A thermal battery system may be provided comprising: a vessel comprising a phase change material and a gas; a heat exchanger configured to transfer heat to and/or from the phase change material; a temperature sensor configured to detect an indication of a temperature of the gas; a pressure sensor configured to detect an indication of a pressure of the gas; and a processor configured to determine a state of charge of the thermal battery system from the indication of the pressure and the indication of the temperature of the gas.