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Unipolar Excitation Signal with Two Phases for Impedance Spectroscopy of Li-ion battery cells
To ensure stability, causality, and linearity in impedance spectrum measurements, excitation signals must be carefully considered. These signals should cover all necessary frequencies down to the mHz range, have sufficient signal energy, and not change the battery state. However, designing a binary excitation signal for dc/dc converters that meets these requirements can be challenging due to the presence of harmonics in the excitation signal and the influence of frequent battery cycling. To address this problem, we propose a design methodology for a discrete interval binary sequence (DIBS) based on binarized multisine signals that are optimized by evolutionary role-playing game theory. Several techniques, including sign,
$\Delta \Sigma $
, and pulsewidth modulation (PWM), have been studied to implement an appropriate binarization procedure. DIBS-
$\Delta \Sigma $
has shown a high signal-to-noise and distortion (SINAD) of 83 dB, but it has a high switching rate (~76%), which leads to more power consumption. The switching rate of the DIBS signal is only ~9%, but it has higher charge changes and a low SINAD of 23 dB. The proposed signals have been experimentally validated using a Samsung INR-18650-25R Li-ion battery cell. With a frequency density of ten frequencies per decade and a measurement time of 200 s, DIBS-
$\Delta \Sigma $
has an rms deviation of 0.05%. For DIBS-Sign, the accuracy is 0.11%. These deviations can be reduced to 0.02% and 0.03%, respectively, with sufficient stabilization time.
In embedded impedance spectroscopy, when the available DC/DC converter in power systems is used to generate excitation signals, a bias current is typically present, leading to unavoidable charging and discharging processes. This violates a crucial condition for impedance spectroscopy, which is to maintain system stability during the measurement, and thus leads to systematic deviations. In this paper, we investigate the stability and characterize the influence of charging/discharging during measurement on the impedance spectrum. Based on this investigation, we propose an excitation sequence followed by a two-step procedure to correct the influence of the unavoidable charging/discharging process and enable a correct impedance spectroscopy measurement. First, the influence of the bias current on the impedance in the diffusion process is identified and neutralized using the distribution of relaxation times, followed by an averaging procedure to minimize the charge state discrepancy between measurement points. The proposed method is validated on a Samsung INR-18650-25R Li-ion battery cell, which was able to provide stable measurements for both 1-quadrant and 2-quadrant settings, with RMS magnitude and phase accuracy of 0.16% and 0.02°, and 0.021% and 0.38°, respectively.
For diagnosis of batteries and fuel cells based on impedance spectroscopy, excitation signals are required, including low frequencies down to the mHz range. This leads to a long measurement time and compromises the stability condition for impedance spectroscopy. Multisine excitation signals with logarithmic frequency distribution can significantly reduce the measurement time but need optimization of the crest factor to realize a high signal-to-noise ratio at all excitation frequencies and maintain at the same time the linearity and stability conditions of impedance spectroscopy. Crest factor optimization is challenging, as the obtained results strongly depend on the initial phase values and many trials are necessary. It takes a very long time and can not be easily performed automatically up to now. In this paper, we propose a time-efficient hybrid stochastic-deterministic crest factor optimization method for multisine signals with logarithmic frequency distribution. A sigmoid transform on the multisine signal gradually transforms the multi-frequency signal into a binary-alike signal. The crest factor is significantly decreased, but the phases of the singular frequency signals remain sub-optimal. Further optimization based on the Gauss-Newton algorithm can determine the final phases, realizing a lower crest factor. The proposed method is less sensitive to initial phase values and provides more reasonable results in a reasonable time. The validation on a Samsung INR-18650-25R Lithium-ion battery cell shows that the crest factor of the optimized multisine signals has a median of 3.62 ± 0.7 within 6 min of run time, which is significantly better than the best previous work in the state-of-the-art of 3.85 ± 0.11 for the same run time.
