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Equivalent Circuit Dynamic Modeling and Parametrization of Lithium-Ion Cells

Bedürftig, Benjamin (2024)
Equivalent Circuit Dynamic Modeling and Parametrization of Lithium-Ion Cells.
Technische Universität Darmstadt
doi: 10.26083/tuprints-00026773
Ph.D. Thesis, Primary publication, Publisher's Version

Abstract

Lithium-ion (Li-ion) battery cell simulation models have several vital uses in the development of new battery systems. These uses range from assisting in cell and battery system design, to estimating a cell’s state of health and charge, as well as developing charging and operating strategies. Thus, it is crucial that any simulation model accurately predicts the modelled cell’s system dynamics. An accurate model is also important to developers due to the ever growing demands on Li-ion battery systems in the areas of safety, energy density, and power density.

Current models include the Newman model (a first principle model based on physical insights), equivalent circuit models, and data-driven models. To complement these, an equivalent circuit model with electrochemical consideration was developed within the scope of this work to simulate the electrical and thermal dynamics of Li-ion cells. All aspects of the model—i.e., the development, modeling effort, simulation time, and implementation effort—required measurement technology and parameterization to be considered collectively. This facilitated the formulation of an overall approach meeting both industrial and scientific objectives.

The developed electrochemical equivalent circuit is based on impedance measurements. In general, the impedance of electrochemical systems, such as Li-ion cells, describes the time-dependent electrical resistance in the frequency domain and enables a deeper insight into the system dynamics. The impedance is the quotient of voltage and current. It is typically used to simulate the voltage response and the irreversible heat released when the electrochemical system is excited by a current. To consider the temperature distribution and the geometric impact of the cell components, a thermal model is coupled with the electrical model, which is also realized as an equivalent circuit model. In addition to irreversible heat, reversible heat is also modeled to reproduce cell dynamics. Furthermore, the electrical cell model describes the open-circuit voltage.

Tailored measurement methods, systems, and algorithms were designed for this work to identify electrical and thermal model parameters and the specific electrochemical processes of the modelled cell. One example is a new calorimetric measurement method based on double pulse measurements which was developed to measure reversible heat. Another is the automated parameter identification method which was designed for fast and reliable model generation. For this purpose, a measurement system was developed that performs automated impedance measurements for Li-ion cells with large- and small-signals with high accuracy in the relevant frequency range. A generic cell model was generated to determine suitable initial parameters for parameter identification.

To validate the cell models, simulations that imitate real-world problems were performed. The results show that the cell specific parameterized model can successfully (i.e., accurately) simulate the cell dynamics over a wide operating range.

The cell model developed for this work enables dynamic time-domain and frequency-domain simulations of the relevant electrical and thermal quantities. The model is suitable for the simulation of battery systems, enabling optimizations of the overall system by rapidly mapping the interactions between interconnected cells. In addition, the model can be used in a battery management system to estimate the state of charge, state of health, aging, internal resistance, energy content, and open-circuit voltage. This is possible because the model captures: the open-circuit voltage hysteresis, transition curves between charge and discharge directions, and relaxation processes. Further, the model can be used in the development of optimal operating strategies, i.e., to increase efficiency, usable energy, and lifetime. The simulation of cell impedance in the frequency domain is needed in the development of charging technology and electronics. In addition, the model can be used in cell development to extrapolate results from small experimental level cells to large-scale industrial cells.

Early battery development phases require estimations of cell dynamics which can be simulated using the generic model. In addition, the model can be used in tailored versions for power supplies to emulate the dynamics of a battery as often needed to validate system components in early development phases. Use cases of the model are the simulation of the temperature distribution and its dynamics within the battery and the cell, which allows e.g., for the evaluation of cooling and fast charging concepts.

Item Type: Ph.D. Thesis
Erschienen: 2024
Creators: Bedürftig, Benjamin
Type of entry: Primary publication
Title: Equivalent Circuit Dynamic Modeling and Parametrization of Lithium-Ion Cells
Language: English
Referees: Findeisen, Prof. Dr. Rolf ; Braatz, Prof. Dr. Richard ; Krewer, Prof. Dr. Ulrike
Date: 20 March 2024
Place of Publication: Darmstadt
Collation: xiv, 188 Seiten
Refereed: 21 December 2023
DOI: 10.26083/tuprints-00026773
URL / URN: https://tuprints.ulb.tu-darmstadt.de/26773
Abstract:

Lithium-ion (Li-ion) battery cell simulation models have several vital uses in the development of new battery systems. These uses range from assisting in cell and battery system design, to estimating a cell’s state of health and charge, as well as developing charging and operating strategies. Thus, it is crucial that any simulation model accurately predicts the modelled cell’s system dynamics. An accurate model is also important to developers due to the ever growing demands on Li-ion battery systems in the areas of safety, energy density, and power density.

