How to utilise solar energy when the temperature is below freezing

DIY
, ,
1

I have following off-grid setup

Victron Multiplus II 48/5000
Cerbo GX
GX Touch 50
2 x Victron Smartsolar MPPT 250/100
2 x Pylontech US 5000 LiFePo4 batteries
16 x 405 W sola panels at 20° angle (along the roof).

This was installed last summer so I only have experience from one winter.

For the winter I turn off the inverter to save the idle consumption (18 Watts or more).

The two MPTTs and the Carbo GX and Touch 50 consume some IDK say 10 Watts so there will be some consumption over the winter months, around 300 Wh/day or 9 kWh/month.

So it would look like the batteries were saved by the BMS of the Pylontech shutting discharge down. That is my assumption, does this sound reasonable?

OTH There is theoretically on average 700 Wh/day solar power available at my latitude on those months, clouds and snow not withstanding.

The temperature is around 0°C in Dec then Jan/Feb around -5°C.

So I really don’t know what happens.

Well that is the background.

As there is possibly (otherwise this experiment would not make any sense) a surplus of solar energy (700 Wh/day produced best case/theory, about half of that consumed) I would like to know and experiment and add GSM router + a weather station + a camera or two and see if the systems stays live (inverter off) over the winter months.

The surplus energy (if any) does not make this work unless I can use it to recharge the batteries and below freezing the LiFePo4 BMS shuts the charging down.

So when the batteries are below freezing I would like to use all the solar energy and only that, not stored energy from the batteries, to warm up the batteries. Also when/if (fat chance) the batteries are at 100% SOC it would make sense to use the solar energy to heat the battery compartment (up to 40°C, not likely reached during winter) to store some heat energy for the next cold and sunny spell.

But how can I detect those conditions?

I have an electronics and software background so I can think of various methods but the details of how the BMS and MPPT and Cerbo operate seem to be rather sparse.

2

Let me elaborate on that.

Can I run some script or such on Cerbo to do this kind of thing or do I need some additional hardware for that say RPi?

Is the battery state available?

If the state is “not accepting charge” then I can presume the temp is below freezing?

But that alone is not enough if the battery is still able to supply power while not accepting power cause I obviously don’t want to heat with batteries.

Is that state available?

An old school method would be to isolate the solar panels with diode, tap the heating power before the diode, add a relay controller by an Arduino and temp sensor. But then in the summer that would loose 50W on that diode even if schottky. This would lack the theoretical possibility of using excess solar power when batteries are full but that I think is not really realistic anyway in winter.