Equilibria evolving over time, also called trends, are often present in ongoing measurements of real-life systems. These trends are considered as disturbances when computing the Best Linear Time-varying Approximation (BLTVA) of the
system. Current techniques for dealing with trends in BLTVA measurements consist of modelling the trend with a finite number of basis functions. However, in measurements with dominant trends, the trend cannot always be captured well enough by this set of basis functions, and, hence, the uncertainty on the BLTVA increases. As a consequence, one looses low frequency information. In this paper, the state-of-the-art method for estimating the BLTVA is extended by removing the trend with a differencing operator. It is shown that with this novel technique, low frequency information becomes more visible. Moreover, the novel method decreases the variance on the BLTVA, and allows to measure
fewer periods. Hence, the novel technique improves the route for treating arbitrary out-of-equilibrium, or also called operando, measurements. As an illustration, it is applied to operando time-varying impedance measurements of three electrochemical processes: the charging of a Li-ion battery cell, the electrorefining of copper and the anodising of aluminium.
Several studies show that impedance spectroscopy is a suitable method for online battery diagnosis and State-of-Health (SoH) estimation. However, the most common method is to model the acquired impedance spectrum with equivalent circuits and focus on the most sensitive parameters, namely the charge-transfer resistance. This paper introduces first a detailed model of a battery cell, which is then simplified and adapted to the observable spectrum behavior. Based on the physical meaning of the model parameters, we propose a novel approach for SoH assessment combining parameters of the impedance spectrum by building the ratio of the solid electrolyte interphase (SEI) resistance to the total resistance of SEI and the charge transfer. This ratio characterizes the charge-transfer efficiency at the electrodes’ surfaces and should decrease systematically with SoH. Four different cells of the same type were cycled 400 times for the method validation, and impedance spectroscopy was performed at every 50th cycle. The results show a systematic correlation between the proposed ratio and the number of cycles on individual cell parameters, which build the basis of a novel online method of SoH assessment.
For embedded impedance spectroscopy, a suitable method for analyzing AC signals needs to be carefully chosen to overcome limited processing capability and memory availability. This paper compares various methods, including the fast Fourier transform (FFT), the FFT with barycenter correction, the FFT with windowing, the Goertzel filter, the discrete-time Fourier transform (DTFT), and sine fitting using linear or nonlinear least squares, and cross-correlation, for analyzing AC signals in terms of speed, memory requirements, amplitude measurement accuracy, and phase measurement accuracy. These methods are implemented in reference systems with and without hardware acceleration for validation. The investigation results show that the Goertzel algorithm has the best overall performance when hardware acceleration is excluded or in the case of memory constraints. In implementations with hardware acceleration, the FFT with barycentre correction stands out. The linear sine fitting method provides the most accurate amplitude and phase determinations at the expense of speed and memory requirements.
Lithium-based batteries are a class of electrochemical energy storage devices where the potentiality of electrochemical impedance spectroscopy (EIS) for understanding the battery charge storage mechanisms is still to be fully exploited. Generally considered as an ancillary technique, the application of EIS should be promoted focusing on improved experimental design of experiments and advanced data analysis using physics-based models. Electrochemical impedance spectroscopy is a key technique for understanding Li-based battery processes. Here, the authors discuss the current state of the art, advantages and challenges of this technique, also giving an outlook for future developments.
Wideband excitation signals are essential in bioimpedance spectroscopy for measurements in a time ensuring a quasi-stable measurement condition. In particular, for wearable biomedical systems, due to limited system resources, several aspects regarding measurement time, crest factor, slew rate requirements, frequency distribution, amplitude spectrum, and energy efficiency need to be thoroughly investigated. In this paper, we present an investigation of excitation signals, which includes not only the theoretical aspects but also aspects of real implementation on microcontroller-based systems. At a fixed number of samples and sampling rate, we investigate the implementability of signal frequencies and the resulting spectral efficiency. We focus on sources of signal distortion due to timer and amplitude deviations. The results show that for 4096 samples and a sampling frequency of 1 MHz, wideband signals are 2.76 times faster than a stepped frequency sweep. The multisine signal provides a better energy efficiency and has a lower slew rate requirement on hardware (around 0.3 V µs ⁻¹ ), but has a relatively high crest factor, even after optimization. An exemplary investigation of the distortion of the time/frequency and amplitudes following implementation on a standard industrial advanced RISC machines microcontroller has shown that a sampling rate compensation is required to overcome timer inaccuracies. Furthermore, non-return-to-zero binary signals are more sensitive to distortion due to hardware-related issues and have a lower signal-to-distortion-and-noise (SINAD) ratio than 24 dB, which is lower than the multisine signal, having a SINAD of 31 dB.
