Hi there, I am using a SmartSolar MPPT 75|15 and read in the specs that it has a temp sensor built-in. I tried to use it for a low temp cut-off but couldn't trigger it as an attached temp sensor is needed for this feature? (see also my video of my test https://youtu.be/S9qw9-SSplo?t=300)
Is this correct and why couldn't the built-in sensor be used for a low-temp cut-off if the temperate is already measured? Is it due to the heat that is produced by the charge controller?
thanks, Lupo
Hi @Lupo,
the internal temperature sensor of the MPPT is only for temperature compensation (primarily used for lead acid batteries) and cannot be used to trigger the MPPT to shut off in case of low temperature.
To make the MPPT shut off in case of low temperature of your Lithium battery you can set up a VE.Smart Network, connect your MPPT and at least a Smart Battery Sense or a BMV with temperature sensor to the same network, so that the MPPT gets the temperature reading directly from the battery.
See also the manual for Low Temperature cut-off, Page 21: https://www.victronenergy.com/upload/documents/Manual_SmartSolar_MPPT_75-10_up_to_100-20/MPPT_solar_charger_manual-en.pdf
As @Stefanie mentions the manual described the internal temperature sensor use in the manual is described as temp compensation charging and a low temps allows small current charge as you were seeing.
The protections of the mppt are described in the technical section of the manual. Low temperature cut off is not one of them as a stand alone item.
Your bms should have cut off charging if it was not happy with received charge in the low temps in anycase?
Here is a guy who tested the low cut off here with the smart network.
thanks @Stefanie and @Alexandra for the information and help. I think I read somewhere in the DIY Solar forum that the built-in sensor worked and therefore, even didn't look into the manual.
The questions remains, why the charge controller don't use the built-in sensor. It would be just a small adaption in the firmware, as all the logic is already present for the low-temp cut-off using VE.Smart Network and the temperature compensation. Might be a good feature? What do you think?
Sorry, I forgot to answer that question. The internal sensor gets warm during PV production and charging and in the end doesn't reflect the battery and/or environment temperature at all, thus not useable for low temperature cut-off.
The temperature of the battery pack is most important since it is not the mppt that cannot charge below freezing but rather the lithiums are the ones that cannot be charged.
Some people have battery warming kits installed so would make the internal mppt temperature not useful. There are too many other variables some have been mentioned. And the fact that some people don't install them correctly, or even in the same room as the batteries.
Will prowse did a basic video on building a pack with the smart network for low temp cut off.
For you, everything is in a box, but maybe someone else has those in separate buildings.
That is why having a known local battery temperature sensor is more reliable.