Current models include the Newman model (a first principle model based on physical insights), equivalent circuit models, and data-driven models. To complement these, an equivalent circuit model with electrochemical consideration was developed within the scope of this work to simulate the electrical and thermal dynamics of Li-ion cells. All aspects of the model—i.e., the development, modeling effort, simulation time, and implementation effort—required measurement technology and parameterization to be considered collectively. This facilitated the formulation of an overall approach meeting both industrial and scientific objectives.

The developed electrochemical equivalent circuit is based on impedance measurements. In general, the impedance of electrochemical systems, such as Li-ion cells, describes the time-dependent electrical resistance in the frequency domain and enables a deeper insight into the system dynamics. The impedance is the quotient of voltage and current. It is typically used to simulate the voltage response and the irreversible heat released when the electrochemical system is excited by a current. To consider the temperature distribution and the geometric impact of the cell components, a thermal model is coupled with the electrical model, which is also realized as an equivalent circuit model. In addition to irreversible heat, reversible heat is also modeled to reproduce cell dynamics. Furthermore, the electrical cell model describes the open-circuit voltage.

Tailored measurement methods, systems, and algorithms were designed for this work to identify electrical and thermal model parameters and the specific electrochemical processes of the modelled cell. One example is a new calorimetric measurement method based on double pulse measurements which was developed to measure reversible heat. Another is the automated parameter identification method which was designed for fast and reliable model generation. For this purpose, a measurement system was developed that performs automated impedance measurements for Li-ion cells with large- and small-signals with high accuracy in the relevant frequency range. A generic cell model was generated to determine suitable initial parameters for parameter identification.

To validate the cell models, simulations that imitate real-world problems were performed. The results show that the cell specific parameterized model can successfully (i.e., accurately) simulate the cell dynamics over a wide operating range.

The cell model developed for this work enables dynamic time-domain and frequency-domain simulations of the relevant electrical and thermal quantities. The model is suitable for the simulation of battery systems, enabling optimizations of the overall system by rapidly mapping the interactions between interconnected cells. In addition, the model can be used in a battery management system to estimate the state of charge, state of health, aging, internal resistance, energy content, and open-circuit voltage. This is possible because the model captures: the open-circuit voltage hysteresis, transition curves between charge and discharge directions, and relaxation processes. Further, the model can be used in the development of optimal operating strategies, i.e., to increase efficiency, usable energy, and lifetime. The simulation of cell impedance in the frequency domain is needed in the development of charging technology and electronics. In addition, the model can be used in cell development to extrapolate results from small experimental level cells to large-scale industrial cells.

Early battery development phases require estimations of cell dynamics which can be simulated using the generic model. In addition, the model can be used in tailored versions for power supplies to emulate the dynamics of a battery as often needed to validate system components in early development phases. Use cases of the model are the simulation of the temperature distribution and its dynamics within the battery and the cell, which allows e.g., for the evaluation of cooling and fast charging concepts.

Alternative Abstract:
Alternative abstract Language

Simulationsmodelle für Lithium-Ionen-Zellen (Li-Ionen-Zellen) sind für die Entwicklung neuer Batteriesysteme von entscheidender Bedeutung. Sie werden in immer weiteren Einsatzgebieten verwendet, von der Entwicklung der Zellen und Batteriesystemen über die Abschätzung des Gesundheitszustands und Ladezustands bis hin zu Lade- und Betriebsstrategien. Wachsende Anforderungen an die Sicherheit, Energiedichte, Leistungsdichte und dem Preis von Li-Ionen Batteriesystemen erfordern eine immer bessere Vorhersagequalität der Systemdynamik, um optimale Batteriesysteme zu entwerfen.

Zur Ergänzung bestehender Modellansätze, wie die auf physikalischen Erkenntnissen basierenden First-Principle-Modelle, wie z.B. das Newman-Modell, Ersatzschaltbildmodelle und datengetriebene Modelle, wurde im Rahmen der Arbeit ein Ersatzschaltbildmodell mit elektrochemischer Betrachtung entwickelt, um die elektrische und thermische Dynamik von Li-Ionen-Zellen zu simulieren. Bei der Modellentwicklung wurden der Modellierungsaufwand, die notwendige Simulationszeit, der Implementierungsaufwand, die Messtechnik und die Parametrierung betrachtet. Dies erlaubte die Erarbeitung eines fundierten Gesamtkonzepts, das sowohl industriellen als auch wissenschaftlichen Zielen gerecht wird.

Das entwickelte elektrochemische Ersatzschaltbild basiert auf Impedanzmessungen. Im Allgemeinen beschreibt die Impedanz eines elektrochemischen Systems, wie z. B. Li-Ionen-Zellen, den zeitabhängigen elektrischen Widerstand im Frequenzbereich und ermöglicht einen tieferen Einblick in die Systemdynamik. Die Impedanz ist der Quotient aus Spannung und Strom. Sie wird typischerweise bei einer Stromanregung des elektrochemischen Systems verwendet, um die Spannungsantwort und die irreversible Wärmefreisetzung zu simulieren. Um die Temperaturverteilung und den geometrischen Einfluss der Zellkomponenten zu berücksichtigen, wurde ein thermisches Modell aufgebaut, welches mit dem elektrischen Modell gekoppelt ist. Das thermische Modell wurde hierbei ebenfalls als Ersatzschaldbildmodell realisiert. Neben der irreversiblen wird auch die reversible Wärme modelliert, um die Zelldynamik zu reproduzieren. Weiter beschreibt das elektrische Zellmodell die Leerlaufspannung.

