I put together a battery heating system to keep my LiFePO4 batteries above freezing so I can charge during cold=weather camping. Basically there are 2 heating pads under each battery connected to an adjustable temperature controller that turns the heating pads on or off depending on the temperature of the battery container (based on a temperature sensor). I set it up so there is a separate heating system for each battery (I have 2 100AH batteries wired in parallel). I surrounded the batteries and heating pads with some rigid foam insulation. Testing so far shows that the heating pads do not get so hot that I need to worry about melting anything and the system draws approximately 2 amps when the pads in both systems are actively heating -- heating cycles on and off once desired temperatures are reached. There is an on-off switch for each system so I can fully shut it down when not needed. Here are some images to show what I did. I will continue evaluating how well it works once colder weather gets here.

Downers Grove, IL
2017 T@B S Max, Blue and Silver -- "The Blueb@rry"

Comments
2017 T@B S Max, Blue and Silver -- "The Blueb@rry"
2020 Audi SQ5
Item
Description
Source
Silicone Rubber Heating Pads
15cm x 5 cm; 12V; 20W; set of 4 pads
Amazon
Project Box
Junction Box IP65 Waterproof Dustproof ABS Plastic Universal Electric Project Enclosure with Fixed Ear Black 6.2x3.5x2.5 inch (2pc)
Amazon
20-Gauge Galvanized Tie Plates
Used as heat distribution for heating pads. Placed one on each side of the heating pads.
Home Depot
Toggle switches
Waterproof Round Rocker Switch with Shell 12V Blue LED Rocker Toggle Switch Lighted ON
Amazon
Rigid Foam Board Insulation
Pro Select R-Matte Plus-3, 0.5 in. x 48 in. x 8 ft. R-3.2 ISO Rigid Foam Board Insulation
Home Depot
Temperature Controllers
wDC 12V Electronic Temperature Controller Programmable -50 to 110 Degree Celsius Heating/ Cooling Thermostat Control Switch
Amazon
2017 T@B S Max, Blue and Silver -- "The Blueb@rry"
2020 Audi SQ5