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Old 20-03-2022, 05:26   #1
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Parallel two solar controllers??

Upgrading my solar panels and the new panels exceed the rated capacity of my current controllers. The two controllers are rated 15 amps each and the new panels are rated 27 amps.

I have a new controller on hand for one panel but have two panels.

Wondered if I could use the two 15 amp controllers in parallel to take the output of one 27 amp panel. Would the controllers share the output equally or would a difference in internal resistance of the controllers send all the panel output to one or the other?
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Old 20-03-2022, 05:52   #2
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Re: Parallel two solar controllers??

Could you let us know the brand/models of the panels and controllers?
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Old 20-03-2022, 05:59   #3
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Re: Parallel two solar controllers??

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Originally Posted by BjarneK View Post
Could you let us know the brand/models of the panels and controllers?
Hi Bjarne,

Victron 75/15 MPPT controllers. 75Voc max, 15 amps.

Not programmable but small, very efficient and a great deal.
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Old 20-03-2022, 06:28   #4
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Re: Parallel two solar controllers??

I also use Victron MPPT 75/15 controllers. They are actually very programmable. They can do all sorts of things. I have used them to charge lead acid, LiFePO4 and Li Ion batteries. They are also super flexible on the input side: they can be used with solar panels, batteries and power supplies for input. Note that the 15 A of the 75/15 controller refers to the maximum battery current.

I have not tried paralleling two controllers on the same panel. I would expect the MPPT algorithms to interfere with each other and for it not to work as hoped. Say one controller figures out that the MPPT voltage is 40 volts. You attach another controller. It sees no current at 41 volts, but lots of current at 39 volts. So it decides that 39 is a good MPPT voltage. Then the other one lowers the panel voltage to 38 volts etc. This results in a "race to the bottom" of the panel voltage and would not give much power. This is just my guess, though.

Here is what I do: I have 300 W panels and a 12 Volt DC system. Ideally, this could result in 22.5 A at 13.33 volts for the batteries. The controller just limits the battery current to 15 A and no damage is done even with a panel that can deliver more power. This may seem like a waste, but it is not really. In all the situations where the panel can only deliver 200 W or less, there is no loss. It is only when the sun is very high in a clear sky that any loss is incurred. In those situations, I have too much power anyway and use it to heat water. You are on your way to Europe, right? Even with a bigger controller, I doubt you would see over 200 W most of the time over there anyway. So just having one controller per 300 W panel works great for me.
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Old 20-03-2022, 06:40   #5
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Re: Parallel two solar controllers??

Another thing: When you oversize the panel, the controller may spend quite a bit of the day at the maxiumum output of 15 A. This will make it hot. Strangely, the 75/15 alu heat sink is a plate that will go right up against the wall when you mount it. It seems like a bad way to get rid of the heat. So I put 15 mm spacers between the wall and the controller to leave some air behind it. That helps a lot to avoid overheating. If it overheats, the output is lowered and no damage is done (except long term heat which is always bad).

I even went so far as to mount a fan to cool my three controllers. It runs from 120 minutes after sunrise until 60 minutes before sunset, but only when the output voltage is over 13.2 V. This was done using the programmable load output of one of the controllers. It would be nice if the load could be controlled from the internal temperature, but that is not possible. With the spacers, the fan may not be strictly necessary, but it is always good to keep electronics as cool as possible.
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Old 20-03-2022, 06:51   #6
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Re: Parallel two solar controllers??

That's a very good idea @Bjarnek. A small CPU pancake fan certainly doesn't use much electricity. They are great for forcing a little bit of air circulation around refrigerators, behind consoles, etc.
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Old 20-03-2022, 06:52   #7
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Re: Parallel two solar controllers??

I have 2 310watt panels mounted in parallel with 2 each 75/15 Victron controllers. Seems to he working just. 3 years now.

If one cell is one panel is shaded it kills the whole panel, or both if mounted in series. In parallel then you loose just one. I like the added reliability of duplicate systems. I will give il some small efficiency gains to achieve redundancy.

But I am more of a pragmatist than a purist.

I have been laying to a dock recently. My heading is about NNE. So morning sun shades my stb panel with my wind gen. Then some days I hook up a 2W trickle charger.
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Old 20-03-2022, 08:25   #8
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Re: Parallel two solar controllers??

Well I lied but just a little bit.

One of the controllers is the older 70/15 that I'm pretty sure isn't programmable. The other is the newer 75/15 but still several years old. Will have to check that one as it might be.

I did notice the heat sinks if one may call a flat metal back a proper heat sink mount against the wall but as the old panels max around 12 amps and in real world usually put out 8-10 overheating wasn't a problem.

But still begs the original question. Any guess what will happen with two 15 amp controllers paralleled with one 26 amp panel? Smoke and flames or not?
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Old 20-03-2022, 08:29   #9
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Re: Parallel two solar controllers??

