Abstract: A `heat battery` or heat storage device based on the phenomena that at a particular temperature and pressure water becomes supercritical and can absorb increasing quantities of heat without a corresponding increase in either temperature or pressure. The device may operate a heat engine such as the Stirling engine or a steam turbine engine in an automobile. The battery includes a pressure vessel for maintaining a charge of water in the vessel at approximately 374 degrees Centigrade and 221 bars of pressure. A heat exchanger coil is positioned in the vessel from which useful heat is extracted and applied to end use such as a heat engine. An electric heater for inputting heat into the charge of water is positioned in the vessel and external connections from the vessel allow for changing the charge of supercritical water and for substituting high supercritical heat containing water for a charge that has been substantially depleted of supercritical heat.