Online Electrochemical Impedance Measurement for Battery State Estimation
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Solution Overview
Problem
Current battery management systems (BMS) face challenges in accurately characterizing battery state, particularly in lithium iron phosphate (LFP) batteries, due to voltage hysteresis and a flat state of charge (SOC) curve, leading to inaccurate range estimation and safety margins, and there is a need for online electrochemical impedance spectroscopy (EIS) measurements in vehicles to improve estimation accuracy.
Innovation Solution
A power system comprising an EIS measurement circuit and control circuitry for synchronously measuring cell voltages and currents between battery subcircuits, using a transformer and switches to transfer energy and apply excitation current at varying frequencies, with a BMU or CMU controller for impedance calculation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conservative operating boundaries are selected to ensure safety margins, then safety is improved, but productivity is reduced due to limited charging/discharging rates
Solution Approach 1:
The system dynamically changes operating parameters (current, voltage, temperature) based on real-time battery state characterization through EIS measurements. By continuously updating the battery model with actual impedance data, the system can safely operate at higher charging rates without exceeding damage thresholds, thus resolving the contradiction between safety and productivity
Solution Approach 2:
The system implements closed-loop feedback by continuously measuring battery impedance through EIS, comparing actual state against safe operating boundaries, and adjusting charging parameters accordingly. This real-time feedback enables aggressive charging when conditions permit while maintaining safety margins, eliminating the need for conservative fixed boundaries
2Measurement precision
If online EIS testing is implemented to improve measurement accuracy, then measurement precision is improved, but device complexity increases due to additional circuitry
Solution Approach 1:
The EIS measurement circuit is integrated into the existing battery management system infrastructure, using the same voltage and current measurement channels for both normal operation monitoring and EIS testing. The transformer and switch network serve dual purposes: power transfer during normal operation and excitation signal generation for EIS measurements, thereby improving measurement precision without proportionally increasing device complexity
Solution Approach 2:
The system uses its own operational energy to perform EIS measurements by transferring energy between battery subcircuits through the transformer. The battery cells themselves serve as the energy source for excitation signals, eliminating the need for external test equipment and reducing overall system complexity while maintaining high measurement precision
3Use of energy by moving object
If energy transfer between battery subcircuits is used for EIS testing, then use of energy by moving object is improved, but loss of energy increases during measurement
Solution Approach 1:
EIS measurements are performed periodically at selected operating points rather than continuously, using sinusoidal excitation signals at multiple frequencies. This periodic approach allows the system to efficiently use energy by conducting measurements only when the battery is in stable operating conditions, minimizing energy loss while still gathering sufficient data for accurate state estimation
Solution Approach 2:
The system recovers energy during EIS measurements by transferring it back to the battery through the transformer rather than dissipating it. The bidirectional energy transfer allows the excitation energy to be reclaimed and returned to the battery, significantly reducing net energy loss while maintaining high energy utilization efficiency
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
Enables accurate and efficient EIS testing in vehicles, reducing errors in battery state estimation and enhancing safety by providing precise SOC determination and impedance analysis, thereby improving battery management.
Implementation Method 1
a transformer having a primary winding and a secondary winding
Implementation Method 2
Electrochemical Impedance Spectroscopy (EIS) is a non-destructive technique for measuring electrical impedances of a material at multiple frequencies
Data Source
AI summary
Online electrochemical impedance spectroscopy (EIS) circuitry and methods. For mobile installations, such as in a hybrid or electric vehicle, battery management units or battery management systems having EIS circuits can provide additional data regarding the state, health and potential failure of batteries. New topologies for the EIS circuitry are proposed, allowing voltage controlled and current controlled EIS to be performed.