NOVELTY - The system (20) has internal thermocouple (44) that is positioned within battery pack (18) to measure temperature of battery cells (42). A controller (30) is provided with processor (34) and tangible non-transitory memory (36) on which thermal model of battery pack and instructions for diagnosing performance of flow device (40) are recorded. The instructions are executed to receive and process the inlet and battery cell temperature signals from inlet thermocouple (50) and internal thermocouple respectively using thermal model so as to diagnose performance of flow device.    USE - Thermal system for heating or cooling rechargeable high-voltage DC battery pack for energizing high-voltage electric traction motor used as polyphase alternating current (AC) induction machine connected to internal combustion engine of vehicle such as hybrid electric vehicle, extended range electric vehicle and battery electric vehicle.    ADVANTAGE - Several internal thermocouples can be used in thermal system to provide improved level of fidelity and more accurate diagnostic results. The performance of the thermal system, mechanical and/or electrical functionality of the flow device, and geometry and other qualities of the conduit loop can be diagnosed efficiently based on recorded thermal model of the battery pack. The heat transfer fluid can be circulated to the battery pack through the conduit loop so as to heat or cool the battery pack. The optimal coverage can be obtained using three or four thermocouples. The temperature of the heat transfer fluid can be determined efficiently.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for method for diagnosing performance of thermal system.    DESCRIPTION OF DRAWING(S) - The drawings show the block diagram of the vehicle with high voltage battery and schematic view of the thermal system.    Battery pack (18)    Thermal system (20)    Controller (30)    Processor (34)    Tangible non-transitory memory (36)    Flow device (40)    Battery cell (42)    Internal thermocouple (44)    Inlet thermocouple (50)