A primary reserve thermal battery includes a primarily CF.sub.x cathode, a lithium-based anode, and a solid electrolyte between the cathode and the anode. A high heat source positioned proximate the electrolyte is capable upon activation to heat the electrolyte to a temperature below about 250.degree. C., and more precisely to the melting point of the electrolyte. At that point, the electrolyte undergoes rapid melting and becomes highly conductive, whereupon to cause the battery to produce a burst of power for delivery to an external load.