Methods and system for operating a thermal storage device of a vehicle system are provided. In one example, a method comprises estimating a temperature of a thermal battery after the battery and coolant included therein have reached thermal equilibrium, and determining a state of charge of the battery based on the estimated temperature and one or more chemical properties of two phase change materials included within the battery. Specifically, the thermal battery may include two phase change materials with different melting points for providing thermal energy to warm coolant in a vehicle coolant system.