[Jack Houpe]

Your batteries BMS should have low temp cut off but that only saves the batteries and your without electric so I installed two 200 watt ceramic heaters that are controlled by a thermostat and a relay. My system is 24v so you may have to install a few more of these. There are also thermo blankets that you can install under or around them that are controlled by the same type thermostat below. You will have to use a relay as the heaters will draw more current than the thermostat is capable of. 

https://www.amazon.com/Constant-Temperat...191&sr=8-3

https://www.amazon.com/bayite-Fahrenheit-Digital-Temperature-Controller/dp/B011VGAPOC/ref=sr_1_3?crid=3VAOV9SXNS5SB&dib=eyJ2IjoiMSJ9.ET1qFEPz7pg9QH6tHpMpGofjMwUzQEcRtATX9Rg2Qwi_EXo5KPLxOF8y2eJ8m9A5vdSBFnlcxbPo4SOhA2EpqWgzWk74ybkhKhr9WXa3rmMiTiX60DCLJ_cItK1_R-B9eJcH2cJl3KlOUbWsdpgk1cZmNCTyJz46s55BSvOfB-tseUXdLPAw50qc_8eDL9mn0WJFI_t6iZW-kg1tpnPtpcOJF-E1ddaUAHfW4GOhhJwfa4TgBVAA6vOO1paPCSFYvRNA3sUjI_vMI2R3tz8V21TuJ40BjNNahNscH_Ze3QM.3akeeGkl8pbhMz9r6lFoHVYEv3VNgOSJM2pAXFUy8kI&dib_tag=se&keywords=12v+thermostat&qid=1773656539&s=home-garden&sprefix=12v+thermostat%2Cgarden%2C177&sr=1-3