Electrochemical Impedance Spectroscopy (EIS) is a valuable tool for the characterization of electrical, thermal and aging behavior of batteries. In this paper, an EIS measurement technique to acquire impedance spectra with high time resolution is examined, which can be used to gather impedance data during dynamic operating conditions. A theoretical analysis of the used multi-sine excitation signals is performed in detail and a practical measurement system is presented and validated. Afterwards, EIS measurements during the charging process of a lithium-ion battery are performed and discussed.
Electro-chemical impedance spectroscopy is widely used to analyze electro-chemical systems. Most attention is paid to the double-layer capacitance and the charge-transfer resistance as they describe the electro-chemical process on the surface of the electrode. Both values can provide specific information about aging mechanisms, which diminish the surface area. This is of interest when capacity tests are restricted to determine the aging. For lead-acid batteries, for example, this is the case in applications like micro-hybrid vehicles or uninterruptible power supply systems. However, the interpretation of impedance spectra of lead-acid batteries necessitates proper measurements, elaborated verification of measurement validity, and a sufficient model of electro-chemical processes. In this work, impedance spectra, recorded on lead-acid test cells, are processed to identify the ohmic resistance, the double-layer capacitance, and the parameters of the charge-transfer reaction of the negative electrode. This electrode suffers from sulfation, a common aging mechanism in current applications. The aim of the paper is to define a correct processing of impedance spectra for lead-acid batteries, and to depict challenges. Furthermore, possible equivalent electrical circuit models for the negative electrode are evaluated regarding their dependencies on state of charge and current rate. Many of these aspects can be transferred to other electro-chemical systems.
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The aging mechanisms of lead-acid batteries change the electrochemical characteristics. For example, sulfation influences the active surface area, and corrosion increases the resistance. Therefore, it is expected that the state of health (SoH) can be reflected through differentiable changes in the impedance of a lead-acid battery. However, for lead-acid batteries, no reliable SoH algorithm is available based on single impedance values or the spectrum. Additionally, the characteristic changes of the spectrum during aging are unknown. In this work, lead-acid test cells were aged under specific cycle regimes known as AK3.4, and periodic electrochemical impedance spectroscopy (EIS) measurements and capacity tests were conducted. It was examined that single impedance values increased linearly with capacity decay, but with varying slopes depending on the prehistory of the cell and measurement frequency of impedance. Thereby, possible reasons for ineffective SoH estimation were found. The spectra were fitted to an equivalent electrical circuit containing, besides other elements, an ohmic and a charge-transfer resistance of the negative electrode. The linear increase of the ohmic resistance and the charge-transfer resistance were characterized for the performed cyclic aging test. Results from chemical analysis confirmed the expected aging process and the correlation between capacity decay and impedance change. Furthermore, the positive influence of charging on the SoH could be detected via EIS. The results presented here show that SoH estimation using EIS can be a viable technique for lead-acid batteries.
In order to match the voltages between high voltage battery stacks and low voltage super-capacitors with a high conversion efficiency in hybrid energy sources electric vehicles (HESEVs), a high ratio bidirectional DC-DC converter with a synchronous rectification H-Bridge is proposed in this paper. The principles of high ratio step-down and step-up operations are analyzed. In terms of the bidirectional characteristic of the H-Bridge, the bidirectional synchronous rectification (SR) operation is presented without any extra hardware. Then the SR power switches can achieve zero voltage switching (ZVS) turn-on and turn-off during dead time, and the power conversion efficiency is improved compared to that of the diode rectification (DR) operation, as well as the utilization of power switches. Experimental results show that the proposed converter can operate bidirectionally in the wide ratio range of 3~10, when the low voltage continuously varies between 15V and 50V. The maximum efficiencies are 94.1% in the Buck mode, and 93.6% in the Boost mode. In addition, the corresponding largest efficiency variations between SR and DR operations are 4.8% and 3.4%. This converter is suitable for use as a power interface between the battery stacks and super-capacitors in HESEVs.
