title: Equivalent Circuit Dynamic Modeling and Parametrization of Lithium-Ion Cells
Creators: name: Bedürftig, Benjamin
Publisher: name: UNSPECIFIED
Year: 2024

description: 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.