Battery Cell State of Charge Balancing with Calibrated Controller
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Solution Overview
Problem
Current methods for balancing the state of charge in vehicle battery packs, especially in electric and hybrid electric vehicles, are inadequate when some cells have been replaced at different times, leading to suboptimal balancing.
Innovation Solution
A method and system that determine initial and subsequent state of charge values for each cell unit, measure the battery pack current, and use a controller to balance the state of charge around a predetermined calibrated value, employing voltage sensors, current sensors, and transistors to adjust cell voltages and currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If current balancing techniques are used, then the system is simple to operate, but the cell balancing is suboptimal when cells are replaced at different times
Solution Approach 1:
The system performs preliminary characterization of each cell unit by measuring voltage at multiple state of charge points and determining capacity and open circuit voltage characteristics before normal operation. This preliminary action enables the balancing algorithm to account for individual cell variations without requiring complex real-time adjustments during operation.
Solution Approach 2:
The system changes the approach from fixed voltage-based balancing to a dynamic model-based balancing that uses measured parameters (capacity, OCV characteristics) of each individual cell unit. The balancing decision is based on comparing predicted state of charge values derived from these parameters rather than simple voltage thresholds, allowing optimal balancing for mixed-age cell populations.
2Measurement precision
If individual cell monitoring is implemented, then the state of charge determination is more accurate, but the measurement and control complexity increases
Solution Approach 1:
The controller performs multiple functions using the same voltage measurement infrastructure: it measures voltage for state of charge determination, characterizes cell capacity and OCV characteristics, and enables balancing decisions. This multi-functionality achieves high measurement precision without proportionally increasing hardware complexity, as the same sensors serve multiple purposes throughout the cell lifecycle.
Data Source
AI summary
Methods and systems for balancing a state of charge of cell units of a battery pack of a vehicle are provided. Voltage sensors each measure a first value of state of charge of respective cell units at a first time, and a second value of state of charge of the respective cell units at a second time that is subsequent to the first time. A current sensor measures a current of the battery pack. A controller is coupled to the voltage sensors and the current sensor. The controller balances the state of charge around a predetermined calibrated value using the first values of state of charge, the second values of state of charge, and the current.