The lithium titanate battery, which uses Li4Ti5O12 (LTO) as its anode instead of graphite, is a promising candidate for fast charging and power assist vehicular applications due to its attractive battery performance in rate characteristics and chemical stability. Unfortunately, commonly used battery models, including a large number of enhanced electrical models, become problematic when describing current-voltage characteristics of lithium titanate batteries. In this paper, a novel Butler-Volmer equation-based electric model is employed to outline unique phenomena induced by changing rates for high-power lithium titanate batteries. The robustness of the proposed model for three types of lithium titanate batteries under varying loading conditions, including galvanostatic test and Federal Urban Dynamic Schedule test, is evaluated and compared against experimental data. The experimental results of three types of lithium titanate batteries with common anode materials but differentiated cathode materials show good agreement with the model estimation results with maximum voltage errors below 2%.
This paper presents a fast cost-effective technique for the measurement of battery impedance online in an application such as an electric or hybrid vehicle. Impedance measurements on lithium-ion batteries between 1 Hz and 2 kHz give information about the electrochemical reactions within a cell, which relates to the state of charge (SOC), internal temperature, and state of health (SOH). We concentrate on the development of a measurement system for impedance that, for the first time, uses an excitation current generated by a motor controller. Using simple electronics to amplify and filter the voltage and current, we demonstrate accurate impedance measurements obtained with both multisine and noise excitation signals, achieving RMS magnitude measurement uncertainties between 1.9% and 5.8%, in comparison to a high-accuracy laboratory impedance analyzer. Achieving this requires calibration of the measurement circuits, including measurement of the inductance of the current sense resistor. A statistical correlation approach is used to extract the impedance information from the measured voltage and current signals in the presence of noise, allowing a wide range of excitation signals to be used. Finally, we also discuss the implementation challenges of an SOC estimation system based on impedance.
The bioimpedance measurement/identification of time-varying biological systems Z(ω, t) by means of electrical impedance spectroscopy (EIS) is still a challenge today. This paper presents a novel measurement and identification approach, the so-called parametric-in-time approach, valid for time-varying (bio-)impedance systems with a (quasi) periodic character. The technique is based on multisine EIS. Contrary to the widely used nonparametric-in-time strategy, the (bio-)impedance Z(ω, t) is assumed to be time-variant during the measurement interval. Therefore, time-varying spectral analysis tools are required. This new parametric-in-time measuring/identification technique has experimentally been validated through three independent sets of in situ measurements of in vivo myocardial impedance. We show that the time-varying myocardial impedance Z(ω, t) is dominantly periodically time varying (PTV), denoted as Z(ω, t). From the temporal analysis of Z(ω, t), we demonstrate that it is possible to decompose Z(ω, t) into a(n) (in)finite sum of fundamental (bio-)impedance spectra, the so-called harmonic impedance spectra (HIS) Z(ω)s with [Formula: see text]. This is similar to the well-known Fourier series of a periodic signal, but now understood at the level of a periodic system's frequency response. The HIS Z(ω)s for [Formula: see text] actually summarize in the bi-frequency (ω, k) domain all the temporal in-cycle information about the periodic changes of Z(ω, t). For the particular case k = 0 (i.e. on the ω-axis), Z(ω) reflects the mean in-cycle behavior of the time-varying bioimpedance. Finally, the HIS Z(ω)s are directly identified from noisy current and voltage myocardium measurements at the multisine measurement frequencies (i.e. nonparametric-in-frequency).
Electrochemical impedance spectroscopy has become a mature and well-understood technique. It is now possible to acquire, validate, and quantitatively interpret the experimental impedances. This chapter has been addressed to understanding the fundamental processes of diffusion and faradaic reaction at electrodes. However, the most difficult problem in EIS is modeling the electrode processes, which is where most of the problems and errors arise. There is an almost infinite variety of different reactions and interfaces that can be studied (corrosion, coatings, conducting polymers, batteries and fuel cells, semiconductors, electrocatalytic reactions, chemical reactions coupled with faradaic processes, etc.) and the main effort is now being applied to understanding and analyzing these processes. These applications will be the subject of a second review in a forthcoming volume in this series.
