There is a need for techniques for efficient and accurate measurement of the impedance of rechargeable batteries at extra-low frequencies (ELFs, of the order of microhertz), as these reflect real usage and cycling patterns, and their importance in fractional battery circuit modeling is becoming increasingly apparent. Major impediments include the time required to perform such measurements and “drift” in impedance values when measurements are taken from the same battery at different times. Moreover, commercial impedance analyzers are generally unable to measure at these frequencies. We describe here our use of programmable two-quadrant power supplies to deliver multiple small-signal measurement tones in the presence of large-signal “working” currents, and our use of these data to generate impedance measurements with good precision and in reasonable time. The improvement in the quality of electrochemical impedance spectroscopy (EIS) data is verified through root-mean-square error (RMSE) when fitting equivalent-circuit models (ECMs).