever. The Blue Sky also has a built in LCD meter so at the push of a button you can view charge, panel state, battery state, battery temp all very helpful. PM me if you want further info.

 

[Tim Good]

The Victron MPPTs have a switch mode converter apparently, so you are not limited to the power of any one panel but at the higher voltage.
Thus four 12v 50w panels in series is 200watts, the controller converting the 48v at 4amps into 12v at 16 amps.
(rough and ready numbers).
I'd be interested to see a reply from Victron confirming the above which is what I am repeating from a earlier thread....

That's absolutely fine and the whole point of MPPT. My issue is 2 panels in series and then another 2 panels in series and then the two "Sets" connected in parallel. So that's basically 24V + 24V rather than 48V.

I'm trying to determine if this will work before I splash out on an expensive Victron Controller.

 

[dunedin]

Apologies for w related daft laddie question - we are also in the course of adding a Victron controller, and planning to use 2 x 100W panels into a 12V battery bank.
I had thought these panels would be better wired in parallel rather than series - but now wondering what the pros and cons of parallel vs serial are for such panels?

 

[Tim Good]

Apologies for w related daft laddie question - we are also in the course of adding a Victron controller, and planning to use 2 x 100W panels into a 12V battery bank.
I had thought these panels would be better wired in parallel rather than series - but now wondering what the pros and cons of parallel vs serial are for such panels?

Wire in series means your wire can be thinner as less amps and voltage will be double therefore potentially less voltage drop. If in parallel then arguably you don't need an expensive MPPT controller is my inderstanding. Also if panels are likely to get unequally shaded or unequal sun then series is no good. Only in series if they are in the same plane as if a single panel.

 

[dunedin]

Wire in series means your wire can be thinner as less amps and voltage will be double therefore potentially less voltage drop. If in parallel then arguably you don't need an expensive MPPT controller is my inderstanding. Also if panels are likely to get unequally shaded or unequal sun then series is no good. Only in series if they are in the same plane as if a single panel.

Very helpful thanks
PS Why the reference to MPT controllers as expensive - as far as I can see a Victron 75/15 is only around £75 - a relatively minor part of an overall installation

 

[Tim Good]

Very helpful thanks
PS Why the reference to MPT controllers as expensive - as far as I can see a Victron 75/15 is only around £75 - a relatively minor part of an overall installation

True but it depends if you bought "marine panels" at a fortune or normal domestic panels at £80. Lots of threads on here to suggest normal domestic ones perform equally well. My two new ones (2x120w) were £160 for pair. Apart from coating the connectors in the box in contralune and sealing the box well I'm sure They'll be ticking over nicely in 10 years.

But I agree in the grand scheme of boating it is not much.

 

[ianj99]

That's absolutely fine and the whole point of MPPT. My issue is 2 panels in series and then another 2 panels in series and then the two "Sets" connected in parallel. So that's basically 24V + 24V rather than 48V.

I'm trying to determine if this will work before I splash out on an expensive Victron Controller.

I can see no reason why that won't work but Victron are the ones who should know. It also has the advantage that if one panel packs up, the other pair will still be working.

 

[Tim Good]

but Victron are the ones who should know.

The haven't been very forthcoming. I get one word answers from a chap and at the foot it says "sent from my iphone". I may just try and see what happens.