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Originally Posted by mako View Post
That's a very good idea @Bjarnek. A small CPU pancake fan certainly doesn't use much electricity. They are great for forcing a little bit of air circulation around refrigerators, behind consoles, etc.
Yep. Use one in my fridge. It works fine until it's jammed full at the start of a trip, then not enough air circulation so the stuff in the bottom freezes. Installed a 2" PVC pipe vertically in the front corner of the box with a 2" CPU fan on top. Now all is good, even with the fridge completely jammed full.
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Old 20-03-2022, 08:34   #10
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Re: Parallel two solar controllers??

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Originally Posted by hpeer View Post
I have 2 310watt panels mounted in parallel with 2 each 75/15 Victron controllers. Seems to he working just. 3 years now.

If one cell is one panel is shaded it kills the whole panel, or both if mounted in series. In parallel then you loose just one. I like the added reliability of duplicate systems. I will give il some small efficiency gains to achieve redundancy.

But I am more of a pragmatist than a purist.

I have been laying to a dock recently. My heading is about NNE. So morning sun shades my stb panel with my wind gen. Then some days I hook up a 2W trickle charger.
So if I understand correctly, you have two 310 panels and four 75/15 controllers. Two controllers paralleled together on the output of each panel? And so far no emeltdowns or major conflagrations?

If so then, at least temporarily, I'll give this a go. But, if my boat explodes I'm giving you a call.
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Old 20-03-2022, 08:44   #11
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Re: Parallel two solar controllers??

Quote:
Originally Posted by BjarneK View Post
Here is what I do: I have 300 W panels and a 12 Volt DC system. Ideally, this could result in 22.5 A at 13.33 volts for the batteries. The controller just limits the battery current to 15 A and no damage is done even with a panel that can deliver more power. This may seem like a waste, but it is not really. In all the situations where the panel can only deliver 200 W or less, there is no loss. It is only when the sun is very high in a clear sky that any loss is incurred. In those situations, I have too much power anyway and use it to heat water. You are on your way to Europe, right? Even with a bigger controller, I doubt you would see over 200 W most of the time over there anyway. So just having one controller per 300 W panel works great for me.
So your saying the 75/15 is current limited and, if fed too much power from an oversized panel, will still only output the max of the controller IE 15 amps?

In that case I think I'll go with just the one controller for now since, as you point out, except in ideal sun angle, shading conditions, etc the panel won't be putting out the max anyway.

Then I can pass along one panel and one controller to a friend that's putting his boat together.
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Old 20-03-2022, 08:46   #12
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Re: Parallel two solar controllers??

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If one cell is one panel is shaded it kills the whole panel, or both if mounted in series. In parallel then you loose just one
No, that only happens for very old panels that were manufactured before they started adding bypass diodes, or when your bypass diodes have blown up (tend to happen with cheap knock-off diodes used in some panels)

A series system actually only looses one string in a panel with a single shaded cell instead of the whole panel like with parallel connections. Modern panels have two or three strings so only loose half or a third of capacity.
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Old 20-03-2022, 09:07   #13
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Re: Parallel two solar controllers??

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So your saying the 75/15 is current limited and, if fed too much power from an oversized panel, will still only output the max of the controller IE 15 amps?

Yes, that is my experience. This means that you can even feed a Victron MPPT controller straight from a battery. So in case you had a 24 V battery and wanted to charge a 12 V battery from it, you could connect the 24 V battery instead of a solar panel as input to the controller.
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Old 20-03-2022, 09:52   #14
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Re: Parallel two solar controllers??

Here is MY system. Off the top of my head, but I think I am correct.

Solar panels are rated in Watts, not amps. They have a max current.
And there is a lot of optimistic thinking in getting to the advertised watt rating.

Power = current x voltage

If I hooked my 310 watt panels direct to the battery (say 12v) the most current I could get would be about 8.7 amps or 8.7x12=104watts.

The MPPT controller allows you to use the solar panels full voltage and converts it to 12vdc. So instead Of 104W delivered to the battery I get 210W delivered.

The 75/15 is rated for 75 Volts i out so that you can hook 2 each 36W panels in series. But you are still limited to 15 amps out of the controller, I believe.

Below is my system.
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Old 20-03-2022, 10:27   #15
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Re: Parallel two solar controllers??

Quote:
Originally Posted by BjarneK View Post
Yes, that is my experience. This means that you can even feed a Victron MPPT controller straight from a battery. So in case you had a 24 V battery and wanted to charge a 12 V battery from it, you could connect the 24 V battery instead of a solar panel as input to the controller.
Very convenient. Think I will keep the 75/15 for general whatever uses. Sounds like it could even be a back regulator for the alternator,
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