Es wurden geeignete Messmethoden, Messsysteme und Algorithmen entworfen, um die elektrischen und thermischen Modellparameter und elektrochemischen Prozesse zu identifizieren. Beispielsweise wurde für die Vermessung der reversiblen Wärme eine neue kalorimetrische Messmethode basierend auf Doppelpulsmessungen etabliert. Für eine schnelle und zuverlässige Modellerstellung wurde eine automatisierte Parameteridentifikation entwickelt. Hierzu wurde ein Messsystem entworfen, das automatisiert Impedanzmessungen für Li-Ionen-Zellen mit großen und kleinen Signalen mit hoher Genauigkeit im relevanten Frequenzbereich durchführt. Um geeignete Startparameter für die Parameteridentifikation zu ermitteln, wurde ein generisches Zellmodell formuliert.

Zur Validierung der Zellmodelle wurden Simulationen, die praxisrelevanten Frage- und Problemstellungen nachempfunden sind, durchgeführt. Die Ergebnisse zeigen, dass das zellspezifisch parametrierte Modell die Zelldynamik über einen weiten Betriebsbereich reproduzieren kann. Hingegen zeigte das generische Modell bei tiefen Temperaturen und hohen Strömen größere Fehler.

Das entwickelte Zellmodell ermöglicht dynamische Zeitbereichs- und Frequenzbereichssimulationen der relevanten elektrischen und thermischen Größen. Durch die Möglichkeit die Interaktionen zwischen zusammengeschalten Zellen abzubilden und der hierbei schnellen Rechenzeit, eignet sich das Modell für die Simulation von Batteriesystemen, wodurch Optimierungen des Gesamtsystems möglich werden. Zudem kann das Modell im Batterie-Management-System verwendet werden, um den Ladezustand, die Alterung, den Innenwiderstand, den Energieinhalt und die Leerlaufspannung zu schätzen. Dies ist möglich, da das Modell die Leerlaufspannungshysterese, Übergangskurven zwischen Lade und Entladerichtung sowie Relaxationsprozesse abbildet. Weiter kann das Modell bei der Entwicklung optimaler Betriebsstrategien eingesetzt werden, um beispielsweise den Wirkungsgrad, nutzbare Energiemenge und die Lebensdauer zu erhöhen. Die Simulation der Zellimpedanz im Frequenzbereich wird bei der Entwicklung der Ladetechnik und der Elektronik benötigt. Darüber hinaus kann das Modell bei der Zellentwicklung einsetzt werden, um Ergebnisse von Experimentalzellebene auf großformatigen Zellen zu extrapolieren. Bereits in frühen Batterieentwicklungsphasen sind Abschätzungen zur Zelldynamik erforderlich, die mittels des generischen Modells simuliert werden können. Weiter kann das Model in angepasster Form in Stromnetzteilen verwendet werden, um die Dynamiken einer Batterie nachzubilden. Diese Stromnetzteile werden benötigt, um bereits in frühen Entwicklungsphasen Systemkomponenten zu valideren. Weitere Anwendungsfälle des Modells sind die Simulation der Temperaturverteilung und dessen Dynamik innerhalb der Batterie und der Zelle, wodurch beispielsweise Kühl- und Schnellladekonzepte bewertet werden können.

German
Status: Publisher's Version
URN: urn:nbn:de:tuda-tuprints-267735
Classification DDC: 500 Science and mathematics > 500 Science
500 Science and mathematics > 530 Physics
500 Science and mathematics > 540 Chemistry
600 Technology, medicine, applied sciences > 600 Technology
600 Technology, medicine, applied sciences > 620 Engineering and machine engineering
600 Technology, medicine, applied sciences > 621.3 Electrical engineering, electronics
600 Technology, medicine, applied sciences > 660 Chemical engineering
Divisions: 18 Department of Electrical Engineering and Information Technology
18 Department of Electrical Engineering and Information Technology > Institut für Automatisierungstechnik und Mechatronik
18 Department of Electrical Engineering and Information Technology > Institut für Automatisierungstechnik und Mechatronik > Control and Cyber-Physical Systems (CCPS)
Date Deposited: 20 Mar 2024 14:32
Last Modified: 28 Mar 2024 08:46
PPN:
Referees: Findeisen, Prof. Dr. Rolf ; Braatz, Prof. Dr. Richard ; Krewer, Prof. Dr. Ulrike
Refereed / Verteidigung / mdl. Prüfung: 21 December 2023
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