Lithium-Ion Cell Impedance Modeling With High-Frequency Rs Extraction
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Solution Overview
Problem
Existing methods for online diagnosis of lithium ion cell state during ongoing operation face challenges due to complexity and reduced accuracy caused by varying state of charge (SOC) and temperature (T) conditions, as well as limitations in signal quality, synchronicity, and computing power in on-board electronics.
Innovation Solution
A method for determining the parameters of an equivalent circuit diagram for lithium ion cell impedance, which involves performing measurements at specific frequencies to directly ascertain series resistance and capacitance, and optionally series inductance, thereby reducing complexity and improving accuracy by incorporating these values as fixed parameters rather than estimating them.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Extent of automation
If model parameters are determined through numerical optimization during ongoing operation, then online diagnosis capability is achieved, but measurement precision and reliability deteriorate due to varying SOC and T conditions and limited computing power
Solution Approach 1:
The patent extracts the series resistance RS as a separate measurable quantity using high-frequency impedance measurements. By measuring RS independently at frequencies above 300 Hz where the RC element impedance is negligible, the problem of simultaneously optimizing multiple parameters is simplified to optimizing only R1 and C1, thereby improving measurement precision while maintaining online diagnosis capability
Solution Approach 2:
The patent performs preliminary measurement of the series resistance RS at high frequencies before proceeding with the determination of R1 and C1. This preliminary action separates the determination of RS from the optimization process, providing a fixed reference value that improves the accuracy of subsequent parameter estimation during ongoing operation
2Reliability
If all model parameters (RS, R1, C1) are determined through numerical optimization, then comprehensive cell characterization is achieved, but device complexity and computational requirements increase
Solution Approach 1:
The patent segments the parameter determination process into two distinct parts: (1) measurement of series resistance RS at high frequencies above 300 Hz, and (2) determination of R1 and C1 through numerical optimization at lower frequencies. This segmentation reduces the complexity of the overall system by separating the measurement of RS from the optimization process, allowing RS to be used as a fixed parameter in subsequent analysis
Solution Approach 2:
The patent changes the measurement frequency parameter to separate the determination of different model parameters. By measuring at frequencies above 300 Hz where the RC element has minimal influence, RS can be accurately determined. This frequency-based parameter change simplifies the overall determination process and reduces computational complexity while maintaining comprehensive cell characterization
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach allows for more reliable online diagnosis of lithium ion cell health by simplifying the parameter determination process, enhancing accuracy, and reducing the impact of SOC and T variations, while leveraging available on-board electronics capabilities.
Implementation Method 1
Electrochemical impedance spectroscopy (EIS) is an established method that may be used, inter alia, to characterize lithium ion cells. It involves applying an excitation signal to the cell, which may be an AC current signal (I(t), galvanostatic) or an AC voltage signal (U(t), potentiostatic), measuring the corresponding response signal (U(t) or I(t), respectively), and calculating the generally complex impedance Z from the excitation signal and the response signal as U(t)/I(t).
Implementation Method 2
The electrode of an electrochemical cell is typically modeled at least as an RC element, in which C is supposed to reflect the capacitance of the electrochemical dual layer at the electrode interface and R is supposed to reflect the charge transfer resistance.
Implementation Method 3
R is supposed to reflect the charge transfer resistance. The electrical resistance of the electrolyte is represented as a series resistance RS.
Data Source
AI summary
A method for determining the parameters of an equivalent circuit for representation of the impedance of a lithium ion cell is provided. The equivalent circuit includes at least one RC element having an ohmic resistor R1, a capacitor C1, and a series resistor Rs. The series resistor Rs is determined by an impedance measurement.