13.7 Electrochemical Impedance Spectroscopy
Electrochemical Impedance Spectroscopy (EIS) has been used for a number of years to test the SoH and SoC of industrial batteries. EIS is well suited for observing reactions in the kinetics of electrodes and batteries. Changes in impedance readings hint at minute intrusion of corrosion, which can be evaluated with the EIS methods. Impedance studies using the EIS technology have been carried out on lead acid, NiCd, NiMH, Li-ion and other chemistries. EIS test methods are also used to examine the cells on larger stationary batteries.
In its simplest manifestation, measurements of internal battery resistance can be taken by applying a load to a battery and observing the current-voltage characteristics. A secondary load of higher current is applied, again noting the voltage and current. The current and voltage relationship of the two loads can be utilized to provide the internal resistance using Ohm’s Law.
Rather than applying two load levels, an AC signal is injected. This AC voltage floats as a ripple on top of the battery DC voltage and charges and discharges the battery alternatively. The AC frequency varies from a low 100mHz to about 5kHz. 100mHz is a very low frequency that takes 10 seconds to complete a full cycle. In comparison, 5kHz completes 5000 cycles in one second. At about 1000Hz, the load behaves more like a DC resistance because the chemistry cannot follow the rapid changes between charge and discharge pulses. The information about electrolyte mass transport is ascertained at lower frequencies.
Additional information regarding the battery’s condition can be obtained by applying various frequencies. One can envision going through different layers of the battery and examining each level. Similar to tuning the dial on a broadcast radio, in which individual stations offer different types of music, so too does the battery provide different information of the internal processes. The EIS is an effective technique to analyze the mechanisms of interfacial structure and to observe the change in the formation when cycling the battery as part of everyday use.
When applying a sine wave to a battery, a phase shift between voltage and current occurs. The reactive load of the battery causes this phenomenon. The overall battery resistance consists of three resistance types: pure resistance, inductance and capacitance. Capacitance is responsible for the capacitor effect; and the inductance is accountable for the so-called magnetic field, or coil effect. The voltage on a capacitor lags behind the current. This process is reversed on a coil and the current lags behind the voltage. The level of phase shift that occurs when applying a current through a reactive load is used to provide information as to the battery’s condition.
One of the difficulties with the EIS method is interpreting the information. It is one thing to amass a large amount of data, and another to make practical use of it. Although the derived information reflects aging and other deficiencies, the readings are not universal and do not apply in the same way to all battery makes and types. Rather, each battery type generates its own set of signatures. Without a library of well-defined reference readings with which to compare, the EIS method has little meaning.
Modern technology can help. The vector settings of a given battery type can be stored in the test instrument and translated into meaningful readings by software. The readings can further be analyzed by coupling impedance spectroscopy with a fuzzy neuro-adaptive algorithm.
Electrochemical Impedance Spectroscopy is commonly used to research batteries in a lab environment. Best results are obtained on a single cell. EIS is also used in aviation and in-flight analysis of satellite batteries. Closer to earth, the EIS method examines stationary batteries for grid corrosion and water loss. Further refined, the EIS technology has the potential for wider applications, such as testing portable batteries. EIS may one day test batteries in a matter of seconds and achieve higher accuracy than current methods.