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jose-mite avatar image
jose-mite asked

MPPT sizing sanity check - is there enough headroom for cold temperatures?

Hello,

a local company has just finished building an ESS system based on Victron. I'm new to the community, I have to say I really like Victron HW and SW so far. However the more I read the more I'm not sure the company designed and built the Victron system correctly. I have what looks like a standard DC-coupled ESS as Victron describes it in their docs.

I have:

  • 20x 500W panels connected in 2 strings (I think these 500W, complete data sheet can at the bottom of the page as PDF),
  • 2x MPPT (SmartSolar MPPT VE.Can 150/100 rev2).
  • 3x Multiplus II, 5000/48 (3-phase system), all loads of the house on ac-out + ac-out2

I assume they connected the panels in series of 3, since I'm seeing voltage of about 130-135V on each MPPT (I'm waiting on the company to confirm how they connected the panels). It's about 15°C outside these days. Voc of the panels is about 48V at 25°C, so I assume it's 3 panels connected serially. Not sure what they did with the remaining 2 panels after connecting 3x6=18 panels, I'm trying to find out.

I have two main questions / worries:

  1. I put the panels into Victron MPPT sizing calculator`. It says I should use the 250/85 version, because in low temperatures the voltage might go over 150V (and I believe it, I'm now at 135V with 15°C outside, in winter we easily get -20°C). Do I need to force the company to change the MPPTs? Are they crazy to not account for low temperatures when sizing the MPPTs?
  2. Is there any way the company could have connected the 20 panels in series optimally? I'm suspecting they made 6 series of 3 and then one serie with only 2 panels, so these 2 are not producing any power. This would be in line with my observations, the whole system never gives more than 8,5kW of power even with ideal conditions, but panels sum up to 10kWp.

Thanks in advance for any advice, any help would be appreciated.

Edit: links are not displayed properly.

MPPT Controllerssolar sizing
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1 Answer
Phil Gavin avatar image
Phil Gavin answered ·

PV panels have a %Voc temperature coefficient listed in their specs. There is no need to guess.

It is easy to calculate the expected Voc at an expected low temperature.

Normally, it's about 0.3% ish.

So a drop in temperature to - 20C represents 45 degrees.

45 * .3% = 13.5%

48V *1.135 = 54.5V

So 3 panels in series will be 163.5V, which by all accounts will damage your 150V MPPTs.


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jose-mite avatar image jose-mite commented ·

Thanks for the answer. I can see the coefficient in the panel's specs, it's 0.28% (for some reason the links were not displayed properly in my question). The MPPT calculator clearly says these 3 panels serially are way over the max voltage in low temperatures:

https://mppt.victronenergy.com/#djE7Y3VzdG9tLDMsMyw1NCwtMzUsNTAsNDgsMTIuMzEsMC4wNSwxMy4yLDUwMCw0MC42MiwtMC4yOCw0OC44MywxNSw2

Thanks for confirming.

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