The
influence of hindered water removal from a polymer electrolyte fuel cell under constant load was investigated
using electrochemical impedance spectroscopy. The cathodic gas outlet of the cell was closed and impedance
measurements were performed at periodic time intervals. Under the experimental conditions, the investigated
system is far from steady-state conditions. It is shown that enhanced mathematical techniques are required for
evaluation of the impedance data obtained. These techniques, the time course interpolation as well as
the Z-HIT refinement, lead
to quasi-causal spectra which are well
interpreted by means of the porous electrode model.
For measuring the impedance spectra of batteries, several challenges should be overcome to maintain the stability condition, especially at low frequencies in the mHz range. Even if the sine sweep signals realize the highest signal power, they lead to a severe stability problem at low frequencies in the mHz region. In this paper, we propose a novel design method for multisine excitation signals, which reduces measurement time and maintains battery stability. The novel excitation signals are structured in a dummy interval for considering the transient behavior of the battery and three folds of a multisine signal with an optimized crest factor, frequency bins, and measurement time. The system stability is evaluated by a consistency check based on the Linear Kramers-Kronig (LKK) method. The results show that the novel excitation signals outperform the often-used sinusoidal sweep signals by maintaining the stability condition during impedance measurements even at low frequencies in the mHz range. For example, for a signal with six frequencies/decade from 10 mHz to 1 kHz, a root means squares error of the LKK of 31.70 ppm could be realized instead of 201.21 ppm for the sine sweep, and the measurement time could be reduced to 315 s from 1440 s.
Impedance spectroscopy is a powerful measurement method having decisive advantages in several application fields. It allows the measurements of non-accessible quantities, supports high measurement accuracy, and enables simultaneous measurements of more than one quantity. The method provides deep insights into systems and materials, is non-invasive, and is highly efficient, considering measurement time, measurement procedures, and possibilities for implementation in embedded systems for in-field measurements. In this contribution, we point out the potential, recent advances, and future trends of the method to provide insight into the powerfulness of the method for future applications.
Battery powered energy systems such as electric vehicles utilize power electronics for controlling energy flows between the battery and the load or generation, respectively. Therefore, the battery is under high frequency stress due to fast switching power electronic devices. However, most battery models throughout the literature are not able to cope with high frequency excitation. This paper proposes an easy to implement equivalent circuit model that covers aforementioned frequency regions with a series of inductors that are each connected in parallel with an ohmic resistance. This circuit is parameterized by electrochemical impedance spectroscopy (EIS) up to 100 kHz. For further regions that reach regions of megahertz a skin effect model is investigated and compared to the RL-model. It is shown that such semi-empirical models can be motivated by geometrical considerations that can be found in the literature. Moreover, the proposed model is validated by simulating the voltage response from an input current that originates from an actual back-to-back half bridge DC/DC converter. The promising results indicate that such models might be implemented in future battery energy systems to improve insights on how batteries react to perturbations such as EMI noise or high frequency current ripple.
This paper presents an online electrochemical impedance spectroscopy (EIS) technique for a Li-Ion battery. A frequency sweeping method and a precise impedance measurement algorithm are used for the proposed online EIS technique. The proposed algorithm consists of fast Fourier transformations, peak amplitudes, and phase difference estimations. Additional instrumentation with offset clipping and amplifying circuits expand the accuracy of the signal measurement. The offset clipping circuit increases the ripple to measurement voltage ratio without phase delay. The amplifying circuit gives more resolution for the ripple voltage. The proposed instrumentation ensures the precise measurement of ripple voltage and current. The proposed impedance measurement algorithm fulfills the precision level required for industrial Li-Ion battery. To validate the proposed approach, a 40Ah 13.6V Li-Ion battery charger was built using a synchronous buck converter and an Opal-RT based controller. The proposed spectroscopy technique will be useful to track and estimate internal electrochemical conditions of a battery.
Considering transport applications, there is worldwide an increasing interest in the use of hydrogen-energy for supplying electric powertrains. In order to extend the fuel cell lifespan and to increase its reliability and efficiency, the following features are essential to a fuel cell stack connected DC/DC boost converter: low input current ripple, high voltage gain ratio, high efficiency, high compactness, and high redundancy. Moreover, in order to assess in real time the state-of-health of the fuel cell stack, on-line Electrochemical Impedance Spectroscopy (EIS) functionality integrated with the fuel cell stack connected DC/DC boost converter is a real promising solution requiring no additional measurement equipment. From these points of view, this paper presents a comparison analysis of high voltage gain DC/DC boost converters for fuel cell electric vehicles applications. In addition, some comments and guidelines regarding integration issues are also provided.
