question

Check out LiFeYPo4 battery made by Winston.

They can be charged down to -25C.

Charts I got from manufacturer of those cells (Thunder Sky Winston Battery Limited)

The other way to go is to build heated box for the battery.

You can not charge LiFePo4 cells if they are below freezing. Some manufactures of LiFePo4 battery systems do provide internal heating because of this but if they are going to be getting that cold you'll be wasting a lot of energy trying to keep them above freezing.

The operating temperature of the Winston LiFeYPO4 Battery is -45℃~85℃，check it on this website:40Ah Thunder Sky Winston LiFePO4 Battery WB-LYP40AHA (evlithium.com)

They certainly *can* be charged (more slowly!) below 0C, but most cell suppliers don't provide any data to tell you how to do this, or just forbid it -- possibly because if they can't control charging currents (see below) it's safer to just ban it below 0C to stop people killing batteries and then making warranty claims.

Winston have some well-hidden data (quoted above), but allowing charging at 0.3C down to -25C seems *very* optimistic to me, it's *way* more aggressive than anything else I've seen.

Some battery suppliers with their own inbuilt BMS (which takes control) do allow charging at reduced rates below 0C, for example BYD allow the following for a 70Ah LV (3.5kWh) battery:

Discharging -- 50A from -10C to +15C, 60A from +15C to +35C, 50A from +35C to +50C.

Charging -- 15A from -10C to +2C, 20A from +2C to +12C, 40A from +12C to +50C.

This is 0.2C down to -10C (0.3C from +2C to +12C) which looks more reasonable then Winston -- but it's their own LFP cells with their own BMS so they can control exactly what is allowed.

I would just choose the simple solution and put the system in your basement. Below the frost line, you should be good to go (though still keep an eye on it)

I have a new Solar system in Alaska. -15 -20 Fahrenheit last week (-40 on a rare occasion some years). Cabin has no heat when unoccupied. LIFEPO4 12V system. I built an insulated battery box and put a 60W forced air fan PTC heater in it. This heater is designed to be in enclosed distribution boxes. Inside the cabin today is -3.5 deg. The heater comes on for 30 seconds every 3.5 min. I use a thermal snapdisc (40-60) inline with the positive input wire as a thermostat. Temperature is stable at 50 deg +- 1 deg F. I have experimented with the snapdisc location, as in high in the box vs low in the box. It currently sets low in the box. High in the box runs for 12 seconds and off for 4 min. While this is the simplest solution I have other ideas in the works. Car seat warmers placed on the underside of an aluminum plate and batteries sit on top of plate. Snapdisc would again be used for control. Or just plain PTC heater elements with no fan secured to the metal plate.

Read up on the PTC heaters.

and

https://dbkusa.com/blogs/whatisptc/ptc-heater-types-explained

We have BB Lithiums in the Airstream, They stay in it on a disconnect switch.

If the monitor should get below 12.2v we plug in to charge when the temp is above 28*F.

It's only happened once when I forgot the disconnect.

POI it does get cold here in the WNY sno-belt,