question

The 'low temperature cutoff' function is only active if the MPPT is paired with another device that can provide it with a measured battery temperature (BMV, SBS, BMS, STS).

The MPPT internal temperature sensor is not reliable once the MPPT is active as the charger generates it's own heat. Even for voltage compensation due to temperature - it only takes a reading in the morning just before it starts up (which could be significantly different to later in the day).

It could be possible to use the same logic to disable charging commencing for lithiums, but it's still a risk as the MPPT would not know if the temperature dropped later.

On top of that the battery temperature could be significantly different to the initial MPPT measured temperature since;

-The MPPT is commonly located in a different area / compartment to the batteries

- The large thermal mass in a battery means that the battery temperature will significantly lag behind ambient temperature (for example the ambient temperature could easily be >5degC but the battery temperature is still <0degC

- The charge / discharge process will generate heat within the battery (a positive factor in this case)

In summary; it's not worth it for an expensive lithium battery, particularly when you can buy a 'Smart Battery Sense' device fairly cheaply and provide the MPPT with the correct battery temperature throughout the day.

You've made good points as to why an external sensor co-located with the battery is better than the charger internal sensor. However in our case, it'll be once in a blue moon when we experience freezing temperatures (this is in a van with rare snow trips), and the fancy wireless sensor is over the top for this use case (pity there isn't a simple inexpensive wired/passive sensor).

It would have been nice to have the unit simply not charge until the ambient temperature was suitable; however I understand why Victron may have decided to do this, and we can manually work around it by simply disabling charging via the app if it gets too cold.

Thanks for the input!

I searched for the same. Thanks for this.

Also in my case it would be enough to stop charging if the temp. is below e.g. 5°C in the ambient, and restart if it's above.

I do currently the same by disabling charging via the app.

I will need to buy the external sensor - not an issue - but the documentation of the mppt should/could be changed accordingly (mppt 150/35).