A method of operating a RESS thermal system in a vehicle having a coolant loop for directing a coolant through a RESS and a refrigerant loop configured to selectively cool the coolant flowing through a chiller in the coolant loop, including: determining a current target temperature range for the RESS based on a current vehicle operating mode and ambient temperature; determining a temperature of the RESS; determining if the temperature of the RESS needs to increase or decrease to be within the current target temperature range; if the determination is made that the temperature of the RESS needs to increase, determining if an active heating or a passive heating of the coolant will be employed, the active heating using a greater amount of energy over a shorter time period than the passive heating; and activating the determined active heating or passive heating of the coolant.