Battery Cell Impedance Measurement via Selective Frequency Extraction
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Solution Overview
Problem
In series-connected battery packs of electric vehicles, State of Charge (SoC) mismatches between cells lead to reduced battery capacity and lifespan due to susceptibility to leakage current, capacity differences, and inefficiencies in energy conversion, causing over-discharge or over-charge issues, which can damage lithium-ion batteries.
Innovation Solution
A circuit arrangement and method to determine the impedance of battery cells by generating sine and cosine waveforms at specific frequencies, injecting current pulses, and measuring voltage levels to separate real and imaginary components, enabling accurate SoC estimation and balancing to prevent over-charge/over-discharge.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If electroscopy with band-pass filters is used to measure impedance, then unwanted signals are eliminated, but the sampling rate is limited and the complete impedance curve cannot be obtained
Solution Approach 1:
The patent extracts only the necessary frequency components (target frequency and its harmonics) using selective filtering, rather than attempting to measure all frequencies simultaneously. This allows the system to focus computational and measurement resources on specific frequency components, achieving accurate impedance measurement without requiring excessively high sampling rates across the entire frequency spectrum.
Solution Approach 2:
The patent applies periodic excitation signals at specific target frequencies to the battery cell. By using periodic sine wave excitations at predetermined frequencies rather than continuous broadband signals, the system can measure impedance at each frequency point separately through multiple excitation cycles, achieving comprehensive impedance characterization without requiring ultra-high sampling rates.
2Measurement precision
If multiple frequency points are measured to obtain complete impedance curve, then all relevant chemical reactions are captured, but measurement time increases
Solution Approach 1:
The patent pre-selects a finite set of target frequencies based on the known time constants of battery chemical reactions. Rather than attempting to measure all possible frequencies, the system predetermined which frequency points are most relevant for capturing the electrochemical behavior, allowing measurements to be performed efficiently at these specific points while still obtaining complete impedance characterization.
Solution Approach 2:
The patent measures impedance at a discrete set of strategically selected frequency points rather than continuously across all frequencies. This partial measurement approach focuses resources on the most informative frequency regions (including fundamental frequencies and harmonics), achieving sufficient measurement precision for battery management without the time cost of exhaustive frequency sweeping.
3Ease of manufacture
If indirect measurements through conversion resistor are used, then voltage and current can be measured, but the response time is slow and high frequency impedance cannot be determined
Solution Approach 1:
The patent uses a known impedance element (conversion resistor) as an intermediary to infer current measurement from voltage measurement. Rather than directly measuring current which would require complex instrumentation, the system measures voltage across the known resistor and calculates current through Ohm's law, maintaining ease of implementation while achieving the necessary measurement speed for impedance determination.
Solution Approach 2:
The patent changes the measurement approach from direct high-speed current measurement to voltage measurement across a known impedance. By measuring voltage (which can be done at higher speeds with simpler circuitry) and using the known resistor value to derive current, the system achieves faster response times while maintaining the simplicity of voltage measurement circuits.
Data Source
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AI summary
A circuit arrangement for determining impedance of a battery cell is provided. A first circuit is configured to generate sine and cosine waveforms having N sample values per period. A second circuit is coupled to an output of the first circuit and is configured to input a current into the cell in response to the sample values of the cosine waveform. The current has an amplitude proportional to the sample values of the cosine waveform. A third circuit is coupled to the cell and configured to sample voltage levels across the cell resulting from the current being input into the cell. A fourth circuit is coupled to an output of the third circuit and is configured to separate each voltage level sampled by the third circuit into real and imaginary components.