Relion is maybe the only one to do it right, for an internal heater that is internally controlled. Below 0dgC, all incoming charging current is diverted to an internal heater. When the battery is sufficiently warmed, the charging can begin. No battery power is ever used to run the heaters. Perfect! I think Battle Born missed the boat with their system.
Couple ideas here....
1) Do what Relion has done; that when under suitable charge temperatures redirect any incoming charge power to the heaters. Problem is if no charge power coming in batteries are not being heated. The colder they get the longer to get them warmed; and risk to get to a dangerous storage temp if in very cold temp areas.
2) Use battery power, down to some SOC %, to power the heaters to keep batteries in a state they can be charged. This way when chargers come on line you can immediate use that to charge and recover the pack. At some safety SOC cutoff % then disconnect.
The ideal is to combine this I think.
When under a safe charge temp use charger power to heat if it's available or battery power (to a defined SOC %) to heat.
All this assumes you have ample capacity; if your capacity is marginal it could change the approach.