Sanity temp derating check and low temp cutoff

[Adwsail]

I am a virtual newbie when it comes to solar though I have built a teardrop trailer with all victron components that works very well thanks to the Will Prowse videos. I have designed and purchased the following for my off grid barn. EG4 6000xp scc/inverter, EG4 LL-S 48v 100ah battery x 2, and 10​

Canadian Solar CS6R395 panels. I am in NW Arkansas and concerned about my understanding of the solar panel output in winter. By my calculations these panels have a Voc temp coefficient of -0.26°C which gives me a 10s string Voc of 504.348 Vdc at -28°C (-18.4°F) which is clearly over the MPPT spec. At -25°C (-13°F) I’m ok at 475.8 Vdc but just barely. With climate change a fact, imho, I’m concerned that even at -25°C I’m risking the health and welfare of the 6000xp.

So the questions: 1. Are my numbers shown reasonable? 2. Is there a low temp cutoff in the 6000xp that I can set to turn off charging at say 1°C? To be honest I’ve read the 6000xp manual but not the LL-S battery manual which I’m sure can shut itself down at a specified temp. 3. Would a temp monitoring circuit with a suitable DC rated relay be prudent to cut off the PV string at 1°C so there is zero chance of charging below freezing? It would be an overkill processor wise but a Raspberry Pi could easily do the temp sensing and relay control. 4. Anyone have a better approach to protecting the 6000XP from over-voltage AND protecting the LiFePO4 batteries from charging below freezing without cutting out any panels? Thank you for the wealth of information found in this forum. And apologies for cross posting but no one replied where it was and I really could use some help.

 

[robbob2112]

Your numbers are an understatement.

The 6000xp has two MPPT input with max Voc of 480v.

Voltage change per panel works out to 6.57 volts - so 47.7 + 6.57 = 54.27v * 10 = 542.7v -- way over.

switch to two strings of 8s for a string Voc of 434.16 or just two strings of 5 panels for 271.35.

This means you will need a bit more wire to run them separately

from

https://eg4electronics.com/wp-content/uploads/2024/04/EG4-6000XP-Inverter-Spec-Sheet.pdf

The battery manual says they have low temp protection to turn off charging

https://signaturesolar.com/content/documents/EG4/1511009-manual.pdf

For low temp - you will need to manage the battery temperature to keep them at least 32f. Insulate around them and use the inverter exhaust is one way it can be done. Use a RPi to control a fan to keep them up. Use a heating mat with a thermostat in the cord and insulation around the batteries - cut on at 32ish and off at 45ish is about the way they come.

What you don't want to do is switch the batteries on and off based on temp, it will make things unstable and possibly damage the inverter.

It won't hurt the batteries to sit at low temperatures, but you won't be able to charge/discharge them when they are cold.

 

[Adwsail]

Thank you for your reply Robbob2112. I’m struggling trying to understand your numbers. Specifically the 47.7v and the 6.57v added to it. I also realize I didn’t show my work either. Could you possibly point out where I’m going wrong with the following calculations.

Vout = (VOC @ 25°C) * (TC = -0.26%/°C) * (temp differential = -50°C) so
Vout = 36.6v * (-0.0026) * (-50) = 4.758v increase (it’s a positive number) per panel @ -25°C or adding the starting voltage at 25°C of 36.6v I get 41.358v @ -25°C. Seemingly well within spec even in a 10s configuration.

 

[robbob2112]

 

[Adwsail]

Now I understand, the example you are showing is a math example which is correct, but it’s not for the CS6R395 panel despite what it says. I’ve attached a current data sheet from Canadian Solar.