Non-destructive testing of thermal batteries without activating them is a challenging proposition. Molten salt thermal batteries are activated by raising their temperature to above the melting point of the salt constituting the electrolyte. One approach that we have considered is to raise the temperature of the molten salt electrolyte to a temperature below the melting point so that the battery does not get activated yet may provide sufficient mobility of the ionic species to be able to obtain some useful ac impedance measurements. This hypothesis was put to the test for two Li(Si)/FeS2 molten salt batteries with two electrolytes of different melting points—a standard LiCl–KCl eutectic that melts at 352°C and a LiBr–KBr–LiCl eutectic with a melting point of 319°C. ac impedance measurements as a function of frequency and temperature below the melting point are presented for single cells and batteries.