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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.
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