Battery Cell State of Charge Estimation Using Voltage Offset
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Solution Overview
Problem
Existing battery management systems face challenges in accurately estimating the state of charge (SOC) of traction batteries in hybrid-electric and electric vehicles, leading to cell imbalance due to manufacturing variations and differing charging rates, which can result in overcharging or undercharging of cells.
Innovation Solution
A controller is programmed to calculate the cell state of charge based on a voltage offset derived from the difference between cell voltage and average cell voltage, relative to a reference voltage associated with the traction battery state of charge, using a cell model that incorporates battery current and open-circuit voltage characteristics, allowing for cell balancing to maintain optimal charge distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional battery management systems estimate state of charge using standard voltage measurement methods, then the system structure remains simple, but the state of charge estimation accuracy deteriorates due to cell voltage differences caused by manufacturing variations and differing charging rates
Solution Approach 1:
The patent transforms the state of charge estimation problem by changing the voltage parameter from absolute cell voltage to voltage difference (offset from average cell voltage). This parameter transformation allows the system to account for cell variations while using simpler computational methods, resolving the contradiction between measurement precision and device complexity
Solution Approach 2:
The patent creates a simplified model by copying the voltage difference characteristic from individual cells and applying it relative to an average reference. This approach captures the essential variation information without requiring complex individual cell modeling, thereby improving accuracy while maintaining computational simplicity
2Reliability
If the system monitors each cell's state of charge individually to prevent overcharging or undercharging, then the reliability of battery operation improves, but the device complexity and computational burden increase
Solution Approach 1:
The patent changes the monitoring parameter from absolute state of charge values to voltage differences relative to the average cell voltage. This transformation maintains the ability to detect individual cell deviations and prevent overcharging or undercharging, while simplifying the computational requirements for individual cell monitoring and balancing control
Solution Approach 2:
The patent uses the average cell voltage as a reference potential, measuring each cell's state relative to this common reference. This approach enables individual cell monitoring and safety control while using a unified reference framework, reducing the complexity of managing multiple independent monitoring systems
Data Source
AI summary
Hybrid and electric vehicles include a traction battery including many interconnected cells. Effective battery control, such as cell balancing, may rely on an accurate state of charge value for each of the cells. A method to reduce the computational effort of the state of charge calculation is developed. An accurate pack level state of charge calculation is implemented and represents the average cell state of charge. An average cell voltage is based on a pack voltage measurement. A state of charge difference is calculated for each cell based on a difference between a cell voltage and the average cell voltage. The state of charge difference utilizes the pack state of charge and a characteristic voltage and state of charge relationship for the cell. The cell state of charge is the sum of the pack state of charge and the state of charge difference.