Differential Electrical Impedance Spectroscopy for Battery Parameter Measurement
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Solution Overview
Problem
Automotive battery management systems face inaccuracies in measuring state of charge (SOC), state of health (SOH), and temperature due to reliance on precise voltage measurements and external temperature sensors, which can underestimate internal cell temperature, especially during high-current regimes.
Innovation Solution
A dual-cell measurement approach that differentiates the electrical impedance of a battery cell by exciting one cell with a time-varying signal while the other shares a common load current, allowing for load current and DC offset compensation without high pass filters or individual voltage measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If voltage measurements are used to determine state of charge (SOC), then SOC can be tracked, but measurement accuracy is insufficient due to cell voltage not being well-defined and precision requirements
Solution Approach 1:
The patent transitions from measuring DC voltage to measuring AC impedance parameters (real and imaginary components) at different frequencies. By applying an AC excitation signal and measuring the resulting voltage and current, the system obtains frequency-dependent impedance data that provides more reliable SOC information than static voltage measurements.
Solution Approach 2:
The patent introduces an AC excitation signal as an intermediary to probe the battery's internal characteristics. This small-signal AC measurement superimposed on the operating point allows extraction of impedance parameters that reflect the battery's electrochemical state without significantly disturbing the normal operation.
2Temperature
If external temperature sensors are used to measure cell temperature, then temperature can be monitored, but internal cell temperature is underestimated especially during high-current regimes
Solution Approach 1:
The patent replaces direct thermal sensing with electrical impedance spectroscopy to infer temperature. By measuring the frequency-dependent impedance characteristics of the battery, the system can estimate internal temperature based on the known temperature dependence of electrochemical impedance, avoiding the limitations of external thermal sensors.
Solution Approach 2:
The patent uses impedance parameters as an intermediary to indirectly measure internal temperature. The AC impedance measurement serves as a proxy that correlates with internal temperature, providing more accurate thermal information than direct external sensing during high-current operation.
3Measurement precision
If AC excitation is applied to measure impedance, then SOC and SOH accuracy improves, but large DC offset and noisy load current reduce measurement sensitivity
Solution Approach 1:
The patent segments the impedance measurement into separate real and imaginary components through synchronous detection. By using reference signals at the excitation frequency and its quadrature, the system separates the AC impedance response from the DC operating point and noise, enabling precise extraction of impedance parameters despite the presence of large DC offsets and load current variations.
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 method enhances the accuracy of SOC, SOH, and temperature measurements, improving the range, lifetime, and performance of electric vehicles by reducing noise and errors in battery parameter estimation.
Implementation Method 1
determine an electrical impedance of the first battery cell
Implementation Method 2
measuring a voltage difference between voltage across the first battery cell and voltage across the second battery cell
Implementation Method 3
Differential electrical impedance spectroscopy
Data Source
AI summary
A device having a structure to measure a battery parameter of a battery of a set of multiple batteries can be implemented in a variety of applications. The device can be structured to measure alternating current (AC) electrical impedance of a battery cell by processing a difference between the battery cell and another battery cell. The battery cell being measured is subjected to AC excitation while the other one is not, where the two battery cells share a common load current. This differencing method can reduce the measurement's sensitivity to noisy battery load current, which is common to both battery cells. This differencing method also can remove or substantially reduce a large direct current (DC) offset, that is, the battery potential itself under which the AC signal measurement is burdened.