On a clear winter's day you probably get much closer to 600W than 60W.
Even 60W is 0.05C for 1200 Wh battery.
In charts Dzl posted earlier, for one battery at -10C to 0C, max charge rate ranged from 0.05C to 0.10C depending on SoC.
At -15C to -10C, max charge rate ranged from 0.01C to 0.02C
...I glazed over...Soooo far past me...... but, does this mean that the claims of the -20F chargeable LiFePO4 cells could be true? Based on what i’ve read in this forum, not for any practical use. I’m no expert on the matter, though.
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In fact, 600W is 0.50C, and for that charge rate to be acceptable regardless of SoC ...
(for the one table "Gangfen" brand I'm looking at, the only one that allows any charging below 0 degrees C)
1) At no temperature is 0.50C acceptable above 95% SoC
2) Only for 15 degrees C to 45 degrees C is 0.50C charge rate acceptable at all SoC from 0% to 90%
Paralleling heaters and battery - would need diodes or other controls to avoid draining battery to run heater.
I suggest you follow whatever specs Big Battery gives for charging vs. temperature (they have man products, I can't find a spec for what you have right now, but I do see 0 degree to 55 degree C for one of their LiFePO4 products.)
It looks to me like your PV capacity, which can charge the battery at 0.5C, would be excessive if temperature dropped below 7 degrees C (for any SoC), would be excessive if SoC exceeded something like 60% to 90% depending on temperature.
How you could regulate charge current vs. SoC, I don't know. Toward the very top SoC, when charger switches from CC to CV, charge current would taper off. But middle of SoC (like between 60% to 90%), that isn't easy to determine based on voltage. It would take an intelligent BMS that counted coloumbs and communicated desired charge current to SCC. I think some BMS do perform that function, and some chargers follow those instructions.
To keep it simple and safely charge the battery, I would suggest increasing battery bank (or decreasing PV array size) for 0.2C max charge rate and allow charging only between 7 degrees C and 50 degrees C.
You could reduce peak output from a PV array in half without significantly reducing Wh/day captured by orienting half the panels at a 60 degree acute angle from the others. For instance, if two 300W panels totalling 600W, aim one panel at 8:00 AM sun and the other at 4:00 PM sun. That way, maximum PV panel area presented to the sun never exceeds a single panel, so output peaks at 300W but you get more hours production. With your 100 Ah 12V battery, this would still be 0.25C
Some LiFePO4 charts I see look like a CV setting of 14.0V, 3.5V per cell, would have charge current start to taper off prior to reaching 90% SoC (assuming cells remain in balance.)
I suggest switching PV production entirely off below 15 degrees C and using PV to heat the battery.
I don't have the exact charge data for your Big Battery, which may vary.
