ACIR measurement of lithium-ion cells
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What you will learn:
- Details on AC resistance and why ACIR has become a standard measurement for Li-ions.
- The 1 kHz test frequency for ACIR and why it is the reference frequency.
- Comparison between the ACIR test and electrochemical impedance spectroscopy (EIS).
The internal resistance of lithium-ion cells is an important measurement to make because the internal resistance of the cell can determine the suitability of the cell for a particular application. It can be used as a quality gate in manufacturing, as well as to determine cell aging or wear.
Internal resistance can be measured as AC internal resistance (ACIR) or DC internal resistance (DCIR). Internal resistance can also be extracted from the measurement of the cell’s impedance spectrum as captured by electrochemical impedance spectroscopy (EIS) instrumentation.
While ACIR measurements are quite common and somewhat standardized when measuring lithium-ion cells, DCIR measurements are non-standard and generally misunderstood. DCIR is also sometimes referred to as pulse testing interchangeably, which can cause further confusion.
In this article, I will focus on CARI. For more information on DCIR, please take the opportunity to read my DCIR-focused article.
What is CARI?
To measure ACIR, an alternating signal, usually alternating current (Iac), passes through the cell and the voltage response (Vac) of the cell is measured. The alternating current is usually around 100 mA and the frequency is 1000 Hz.
The measurement being carried out with an alternating signal, the result is an impedance measurement; to be a resistance measurement, the stimulus should be DC. The impedance at 1 kHz is calculated as Vac/Iac. There may be a phase shift between Iac and Vac when the impedance is measured. To put it simply, the real part (dc) of the impedance Vac/Iac is called ACIR.
A real application of the cell is unlikely to have a 1 kHz sinusoidal current load on the cell. Therefore, this measurement of ACIR does not reflect the behavior of the cell in a real application. However, ACIR has become a very standard way of assessing cell resistance, especially when comparing cells to determine which has the highest resistance.
Why is 1 kHz used as the test frequency for ACIR?
There are several reasons to use 1 kHz:
- 1 kHz is high enough that low frequency electrochemical processes are effectively invisible to the measurement, resulting in a measurement that captures ohmic resistive components.
- 1 kHz is low enough that any parallel capacitance or inductance of the cell, as well as the capacitance and inductance of the test wiring, will not have a significant impact on the measured resistance value.
- At 1 kHz, the measured resistance values are less dependent on cell state of charge (SoC) or temperature, compared to lower frequencies.
- Instrumentation that makes measurements at 1 kHz is easy to build accurately, reliably, and cost-effectively, compared to instrumentation that operates at lower and higher frequencies.
- Finally, it is traditional to use 1 kHz, as described in the next section.
History lesson: ACIR comes from the LCR meter
Using 1 kHz as an alternate stimulus signal reverts to the traditional method of measuring impedance, the LCR meter. An LCR meter is a type of electronic test instrument used to measure the inductance (L), capacitance (C), and resistance (R) of an electronic component. Since it can measure resistance as a function of impedance, an LCR meter can therefore be used to measure the internal AC resistance of a cell.
The LCR meter applies a 1 kHz sinusoidal current to the cell, then the meter measures the voltage across the current flowing through the cell. From this ratio, the multimeter can determine the magnitude of the impedance. Thus, it measures ACIR just like a dedicated ACIR meter, even though the LCR meter can measure more than ACIR.
For example, the Keysight E4980A Precision LCR meter (Fig.1) can be configured to accurately measure the ACIR of the cell under test. The test setup is described in the Keysight Technologies Impedance Measurement Handbook.
How does ACIR compare to EIS?
The ACIR and EIS pass an alternating current through the cell and the voltage response of the cell is measured. The main difference is that ACIR is complemented at 1 kHz and EIS is complemented over a wide frequency range by sweeping the AC stimulation current from mHz to 30 kHz or even higher.
During the sweep, at each frequency, the impedance Z is the ratio of voltage to current applied to the system Z = V/I and includes amplitude (real) and phase (imaginary). This data sweep is then plotted using a Nyquist chart, which plots the magnitude of the real value on the x-axis versus the phase or imaginary on the y-axis (Fig.2). Interpreting the Nyquist plot requires some physics, intuition, and/or the use of equivalent electrical circuits.
While ACIR only looks at the response of a frequency to determine the internal resistance at that frequency, EIS can reveal much more about the cell under test. (see table below):
- The frequency dependence of the response is used to separate the different contributions from the cell to allow independent observations of processes, such as different electrochemical reactions, ion diffusion in the cell, and material resistance.
- Examine different reaction mechanisms
- Examine the dynamics of the electrode process
- Evaluate the quality and function of the separator
- Study the behavior of the solid electrolyte interphase (SEI)
- Identify electrode corrosion condition
- Evaluate the SoC by observing changes in the EIS as the SoC changes
- Evaluate cell behavior with respect to temperature by observing changes in EIS as temperature changes