Battery Cell Capacity Estimation Using Knee-Point Charging Curves
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Solution Overview
Problem
Existing methods for calculating the state of health (SOH) of lithium-ion cells, such as electric vehicle batteries, are inaccurate due to reliance on state of charge (SOC) estimation, leading to errors and potential safety issues from full charging or discharging, which also reduce cell life.
Innovation Solution
A method and apparatus that utilize knee-point voltage thresholds to calculate cell capacity by detecting knee-point electricity and charged electricity without full charging or discharging, using real-time charging curves and voltage difference analysis to determine cell state accurately.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If full charging or discharging is used to calculate cell capacity, then the calculation can be performed, but it affects cell life and causes safety problems
Solution Approach 1:
The patent applies partial action by performing charging only to a specific knee-point voltage threshold (e.g., 4.2V or 4.35V) rather than full charging to maximum voltage. This partial charging approach allows capacity calculation through integration of charging current and time from the knee-point to full charge, while avoiding the degradation and safety risks associated with repeated full charge-discharge cycles.
2Measurement precision
If charge or discharge in a specific SOC range is used, then the range must be large enough, but this has high requirements for SOC accuracy
Solution Approach 1:
The patent extracts and eliminates the dependency on SOC estimation by using voltage threshold-based knee-point detection. Instead of relying on accurate SOC values to define a charging range, the method identifies the knee-point through voltage thresholds and calculates capacity by integrating from that point to full charge, thereby removing the need for large SOC ranges and high SOC accuracy.
Solution Approach 2:
The patent changes the parameter basis from SOC (state of charge) estimation to voltage threshold detection. By using measurable voltage thresholds to identify the knee-point and define the charging integration range, the method replaces the complex SOC estimation requirement with a simpler voltage-based parameter that can be directly measured.
3Ease of operation
If SOC estimation is used to determine charging range, then the range can be selected, but inaccurate SOC estimation leads to large errors in results
Solution Approach 1:
The patent introduces voltage threshold as an intermediary parameter between the charging control system and the capacity calculation. The knee-point voltage threshold serves as a measurable mediator that defines the start point of integration, replacing the inaccurate SOC estimation with a directly measurable voltage parameter that accurately marks the transition point for capacity integration.
Data Source
AI summary
A method for calculating a cell state includes: acquiring a first knee-point voltage threshold of a cell, a first knee-point electricity corresponding to the first knee-point voltage threshold, a second knee-point voltage threshold of the cell and a second knee-point electricity corresponding to the second knee-point voltage threshold; recording a first charging curve of the cell in real time; obtaining a target knee point according to the first charging curve, the first knee-point voltage threshold, and the second knee-point voltage threshold, and obtaining a target-knee-point electricity according to the first knee-point electricity or the second knee-point electricity; detecting the electricity of the cell until the charging is completed, and obtaining a charged electricity of the cell from the target knee point to the completion of charging; and obtaining the capacity of the cell according to the target-knee-point electricity and the charged electricity.