Electrochemical impedance spectroscopy (EIS) is being used by battery manufacturers and OEMs to verify battery performance and warranty claims. EIS evaluates the electrochemical characteristics of a battery by applying alternating current (AC) at varying frequencies and measuring the current response of the battery cell. Frequencies range from 100 microhertz (mHz) to 100 kilohertz (kHz). 100 mHz is such a low frequency that it takes more than 2 hours to complete a full cycle. By contrast, applying a 100 kHz frequency pulse completes 100,000 cycles in 1 second.
The idea of applying various frequencies is to test the different layers of the battery and examine its characteristics at all levels. Battery resistance falls into three categories: pure Ohmic resistance, inductance, and capacitance. Capacitance is responsible for the capacitor effect, whereas the inductance is accountable for so-called magnetic field, or coil effect. While the voltage on a capacitor lags behind the current, the opposite is true on a magnetic coil, where the current lags behind the voltage. When applying a sine wave to a battery, the reactive resistance produces a phase shift between voltage and current. This is the basis used by EIS to evaluate battery performance.
EIS is not new and was used originally to perform in-flight analysis of satellite batteries, providing an estimate of such factors as grid corrosion and H2O loss. Because this type of testing provides the ability to observe the kinetic reaction of the electrodes, it has gained acceptance as the most reliable means of evaluating performance and expected remaining battery life. Increases in impedance readings can suggest intrusion of corrosion and other deficiencies. Impedance studies using the EIS methods have been used for a number of years on lead-acid batteries and they have generated a good database for reliably determining battery capacity and predicting life span.