I personally have my 9.5 KW batteries outside in a small enclosure, I have then placed some greenhouse heaters below them ( Tubular Heater 1ft, 2ft, 3ft, 4ft Tube Heater Green house Bathroom Conservatory (2FT (120w)) : Amazon.co.uk: Garden )
These are low power and I use Home Assistant to monitor the battery temp and turn them on when they start getting cold and turns them off once the batteries are warm enough
So far no issues at all with my batteries