Measurement is a more straightforward method of obtaining impedance. When electric vehicles are running, the operating conditions and status of the battery are changing all the time, and it is difficult to ensure the linearity, stability and causality conditions of impedance measurement. Most of the impedance measurements mentioned in the article are done during the shutdown process. At this time, the uneven temperature distribution in the battery pack, the equalization current of the battery cell, and the ripple of the converter are all controllable or even negligible. In order to measure battery impedance, a system with excitation generation, voltage, current signal measurement, and impedance calculation functions is necessary.
Excitation device. The power battery pack of electric vehicles is often composed of many battery cells or modules connected in series. Depending on whether the excitation is loaded into the battery pack or both ends of the single cell, it can be divided into two types: centralized and distributed measurement schemes. Centralized schemes can stimulate battery cells or modules at the same time, while distributed solutions need to be stimulated separately. The former is mostly integrated into high-power DC-DC converters, especially chargers. The latter are mostly combined with active equalization circuits, or single-chip solutions similar to those proposed by NXP and Panasonic. In contrast, centralized solutions provide stronger excitation signals and are more suitable for large-capacity batteries or modules. However, the distributed scheme needs to have higher requirements in signal detection accuracy due to power limitations.
Excitation signals. Many excitation signals that can be used for impedance measurements have been reported, including single-frequency sinusoidal, multi-frequency synthetic sinusoid, square wave, triangle wave, pseudo-random sequence, step signal, etc. These signals are divided into two forms, one is a signal containing a single frequency, this kind of excitation signal can easily ensure the signal-to-noise ratio during the measurement process, which is conducive to improving the measurement accuracy. However, due to the superposition of the various frequencies in sequence, the measurement speed of impedance over a wide range is relatively slow. This form of excitation is suitable for applications where real-time performance is low and accuracy is required. In some cases, signals with rich harmonic components are used as excitations for fast impedance measurements. Compared with a single-frequency signal, the amplitude of the harmonic component in such signals is often not optimized, resulting in a low harmonic signal-to-noise ratio of some frequencies, resulting in a large impedance error. In addition, excessive current will cause the temperature of the battery to change, and the presence of direct current will also cause the SOC of the battery to shift. When selecting an excitation signal, attention should be paid to the influence of different amplitudes, DC currents, and settling time on impedance.
Voltage and current measurement. Generally, for linearity and stability, impedance is measured with small perturbations. The response voltage of the battery under the action of excitation current is about 10mV. Considering that the terminal voltage of the ternary battery itself is between 2.5-4.2V, the detection of weak response voltage directly under large voltage bias puts forward very high accuracy requirements for the measuring device. Generally, the method of removing the DC component is used and the AC response voltage is further amplified to ensure measurement accuracy. In addition, in order to accurately measure impedance, according to Shannon’s sampling theorem, the sampling frequency of voltage and current needs to be at least twice the highest frequency of the impedance being measured. In a real system, this multiple would be higher, 2-5 times the highest frequency. This places high demands on the sample rate of the analog front end. Existing battery management chips designed to monitor voltage are still not sufficient for this application. Moreover, in series packs, the voltage and current of the cells are often measured by the local controller of the battery pack and the central controller of the battery pack, respectively, and the different controllers need to complete clock synchronization before the measurement to ensure that the calculated impedance phase is not affected.
Impedance calculation method. For sinusoidal perturbations at a single frequency, a lock-in amplifier can be used to extract the amplitude and phase of the voltage and current signals to calculate the impedance. For periodic harmonic signals that contain multiple frequencies, the Fourier transform can be used for impedance calculation. For non-periodic harmonic signals, the windowed Fourier transform can be used. Since the width of the window function with the Fourier transform is fixed, it is difficult to realize the adaptation of the window width and the frequency of the analyzed signal when performing multi-frequency analysis. The wavelet transform method is also used in impedance calculations. With the powerful harmonic extraction capability of the wavelet transform, broadband impedance calculations can be easily achieved using step signals.
Other effects. The measurement of impedance also has to consider other influencing factors. For battery cells or modules connected in series, their voltage measurement cannot be like the four-wire system of the Kelvin connection method used for EIS measurement in the laboratory, and the voltage division on the contact impedance between the connector and the battery (points B and C) will be measured but cannot be distinguished from the terminal voltage of the battery. When the battery pack has a connection failure, the contact impedance changes, causing the measured impedance to deviate from the true impedance. This impedance tends to have only an effect on high-frequency ohmic resistance. Therefore, a simple solution is to directly consider this contact impedance as the impedance of the battery. However, there has been relatively little research in this area. How contact resistance affects the impedance application of a battery requires an in-depth study.
Battery impedance can be obtained by estimation and measurement, which is challenged in terms of model accuracy, harmonic abundance under working conditions, and multi-time scale identification. The latter is a more direct and effective method, but it still needs to overcome technical problems such as high-precision and high-speed analog front end and the implementation of low-cost and high-compatibility systems.
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