Can't charge lithium batteries when cold?

I've heard that 12V RV heater mats to keep a water heater from freezing work well for batteries and have built in thermostat.

Coyyote said:

Has anybody had any experience using thermostatically controlled mats used for germinating seedlings?  Relatively inexpensive and use 20-40 watts. Thought about strapping them onto the side of the battery. 

I've done it. The details are in another forum. Not sure if it is OK to post a link here. Bill ?

Summarizing: I built an insulated battery box with electronic thermostats, and a couple of small 12W 12V heating pads (my battery is 24V). I tested it in some pretty extreme temps over last winter, and it was able to keep the LiFePO4 cells between 50°F and 60°F with less than 5Ah (@ 24V) per 24 hr period. I had a couple of fail-safes, in that the heater won't run if the battery is below about 40% SoC (but can run direction off the SCC in that case), and the BMS has a low-temp cutoff that will prevent charging if the cells get down to 35°F.

Ok, thanks Bill.

Here's a thread where I posted my battery upgrade from lead acid to LiFePO4 (the first post was the original system in 2017 that was already posted here, everything after is about the upgrade):

https://diysolarforum.com/threads/horseflys-cabin-solar-lifepo4-upgrade.27472/

The details are in that thread, but I'll post some of the main stuff here.  Some main takeaways:

1. The physics tells us that how heat escapes the box you have the cells in is the main issue. If there is no heat escaping, you would just have to heat it once over the summer and leave it alone. But that isn't the case. Insulation is pretty important if you want to minimize how much energy you use to heat.
2. Keeping LiFePO4 warm is a bit like smoking a good pork shoulder: Low and slow wins the day. I only used 24W of heating pads for my roughly 2'x2'x2' box. With the temperature outside the box in the low teens or single digits, it took a bit over 3 hours to warm the cells from 50°F (where the thermostat turned ON) to 60°F (where the thermostat turned OFF). But that is only 3Ah pulled from my 230Ah 24V battery.
3. The thermal mass of the cells is high enough that it takes quite a while for them to cool off. After warming the cells to 60°F, it took between 15 and 21 hours for them to cool back down to 50°F.

Here's a pic of the not-quite-completed battery box, with 2" XPS on all sides and on the bottom (the lid is not shown, but it also has 2" XPS). The aluminum plate on the bottom has the two 12W 12V heating pads under it.


Here's a graph of the multi-day test when the temps were in the single digits and low teens outside the box.  


Here the green line is showing when the power was applied to the heating pads. The blue line shows the temperature on the aluminum plate under the cells, and the red line shows the temperature on the top surface of the cells. I had originally worried that the pads might heat up too fast, which would cook the bottom of the cells before the top got warm. But by using just 24W of pads the bottom didn't heat up all that much.

Bottom line: Keeping LiFePO4 cells warm enough to operate through sub-freezing temps is not really difficult, and doesn't consume all that much energy. Even if my panels get covered with snow, I think my setup can keep the cells warm and toasty for a least a couple of weeks.