Heating pad control via SMBS or DEXT16

[alvise mj]

I was thinking to use a heat pad to keep battery above 5 degree Celsius in a mobile application.

My idea is when remote camping I will have maybe sun but cold battery 

SMBS will not allow charging battery because they are cold, I wonder if I can use the diversion from the DSSR20 to a heating pad.

Compare to using directly the battery I will be sure that I do not deplete the battery try to heat, but just using the extra energy of the panels.

Do i need to have thermostat on the line going to the pads or is better using the DEXT16 to receive signal from thermostat?

I don't think i can have the SMBS activating the heating unless I use a EXTIOn, but i am running short on available EXTIO ...
Any idea suggestions ?
Comment on any other part of the setup is always very welcome

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https://drive.google.com/file/d/1b3Qkw0QCkegfX1ItxfiXOSq8qhrxBtdj/view?usp=sharing

[Gordon Bland]

I just use a small heat pad and an external digital temperature controller (dm w3001).  It frees up an extio and at a glance I can see my battery temperature. 

[Dacian Todea]

alvise,

It is not practical to have a battery outside without a insulated and heated box. If the battery cools down bellow freezing it will take a significant amount of time to heated back if you are even able to for the entire day.

The only practical solution is to have the battery in an insulated and heated box. If box is properly done you will not need more than a 10 to 15W heater to keep the battery above +10C

[alvise mj]

sound good, I have the battery inside so i will not probably insulate the box more, I also concern about high temperature in summer , looks like LiFoPo4 lifespan can badly shorten in high temp environment. Inverter and all the other components are in the same space. I start thinking maybe to have just a separate digital temperature controller that has 2 relays , one set for heating and one for cooling in  summer with a small fan that get fresh air from outside in the compartment.

[Dacian Todea]

In typical offgrid use the battery will be just in the 0.2C charge rate and less than 0.5C discharge thus battery will not be more than 1 to 2 degree Celsius above the ambient temperature.

If you have an insulated battery box then heating element will not work when above +10C (assuming that was your set temperature) and as mentioned battery in normal use case conditions will not generate much heat. You can calculate exactly based on your battery internal resistance and connection resistance (but those should be much smaller if done correctly).

Say a proper 8s 24V battery made with 200Ah cells each cell with 0.5mOhm internal + contact resistance will make the total battery pack around 4mOhm so say you charge with 40A then 40A x 40A x 0.004Ohm = 6.4W as heat on the entire battery.

You can have maybe the battery box top open or slightly open so ambient air gets inside but the 6.4W on that entire battery surface will not increase battery to more than 2C above ambient.

In some other applications like EV's battery is stressed at much higher charge discharge rates and those high energy density cells used in EV's have higher internal resistance compared to LiFePO4 thus there is much higher heat loss hundred's of Watt maybe even a few thousand depending on pack and charge/discharge rate.