A dual mode, thermal management system for use in a vehicle is provided. At a minimum, the system includes a first coolant loop in thermal communication with a battery system, a second coolant loop in thermal communication with at least one drive train component (e.g., electric motor, power electronics, inverter), a dual mode valve system that provides means for selecting between a first mode where the two coolant loops operate in parallel and a second mode where the two coolant loops operate in series, and a coolant reservoir that is coupled to both coolant loops when the two coolant loops are operating in series and only coupled to the drive train coolant loop when the two coolant loops are operating in parallel.