SOC Estimation Using Segmented OCV Curves and Linear Interpolation
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Solution Overview
Problem
Existing methods for estimating the State of Charge (SOC) of secondary batteries using Open Circuit Voltage (OCV) are inaccurate due to the hysteresis gap between charging and discharging curves, lacking a clear criterion for selecting the appropriate method based on battery history.
Innovation Solution
A secondary battery system with a controller that estimates SOC by calculating electric quantities charged and discharged, referring to charging or discharging curves based on these quantities, and using a linear approximation relation to supplement SOC-OCV characteristics in the region enclosed by both curves, with temperature and SOC influencing the calculation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If SOC is estimated by referring to charging curve or discharging curve, then SOC estimation can be performed, but accuracy deteriorates when battery state is in the region enclosed by both curves due to hysteresis
Solution Approach 1:
The patent segments the SOC estimation method into three distinct approaches based on battery history: (1) using charging curve when charged from fully discharged state, (2) using discharging curve when discharged from fully charged state, and (3) using linear approximation for states in the enclosed region. This segmentation allows selecting the appropriate method for each operational scenario, resolving the contradiction between measurement precision and adaptability.
Solution Approach 2:
The patent introduces dynamic selection of estimation methods based on real-time battery state and history. The controller dynamically determines which curve or approximation method to use by evaluating whether the battery is in the charging curve region, discharging curve region, or enclosed region. This dynamic adaptation resolves the contradiction by making the system versatile across different states while maintaining accuracy in each specific state.
2Ease of manufacture
If a single SOC-OCV curve is used for estimation, then the method is simple to implement, but accuracy deteriorates due to hysteresis gap between charging and discharging curves
Solution Approach 1:
Instead of using a single SOC-OCV curve, the patent segments the characteristics into charging curve, discharging curve, and linear approximation region. This segmentation maintains relative simplicity by using basic curve storage and linear calculation, while dramatically improving accuracy by selecting the appropriate segment based on battery history and state.
Solution Approach 2:
The patent introduces a linear approximation relation as an intermediary method for the enclosed region between charging and discharging curves. This intermediary approach fills the gap created by hysteresis, providing accurate estimation in regions where neither the charging nor discharging curve alone is sufficient, thus improving overall precision without excessive complexity.
3Measurement precision
If SOC estimation method is selected based on battery history, then accuracy in enclosed region improves, but device complexity increases due to multiple curves and selection criteria
Solution Approach 1:
The patent segments the estimation system into three manageable components: charging curve storage, discharging curve storage, and linear approximation calculation. Each component has a clear, simple function, making the overall system easier to implement despite handling multiple curves. The segmentation reduces complexity by organizing the solution into distinct, well-defined modules.
Solution Approach 2:
The patent performs preliminary action by pre-storing the charging and discharging curves in the controller memory before operation. This preparation allows the runtime selection process to be simple and efficient, comparing current battery state against predefined curves and selecting the appropriate method. The preliminary storage of curves eliminates the need for complex real-time curve generation, reducing device complexity.
Data Source
AI summary
If a charging quantity charged to a battery pack since switching from discharging to charging is more than or equal to a reference charging quantity, an ECU estimates the SOC from the OCV by referring to a charging OCV. If a discharging quantity discharged from the battery pack since switching from charging to discharging is more than or equal to a reference discharging quantity, the ECU estimates the SOC from the OCV by referring to a discharging OCV. If the electric quantity is less than the reference charging quantity or if the electric quantity is more than the reference discharging quantity, the ECU estimates the SOC from the OCV using a straight line for supplementing SOC-OCV characteristics in a region enclosed by the charging OCV and the discharging OCV.