NOVELTY - The system has a rechargeable energy storage systems (RESS) coolant circuit (53) directing a coolant through a battery pack (46). A power plant coolant circuit (75) has a heater core branch (83) with a heating, ventilation and air conditioning (HVAC) pump (74) for pumping the coolant through a coolant heater (76). A heater core (68) is located in a HVAC module (28) to provide heat to a passenger compartment of a vehicle (20). A coolant-to-coolant heat exchanger (66) provides heat transfer between the coolant in a coolant heating branch (57) and the coolant in the heater core branch.    USE - Heating, ventilation and air conditioning system (HVAC) and rechargeable energy storage systems (RESS) thermal management system for a vehicle e.g. plug-in hybrid electric vehicle, electric vehicle and fuel cell vehicle.    ADVANTAGE - The coolant heater provides heat of coolant for both the heater core in the HVAC module and the battery pack in various vehicle modes of operation. The system utilizes the coolant for both passenger compartment heating and battery pack heating without using the additional coolant heater for the battery pack, thus saving the cost and reducing the weight and the complexity. The coolant used to warm the battery pack is separately maintained from the coolant used in the power plant, thus protecting the coolant flowing through the battery pack from the potentially high temperatures and pressures, and debris.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method for thermally controlling a passenger compartment and a battery pack of a vehicle having a power plant.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic illustration of a vehicle having a heating, ventilation and air conditioning system (HVAC) and a rechargeable energy storage systems (RESS) thermal system.    Vehicle (20)    Heating, ventilation and air conditioning module (28)    Battery pack (46)    Rechargeable energy storage systems coolant circuit (53)    Coolant heating branch (57)    Coolant-to-coolant heat exchanger (66)    Heater core (68)    Heating, ventilation and air conditioning pump (74)    Power plant coolant circuit (75)    Coolant heater (76)    Heater core branch (83)