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Old 05-27-2018, 06:52 PM   #1
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THOR #4687
Why multiple MPPT controlers on a solar installation

Trying to get the final design in my head for a solar system for our 25.2 VEGAS...


I am a little "Mystified" about why you would want to have multiple MPPT controllers on a solar system.


We are looking at utilizing six (6) 100 watt panels and I am thinking that will take one (1) 60 AMP MPPT (If that is wrong, someone correct me) or a mixture of smaller MPPTs.


That is where having multiple MPPTs is escaping me at the moment. I understand having more than one as a "backup". but not sure exactly how to design this, or if it is the best/most correct design.


Any help form all you RV GURUs out there would be greatly appreciated.
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Old 05-27-2018, 08:25 PM   #2
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Why do you want to complicate a system solely to have a backup?

Don’t you have other backups in the form of your generator + converter? Or the engine driven alternator that can charge the batteries until you can replace the solar controller in case of a failure?

I can see having 2 if needed because the system is too large for one controller, but can’t see splitting the system for fear of a failure. I’d make it as simple as possible. Just my personal preference to keep things simple.
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Old 05-27-2018, 08:39 PM   #3
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I completely agree on "keeping it simple", I am just having a problem finding any information on why to have more than one.

From what I am seeing the system I am designing should have a 60A controller, so I am wondering why, when I read about other RVers installing their systems, they are using two (2) 30A controllers....

I just have not seen anything on the "why" of designing the system this way..

Thanks for the reply CHANCE.. Just trying to make sure I am not going down the wrong road before I order anything...
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Old 05-27-2018, 09:40 PM   #4
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Almost sure that you cannot wire 6 of any panels in series without violating the max voltage of one MPPT controller. So if you wire 6 in parallel you will have a significant shade problem along with 60 amp wires required. With two sets of three in series and two MPPT controllers you'd get the best performance from the 6 panel system and much less amps down to the MPPT. Best to get 3 larger watt panels that will fit your MPPT's voltage spec.
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Old 05-28-2018, 12:55 PM   #5
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The break even point from a price perspective on choosing MPPT to begin with is around 400 to 500 watts of solar. Since your over that one may have value but I don't see any way to justify the cost of two.
Looking at wiring options 3 panels, each 100 watts, will have a Voc of about 63 volts which should not be a problem for the MPPT CC. You of course have no choice but to use mppt when wiring panels in series.


The way I did it was 5 -100 watt panels in parallel. 3 are portable so I can get around the the shading issues. This allowed me to use a 45 amp PWM CC.
If you did something along these lines for your 6 panels the cost of two PWM cc's would be about the same as a single mppt.


Wiring costs would of course go up.


This maybe another option that lets you set up the system with a spare CC.
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Old 05-28-2018, 03:25 PM   #6
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Howzit fit?

Hey Mo_Mike, how do you fit 6 panels on your roof? I'd love to see your layout plan. Would you be able to walk around to clean them?
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Old 05-28-2018, 03:29 PM   #7
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My 2 cents ....

A major controller manufacturer rates their 60-Amp unit at up to 800 watts of nominal panel capacity (for 12-Volt battery systems). Maximum panel power ratings obviously go up considerably at higher battery voltages, but that doesn’t apply here.

The same manufacturer rates their 45-Amp controller for up to 600-watts of panel capacity (for 12-Volt battery system). That should also work but it’s cutting it close, so I’d personally stay with 60-Amp size.

With 6 panels, there are only so many ways to connect them, and most can be ruled out quickly:

1X6
2X3
3X2
6X1

One string of six panels in series I would rule out because voltage is too high, and it’s also too susceptible to shade.

Connecting all six panels in parallel is best for shade, but current is highest from roof down to controller, so I’d skip that option also.

Of the two remaining options, I’d personally go with 3 strings of 2 panels in series each. The current is a little higher than 2 strings of 3 panels in series each, but the maximum voltage is lower, making it a bit safer. The system would also be a little less susceptible to spot shade.
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