Increasing the loading of active materials by thickening the battery electrode coating can enhance the energy density of a Li-ion cell but the trade-off is the much reduced Li⁺ transport kinetics. To reach the optimum energy and power density for thick electrodes, the effective chemical diffusion coefficient of Li⁺ (DLi) must be maximized. However, the diffusion of Li⁺ inside an electrode is a complex process involving both microscopic and macroscopic processes. Fundamental understandings are needed on the rate-limiting process that governs the diffusion kinetics of Li+ in order to minimize the negative impact of the large electrode thickness on their electrochemical performance. In this work, lithium Ni-Mn-Co oxides (NMC) cathodes of various thicknesses ranging from 100 to 300 µm were used as a model system to study the rate-limiting diffusion process during charge/discharge. The rate-limiting diffusion coefficient of Li+ was investigated and quantified, which was correlated to the electrochemical performance degradation of thick electrodes. It is revealed here that the under-utilization of the active material was caused by the limited diffusion of Li⁺ inside the porous electrode, leading to a critical electrode thickness beyond which the specific capacity was significantly reduced.
Electrochemical impedance spectroscopy (EIS) is a fast, inexpensive and non-destructive tool for identification of electrical and electrochemical loss processes of battery cells in a wide range of frequencies. Among those, particularly interface processes like the migration of Li species through surface films and the charge transfer reaction are easy accessible by EIS. One of the main drawbacks of its application to commercial cells is the limitation to the full cell spectrum due to the absence of an appropriate reference potential to separate cathodic and anodic contributions. Further, the impedance response of cell geometries such as cylindrical cells is characterized by a complex superposition of contributions arising from electrochemistry and cell design. In this study, we address these issues by a planar transmission line model allowing to reconstruct the impedance response of cylindrical cells by impedance data obtained from three electrode measurements on extracted electrodes and geometrical information obtained from microscopical cross sections. The reconstruction results indicate a substantial contribution from the spiral geometry of the current collectors and the position of the current collector tabs along the jelly roll. Furthermore, the impedance response is significantly determined by the power/energy design of the cell, for instance by the ratio of electrode areas and thicknesses. Overall, the proposed methodology presents a sound modeling basis for a deeper understanding of the impedance response of cylindrical cells and a tool for cell design optimization from electrode to cell level.
Micro-hybrid vehicles (μH) are currently starting to dominate the European market and seize constantly growing share of other leading markets in the world. On the one hand, the additional functionality of μH reduces the CO2 emissions and improves the fuel economy, but, on the other hand, the additional stress imposed on the lead-acid battery reduces significantly its expected service life in comparison to conventional vehicles. Because of that μH require highly accurate battery state detection solutions. They are necessary to ensure the vehicle reliability requirements, prolong service life and reduce warranty costs. This paper presents an electrical model based on Butler-Volmer equation. The main novelty of the presented approach is its ability to predict accurately dynamic response of a battery considering a wide range of discharge current rates, state-of-charges and temperatures. Presented approach is fully implementable and adaptable in state-of-the-art low-cost platforms. Additionally, shown results indicate that it is applicable as a supporting tool for state-of-charge and state-of-health estimation and scalable for the different battery technologies and sizes. Validation using both static pulses and dynamic driving profile resulted in average absolute error of 124 mV regarding cranking current rate of 800 A respectively.
The increasing market demand for electric vehicles requires a continuous development and improvement of battery systems. A crucial aspect is the application of analytical methods to characterize the battery systems in the most exact way. At the same time these methods should remain as simple as possible. The current analysis procedures use essentially the battery system- and battery cell-characteristic of the voltage curve in relaxation- and operation-time. These approaches alone are not conclusive enough due to the influences of relaxation time and varying load currents.
In recent years, methods based on electrochemical impedance spectroscopy (EIS) have found application for accurate analysis of occurring electrochemical processes and diagnosis of lithium ion batteries.
