ejrossouw avatar image
ejrossouw asked

SmartSolar 150/45 MC4 - Max input per MC4 port?

Hello. I used the MPPT configurator which suggested above unit for 8 panels of two strings of 4 panels in parallel. Closed circuit voltage/current of one panel is 32.26V and 9,75A. The above unit has two dual MC4 inputs and I would like to now do they work in parallel or series from a total input calculation point of view.

E.g. in parallel current per string will be 4 x 9,75 which will slide in under the 45A capacity, but exceed the 30A MC4 rating. Then there will be two of these strings doubling the current if connected to the two MC4 ports were they in serial resulting in nearly 80A, but still only 32.26 V which is below the 48+5V startup voltage.

What am I missing or how can I best connect these 8 panels? There is no shading risk or anything although I planned to have an East and West facing setup of 4 panels each to ensure morning and afternoon sun it benefitted from.

Any help will be appreciated.

smart solar set-up help
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4 Answers
Mark avatar image
Mark answered ·

I just saw the layout / wiring proposal.

Although it would work it's normally not done this way.

You should wire the panels in series first - so you will have 4 seperate strings of 2 panels in series.

Then you can wire 2 strings in parallel to one of the MC4 inputs & the other 2 stings in parallel to the other MC4 input. You will only need 4 x MC4 Y connectors to achieve this.

Inside the MPPT the MC4 inputs are connected in parallel.

This arrangement will work fine with respect to the MPPT input voltage & current limits.

In terms of how strings of 2 panels in series will work with a 48v system & morning 'wake up' / evening 'shut down'- it will work OK, but it's cutting it a little fine. A higher PV voltage into the MPPT would be of benefit with this setup.

As a worked example - if you are running a 48v system, then float voltage would be set at somewhere ~55.2V. If the battery is still at or near ~55V in the morning then it would require the input PV to exceed ~60V (55V+5V) before the MPPT will start up. Now yout MAX Voc is 79.7V (2 panels in series), so it will get there - but you will need to wait for a reasonable amount of light.

In saying that the Vmp for the same configuration is only 64.5V - but once things are up & running you only need to maintain PV above Vbatt +1V.

In future you could safely buy 4 more panels & increase the panels per string (in series) from 2 to 3 (total of 8 to 12). This would work great during winter & low light conditions, but with only 45A MPPT output limit there would be a fair amount of unused avaliable solar energy during periods of peak sunlight (this is not necessarily such a bad thing).

Something to consider...

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boekel avatar image boekel ♦ commented ·

[quote]You should wire the panels in series first - so you will have 4 seperate strings of 2 panels in series.

I think connecting the midpoint between the series might have some advantages, but that would be worth a seperate topic.

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ejrossouw avatar image ejrossouw boekel ♦ commented ·

Thanks. I saw some examples indeed and will take the advice :)

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Mark avatar image Mark ♦♦ boekel ♦ commented ·

Yes, it would be interesting to run a B2B controlled test with partial shading and individual panel current monitoring to fully understand whats going on.

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ejrossouw avatar image ejrossouw commented ·

Thank you Mark. You confirmed my thoughts the two MC4 ports are wired in parallel. I agree also with the startup voltages being on the low end and thank you for your wiring suggestion. Typically sunshine is not that much of an issue as our property is in Agulhas, South Africa and I am upgrading the existing 10 year old solution which was installed when the house was off grid totally. I will install two rows of 4 x 300W on either side of roof (replace existing in picture) to ensure I also catch the morning eastern sunrise. At least there is still ample roof to play with additional panels etc. and I can also simply opt for a larger MPPT. Incidently, I noticed many installation are a little oversized for improving winter generation so suspect 4 panels in series could also work as MPPT will lower excess to what is required.

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1552044125137.png (890.3 KiB)
Mark avatar image Mark ♦♦ ejrossouw commented ·

Very nice spot & beautiful stone house!

4 Panels in series will have a Voc over the 150v input limit - that configuration is NOT acceptable for your 150/45 MPPT.

The MPPT will limit the output current & consequently the input power - but it will NOT protect for Voc going over the input limit - you will damage the MPPT!

For example - if your batteries are fully charged on a sunny day then the MPPT will limit the PV input current to ~0A & as a result the PV voltage will be close to Voc.

This will also be even worse in cold & sunny conditions (Voc is quoted @ 25degC normally & is not an absolute max)

Also, if you are placing panels in different directions then the most efficient way to do this is to use a separate MPPT for each direction. I would highly recommended this & you could use 2 smaller MPPT's.

That way the maximum power point tracking is optimal for each set of panels & not negatively effected by the other set of panels exposed to more or less light.

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ejrossouw avatar image ejrossouw Mark ♦♦ commented ·

Thank you. Yes, the garage roof also slides open with a concrete platform where the previous owner mounted a telescope.

Thank you for the valued feedback, as it provided a lot a clarity on some things I still had to research further. Next week I will fly out to start mounting all the panels in the same as existing location.

I doubt I will expand beyond its current capacity, as evening consuption will hardly ever exceed 250W per hr with 3.6 kWh of 4kWh available in my BlueNova Lithium, which can deliver 125A continuous if necessary. (inverter would bail out long before battery)


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JohnC avatar image
JohnC answered ·

Hi Ej. The MC4 pairs into the mppt (as I understand it) simply combine into a singe bus inside the unit. So the multiple pairs supplied are for wiring convenience with MC4 rated 6mm2 wire.

But there's something wrong with your figures, and I urge you to recheck. In particular that 32.26V, which can't exist in short circuit conditions. So it must be either Voc or Vmp that you've picked from the spec sheet. If it's Voc, then ok, but that's a strangish Voc. If it's actually Vmp, then the Voc will be higher, maybe close to 40V, and you won't be able to series the panels 4x into a 150/ mppt.

Please come back to confirm. The rest shouldn't be an issue anyway, but let's not go there just yet.

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ejrossouw avatar image ejrossouw commented ·

Thank you for the prompt reply. These are the full specifications

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1551965935478.png (29.7 KiB)
ejrossouw avatar image
ejrossouw answered ·

This is my own design.

Panel connection.jpg

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JohnC avatar image JohnC ♦ commented ·

Yeh. And the 9.75A Isc is matched to a 32.26 Vmp or Voc. Which one?, hey this is actually very important - life or death of your system..

Google's a marvellous thing, so I went for a look and found a 300W JA panel that matched that Isc and V, but it was Vmp. And 4x in series wouldn't suit because the Voc is too high at just under 40V.

Please check. Sorry if I seem picky, but it really is important.

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JohnC avatar image
JohnC answered ·

We crossed over. And yeh, Google is a wonderful thing, cos that's the panel it found for me too.

Note the 32.26 Vmp and the 9.30 Imp. Multiply them together and there's your 300W. Production conditions.

The trouble is that 39.85 Voc. 4x of those in series is 159.4V, exceeding your mppt 150V spec. A BIG nono. You'll be limited to 3x of those in series, and that's typical of what everyone has to do with typical standard-style panels. Yeh it's a gamekiller, and you'll need to rethink your layout.

If you're stuck on 8 (not 9), you could wire 2S 4P and that would work fine into 48V batts. With all the I's considered.

I hate being the bearer of bad news, but best now rather than later. But ask again if you want. :)

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markus avatar image markus ♦♦ commented ·

"Yeh it's a gamekiller, and you'll need to rethink your layout."

Maybe his design picture got overlooked. He anyhow intends to have a 2s 4p.

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JohnC avatar image JohnC ♦ markus ♦♦ commented ·

Quite so. All good then when the correct figures added. Good spot.

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