Hi, my apologies if this has been suggested already. After going through about 18 of the 36 pages of replies here, I decided to just go ahead and describe the dead-simple, cheap and effective battery warming system that I used last winter.
I have an off-grid cabin here in Bethel, Vermont. The cabin temperature frequently reaches subfreezing temperatures for several days/weeks in a row.
Electrical system consists of four 360W solar panels, two Growatt SPF 3000TL LVM-ES inverter/charger/MPPT controllers in parallel (120v). Batteries are one Jakiper 48v 100ah lifepo4 running in parallel with four series-connected 12v 200ah AGM batteries.
I built an easily removable enclosure, similar to the engine covers used with inboard speed boats - 2x4 frame, 1/2" OSB and interior lined with 1 1/2" rigid foam.
In the box I have a $20 120v electric baseboard heater thermostat
like this one connected to the inverter that controls a 120v socket. Heating pad plugged into that and lying on top of the lithium battery. Alternatively, one or two (for redundancy) low wattage (~15w) light bulbs. Not too close to the thermostat. Have thermostat set to turn on when temperature in box drops below 40°-50°F.
An afternoon's work, $100 or so in parts and materials, simple, reliable. Little technical expertise needed. In the spring, take the box off and stick it in a corner.