This publication aims to show how the state of charge can be predicted with the use of EIS and a simplified equivalent circuit for redundant supplementing the OCV-method (open circuit voltage) and the current- counting method. Therefore film batteries with a graphite anode and a Ni1/3 Mn1/3 Co1/3 cathode are charged and discharged according to a specified procedure. The battery is (dis)charged with steps of 10% followed by an EIS measurement. The measured data is used to determine the elements of the simplified equivalent circuit. These elements are analyzed for their different values at various states of charge.
From these measurements can be concluded that EIS can be applied for the estimation of state of charge (in the middle range) as redundant method to other estimation methods. It therefore provides an additionally methods by which statements about the battery conditions can be made.
Impedance Spectroscopy is a well-known measurement technique for electrochemical systems, such as a battery half-cell. Once this method would be implemented in the battery management system and performed online during operation it could provide a monitoring system for the whole pack and increase its lifetime and safe operation. Current solutions are either inaccurate or too big, expensive and energy inefficient. The presented approach proposes a dual use of the battery charger which incorporates a switched mode amplifier to generate the stimuli current necessary to perform an Impedance Spectroscopy. The drawback of this method are the distortions associated with the modulation of the measurement frequency onto a carrier frequency. The influence of the DC-link voltage and the switching frequency on the on the total harmonic distortion are analysed. These theoretical results are compared with the ones from experiments. A custom designed switched amplifier was built up, specifically designed for performing Impedance Spectroscopy on a battery pack. This technology could allow impedance monitoring of every cell in the battery pack and make a better estimation of the state of the battery possible.
This paper deploys electrochemical impedance spectroscopy (EIS) to investigate the impact of temperature and dc bias current on battery impedance characteristics. Measured test results are used to demonstrate that, under conditions where the nonlinear Butler-Volmer equation is necessary to model the electrode charge transfer characteristics, the semicircular trajectory that typically appears in the EIS results shrinks in diameter as the battery's dc bias current increases. For a lithium-based battery, the nonlinearity introduced by the Butler-Volmer relationship is more pronounced at low temperature, while lead-acid batteries typically exhibit this nonlinearity even at room temperature. The impact of dc bias current on the battery model and EIS characteristics are thoroughly investigated using a combination of experimental tests combined with theoretical justification based on the Arrhenius equation. The value of using this observed relationship to improve the accuracy of battery models and condition monitors (e.g., state-of charge, etc.) is discussed in the paper.
Zusammenfassung
Für Batteriemanagementsysteme (BMS) sind Umfang und Verlässlichkeit der Datenbasis von besonderer Bedeutung. Bisher kommt ein Großteil dieser Daten aus Prüfständen und wird dem BMS als Tabelle bereitgestellt. Um den Einfluss dieser Offline-Daten im BMS zu reduzieren und so die Leistungsfähigkeit eines Batteriesystems zu steigern, wurde eine Hardware entwickelt, welche eine Online-Messung des Impedanzspektrums von Batterien ermöglicht. Der Fokus liegt hierbei auf der Adaption der im Labor häufig eingesetzten Impedanzspektroskopie, um diese in mobilen Anwendungen mit Low-cost-Hardware verfügbar zu machen.
Online diagnostics for monitoring of battery cells in a battery pack is necessary in order to determine the state of the battery pack, like its age and safe operation. It could also provide the possibility to adjust the operation strategy of the battery management system and the load to increase the battery lifetime and safety. Impedance spectroscopy is a well-known measurement technique for electrochemical systems, such as a battery half-cell. Once the method is implemented in the battery management system and performed online during operation it could provide a monitoring system for the whole pack. Current solutions are either inaccurate or too big, expensive and energy inefficient. The presented approach proposes a dual use of the battery charger which incorporates a switched mode amplifier to generate the stimuli current necessary to perform an electro impedance spectroscopy. A suitable control is designed to overcome the non-linearities and instabilities introduced by the output filter and the current crossover effects of the electronic switches. This inexpensive, energy efficient technology could allow impedance monitoring of every cell in the battery pack and make a better prediction of the state of the battery possible.
