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For our system installation in Canada, the temp can go very low up to -50 Deg C, and then the Voc will jump high, and can in a clear sky day with low temp jump above the max MPPT voltage,
As we understand it can cause damage to the MPPT if the voltage exceeds the max MPPT voltage (in our case we use 150/45 so it would be 150V).
The Open Circuit Voltage is the metric to measure for the maximum allowable voltage for the MPPT. Configuring a system that could exceed that voltage, even in extreme and unlikely conditions could destroy it.
It is also possible that the unit could work for years without issue, but the conditions of failure are still the same. If the battery is full, the MPPT can push the panels close to the open circuit voltage in normal operation, if that exceeds the rated voltage for the unit, it will fail catastrophically.
You may wish to use a 250V MPPT instead, while still operating around the ~110V range to give sufficient safe headroom.
Unfortunately, the Victron MPPT sizing calculator bottoms out at -40C. That is some exceptionally cold weather, do you really get any solar in at all during those conditions? You may be better to isolate the panels from the MPPT completely during those extreme months if solar yield is negligible anyway.
The Voc can be calculated for any temperature with the temperature coefficient of the panel. Just check the datasheet of the panel, that is being used. It should mention the temperature coefficient in Volts per Kelvin (V/K). Usually -0.3 or -0.4 %/K. It usually mentions the Voc for STC (Standard Test Conditions), which is at 25 degrees Celsius. Fom there you can work out the Voc at any given temperature.
Panel can work surprisingly well at temperatures below -40C as long as it is sunny. And conditions with -40C are usually quite sunny. With added reflections from snow and ice as a bonus.
Sunny yes, but presumably also extremely low sun and very short days?
What "fails catastrophically" when exceeding the maximum Voc at the input? Is it some capacitor, or an IC that can't handle the voltage? I would expect that there would be some overhead build-into the charge controller.
Asking the above question, because I just connected three panels in series to boost the Voc since it will be much too cloudy and snowy for a few days here and I really HAVE to keep a particular installation alive. Tried with 2s2p but the charge controller wouldn't produce enough power. 3s works much better, due to the higher voltage. Problem is, I am running 3 49.89Voc in series on a 150/30 SmartSolar, which means that if the sun shines through a hole in the clouds, and the battery is full, I will be having issues.
Any other ideas, apart from upgrading to a 250V Voc controller? Is there any way I could lower the Voc just a bit? I thought of adding diodes in the series connection, but diodes don't add a voltage drop, unless there's current going through. So, under the above conditions, the controller would still register a too high input voltage.
It will damage the MPPT. Please dimension the PV array accordingly with MPPT max OCV voltage specs
Thanks Guy and Dave.
As Dave mentioned, the panel can work very well in low temp conditions, as it is usually very sunny in that case.
I have used the MPPT calculator and also developed our own model, the results are that by -50 Deg C the Voc is about 111 V DC, which I believe would be acceptable for the 150/45 MPPT, right?
I have attached screenshots and also the datasheet for the PV modules we are using the 320 SP series.
Ca Sunprism MPPT.PNGCa Sunprism MPPT -50.PNG
SP Series 315-350-P72 Data Sheet V3_0 2018.pdf
Be careful with 2 panels in series that you have the sufficient +5V minimum above the battery voltage to operate the MPPT.
This will depend on your batteries, and voltage drop losses, etc.
Thanks for the warning, I have checked it with the Victron tool and it looks acceptable at least from the calculations side.
If there any other way to double check it let me know, please Guy
