Methods and system for operating a thermal storage device of a vehicle system are provided. In one example, a method comprises determining a state of charge of the thermal battery based on an accurate estimation of a melting temperature of one or more phase change materials (PCMs) at a specific aggregate pressure inside the thermal storage device. Variation in melting temperature of the PCM may be minimized by reducing pressure variation inside the thermal storage device by regulating a position of one or more pressure relief valves of the thermal storage device.