The present invention generally relates to a thermal battery testing apparatus and a method for testing a thermal battery. The method includes one or more of the following steps: connecting the thermal battery in series with a resistance; connecting the thermal battery and resistance in series with a sinusoidal voltage source; applying a sinusoidal voltage to the thermal battery, measuring an impedance, reactance and/or capacitance across two terminals of the thermal battery, comparing the measured impedance, reactance and/or capacitance to a reference impedance, reactance and/or capacitance; and indicating whether the tested thermal battery is "in family" or "out of family." The battery testing apparatus may include a testing device configured to apply a sinusoidal voltage to the thermal battery and to measure the impedance, reactance and/or capacitance across two terminals of the thermal battery. The testing apparatus may further be configured to report "out of family" batteries.