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Old 04-02-2015, 06:50   #1
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Mixed solar panel on MPPT controller

I have a 290W panel and a MPPT controller on my boat. I sized the wiring etc when I did the install to have 2 of the panels for later and am at that point that I'm thinking of installing the second panel. The issue is that I can not get a second panel that matches the current one so the new one will be a little different.

I want to install the panels in parallel. My question is are the different Vmp voltages being 0.5V different going to cause the controller to go crazy?
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Old 04-02-2015, 06:51   #2
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Re: Mixed solar panel on MPPT controller

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Originally Posted by sailorboy1 View Post
I have a 290W panel and a MPPT controller on my boat. I sized the wiring etc when I did the install to have 2 of the panels for later and am at that point that I'm thinking of installing the second panel. The issue is that I can not get a second panel that matches the current one so the new one will be a little different.

I want to install the panels in parallel. My question is are the different Vmp voltages being 0.5V different going to cause the controller to go crazy?
the answer it, depends on the controller software but in general the answer is NO. its just that the VMP will not actually be the real VMP
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Old 04-02-2015, 18:40   #3
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Re: Mixed solar panel on MPPT controller

I would think there are lots of boaters with different sized panels out there who could provide input.

In the meantime I'm trying an experiment and send the question to Outback to see what they think (i.e. willing to put in writing)
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Old 04-02-2015, 18:53   #4
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Re: Mixed solar panel on MPPT controller

I have mixed panels on my boat, with an mppt controller, and it works fine. I wouldn't worry as long as the voltages are close (i.e. don't mix a chrystalline panel with an amorphous panel).
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Old 04-02-2015, 19:06   #5
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Re: Mixed solar panel on MPPT controller

.5 volt vmp difference will make very little difference, MPPT controller should work fine. There will be a little loss but very little.
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Old 04-02-2015, 19:59   #6
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Re: Mixed solar panel on MPPT controller

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.5 volt vmp difference will make very little difference, MPPT controller should work fine. There will be a little loss but very little.
I agree. The loss will be almost too small to even think about. Assuming you have the proper sized cabling, you'd be better served by considering tilting the panels toward the sun. The .5v difference is negligible.
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Old 05-02-2015, 03:54   #7
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Re: Mixed solar panel on MPPT controller

The closer the Vmps are the more efficient the system will be. As a general rule if the Vmp's are within 0.5v of each other the loss will be slight.

It is helpful to calculate the actual working Vmp of the two panels. The specifications list the Vmp at 25 deg C which is very unrealistic. You can use the temperature coefficients to calculate the Vmp at a more appropiate panel temperature. It is even possible to, roughly calculate the worst case loss. With the full I/V curve it can even be calculated accuratly, but the rough worst case numbers will give you some idea.

If you list all the specs (the temperature coefficients are needed) for the two panels I can do the rough version.

Other than the (slight) loss in power the voltage mismatch will not cause any other problems. Occasionally controllers can get temporarily stuck tracking the wrong Vmp, but this is more likely to happen with shadows than a Vmp mismatch. Good controllers like the Outback are much less likely to do this.
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Old 05-02-2015, 05:35   #8
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Re: Mixed solar panel on MPPT controller

Why would I need to calculate it as the controller has a setting for how offer it is checking for the max power point?

My 290W panel has been enough the past 2 years (boat out on a mooring with the refrigerator on all week). Tt takes a few days of clouds and a south wind to cause shading for the battery to get to 50% and that rarely happens. So I'm not all the worried about not getting the "most" out of having the second panel. I'm more woried about the effect on the controller.
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Old 05-02-2015, 05:56   #9
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Re: Mixed solar panel on MPPT controller

I would tend to think that with the panels in parallel the new Vmp will be calculated by the controller (the controller doesn't know or care what it's power source is only that at this elevated X voltage it's able to deliver Y amps to the batteries).


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Old 05-02-2015, 06:00   #10
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Re: Mixed solar panel on MPPT controller

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Why would I need to calculate it as the controller has a setting for how offer it is checking for the max power point?
I don't think this is user adjustable on the Outback, but that is not really critical.

The Outback will select the optimum Vmp for the combined effect of both panels and it will do this automatically. The closer the Vmps are between the two panels the closer the total Vmp will be to optimum Vmp for each panel and the better the performance (although the difference will be slight if the voltages are not a long way apart).

All solar manufacturers list the Vmp of their panels, but it is measured in a rather atypical way that does not correspond to actual Vmp you will see in real life. The actual Vmp under normal operating conditions will be a volt or so lower than the specifications. It is worth calculating the "real" Vmp of the two panels at least if you are trying to decide which panel to purchase to best match your existing one.
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Old 05-02-2015, 06:47   #11
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Re: Mixed solar panel on MPPT controller

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All solar manufacturers list the Vmp of their panels, but it is measured in a rather atypical way that does not correspond to actual Vmp you will see in real life. The actual Vmp under normal operating conditions will be a volt or so lower than the specifications. It is worth calculating the "real" Vmp of the two panels at least if you are trying to decide which panel to purchase to best match your existing one.
I can see this. But unless I can find a second panel with pretty much the same temperature curve it seems an anal irritation.
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Old 05-02-2015, 08:10   #12
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Re: Mixed solar panel on MPPT controller

Well Outback says it isn't a problem for my controller


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