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tony-m avatar image
tony-m asked

Understanding MPPT calculator

Is there a way I can use the VictronConnect bluetooth app to check the results I'm actually getting against the results I'm theoretically supposed to get from the Victron MPPT calculator? For example, I see there are many things I can plot (Solar Voltage, Solar Amps, Battery Voltage, Battery Amps, etc...) and I want to use some of these to check whether I'm nearing the limits for my Victron SmartSolar 100/20 MPPT.

Here is the specifics of my situation: I have a campervan filled with four 50W and four 175W renogy solar panels. I'm going to spend the next 2 winter months with a 95Ah AGM battery (as I wait for delivery of my 460Ah LiFePO4 battery), and with all the cloudy and rainy weather lately I want to get the most solar power I can to charge it.

According to the calculator, I can use the MPPT 100/20 with two of the 175W panels connected in series as long as I check "Terminals" instead of "MC4" in the PV Connection section, as shown below.

pv-connection.jpg

But if I choose MC4, the calculator indicates my MPPT 100/20 is not adequate even with just two 175W panels in series. I guess I don't understand what PV Connection is being referred to here? I have ferrules on the wires connecting to the MPPT 100/20 itself and am using the MC4 type of connectors that came with my renogy solar panels. I thought that was the standard way of doing it, if not please explain. That part of your calculator makes a huge difference!


I have actually tested connecting three and even four 175W panels in series under fairly sunny and cold conditions and when I look on the VictronConnect app everything seems well under the "100V" and "20A" specs of the MPPT/20.

I realize that it all depends on the exact sun conditions and temperatures, but my point is that I want a way to check how close I'm actually getting to my charge controller's limits under the actual conditions I'm experiencing.

Because of your calculator, I've decided to play it safe and only have two of my 175W panels connected, but I'm including a screen recording of the results from the calculator when I add a 3rd or 4th panel to show the kind of results it gives.

Thanks for your help

VictronConnectmppt calculator
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3 Answers
Justin Cook avatar image
Justin Cook answered ·

What you're probably seeing is that if you check the "MC4" connector box, the calculator will recommend the smallest Victron MPPT that has MC4 connectors, which if I recall correctly would be the 150/60-MC4 (also available in a -Tr with terminals).

The 100/20 only has terminal connectors, so is being ruled out when you tell the calculator that you need a version with MC4 connectors.

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Alexandra avatar image
Alexandra answered ·
You can check out the mppts on the product pages. The picture help with understanding.

TR is the screw terminals & MC4 is the MC4 connections on the mppt. (Not referring to the solar panels at all which have MC4s)

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tony-m avatar image
tony-m answered ·

OK now I am clear about MC4 connector part -- I hadn't realized Victron sold MPPT's with MC4 connectors directly attaching to them.


But I still feel Victron's MPPT calculator tends to return answers that suggest a larger MPPT, and the focus of my original question is: how can I use the VictronConnect app to see if I'm really likely to exceed the limits of my MPPT?

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regman54 avatar image regman54 commented ·

Just a suggestion but when I was looking to upgrade my charge controller I found this calculator easy to use: https://explorist.life/solar-charge-controller-calculator/

As for using the connect app I think the above calculator is more a more useful tool than the app.

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kevgermany avatar image kevgermany ♦♦ commented ·
The connect app shows what's happening. You can interrogate it to see how close to the limit you are getting.

But... As you say 2 panels in series is OK, putting the panels in a 2P2S configuration will also be ok. Just make sure you don't exceed the Isc rating of the controller.



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regman54 avatar image regman54 kevgermany ♦♦ commented ·

When I was rebuilding my small solar system I had some constraints on what size and how many solar panels I could carry in the storage compartment so that's where I started - the solar panel specs. Using the above calculator I found it easier to determine what size CC was needed without going overboard on hardware which is why I suggested it. Now that I've completed my hardware upgrade I find the Connect app to be very useful and it's why I went from the other brand to Victron.

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