This paper presents a simple online impedance measurement method for electrochemical batteries, including lithium-ion, lead-acid, and nickel–metal-hydride chemistries. By using the proposed online impedance measurement method, there is no need to disconnect the battery from the system or to interrupt system operation, and there is no need to add ac signal injection circuits, costly response measurement, and analysis circuits/devices. In practical battery-powered systems, a power converter is usually used to interface the battery with the load for voltage/current regulation purposes. In this paper, through the control of the power converter and duty-cycle perturbation, the ac impedance of the battery can be determined. The proposed method provides a low-cost and practical solution for the online measurement of the ac impedance of batteries. Moreover, the proposed method can be either continuously or periodically performed without interrupting the normal operation of the battery system and the power converter. In addition, this paper provides an example where the obtained impedance data are utilized for online state-of-charge estimation of lithium-ion batteries. The proposed online impedance measurement method is validated by experiments conducted on a 2.6-Ah 18650-size lithium-ion battery interfaced to the load via a bidirectional dc–dc boost/buck converter.
We present an exclusively thermodynamics based derivation of a Butler–Volmer expression for the inter-calation exchange current in Li ion insertion batteries. In this first paper we restrict our investigationsto the actual intercalation step without taking into account the desolvation of the Li ions in the elec-trolyte. The derivation is based on a generalized form of the law of mass action for non ideal systems(electrolyte and active particles). To obtain the Butler–Volmer expression in terms of overpotentials, it isnecessary to approximate the activity coefficient of an assumed transition state as function of the activitycoefficients of electrolyte and active particles. Specific considerations of surface states are not necessary,since intercalation is considered as a transition between two different chemical environments withoutsurface reactions. Differences to other forms of the Butler–Volmer used in the literature [1,2] are dis-cussed. It is especially shown, that our derivation leads to an amplitude of the exchange current whichis free of singular terms which may lead to quantitative and qualitative problems in the simulation ofoverpotentials. This is demonstrated for the overpotential between electrolyte and active particles for ahalf cell configuration.
The more effects and mechanisms are represented in an impedance spectrum, the more unknown parameters are needed for accurate modeling. The inverse identification problem corresponding to this parameter extraction process becomes thereby more difficult to solve and needs generally a lot of a priori knowledge and trials. The use of evolutionary strategies (EV) can significantly contribute to increase efficiency during this process. With selected examples, many benefits of evolutionary strategy are shown by means of simulations with more than 100 trials.For simple models, the main advantage of evolution strategy is to be nearly insensitive to the chosen search region. For models with a big number of unknown parameters, the combination with the Levenberg–Marquardt (LevMq) method reaches very good results compared with each method alone. The combined method profited of positive properties of both methods. The evolution strategy is, also for an ill-posed inverse problem, able to calculate a parameters vector near the optimum. The Levenberg–Marquardt method profits from it and is then able to calculate the optimum more exactly, because it is already in its next neighborhood. The resulting process is robust and reaches good modeling results even, if only a few a priori knowledge is available concerning expected parameter values.
Impedance spectroscopy is one of the most promising methods for characterizing aging effects of portable secondary batteries online because it provides information about different aging mechanisms. However, application of impedance spectroscopy “in the field” has some higher requirements than for laboratory experiments. It requires a fast impedance measurement process, an accurate model applicable with several batteries and a robust method for model parameter estimation.In this paper, we present a method measuring impedance at different frequencies simultaneously. We propose to use a composite electrode model, capable to describe porous composite electrode materials. A hybrid method for parameter estimation based on a combination of evolution strategy and Levenberg–Marquardt method allowed a robust and fast parameter calculation.Based on this approach, an experimental investigation of aging effects of a lithium ion battery was carried out. After 230 discharge/charge cycles, the battery showed a 14% decreased capacity. Modeling results show that series resistance, charge transfer resistance and Warburg coefficient changed thereby their values by approximately 60%. A single frequency impedance measurement, usually carried out at 1 kHz, delivers only information about series resistance. Impedance spectroscopy allows additionally the estimation of charge transfer resistance and Warburg coefficient. This fact and the high sensitivity of model parameters to capacity change prove that impedance spectroscopy together with an accurate modeling deliver information that significantly improve characterization of aging effects.
Battery management systems, Volume I: Battery modeling
- G L Plett