EV Battery Cell Balancing for Capacity Retention Control
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Solution Overview
Problem
Lithium-ion batteries in electric vehicle battery packs experience uneven deterioration due to variations in cell state of charge and temperature, leading to reduced battery capacity and performance, as existing methods fail to individually control each cell's deterioration effectively.
Innovation Solution
A cell balancing method and system that predicts standing time and estimates capacity retention rates for each cell, adjusting their state of charge to maintain a predetermined difference between maximum and minimum values, using a temperature determination device, state-of-charge determination device, and state-of-charge adjustment device to optimize cell performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the battery is left at high temperature and high state of charge to maintain battery capacity, then the battery capacity is maintained initially, but deterioration progresses with passage of time resulting in decline in battery capacity
Solution Approach 1:
The system performs preliminary action by predicting standing time and estimating capacity retention rates before actual deterioration occurs. It proactively adjusts state of charge values during stoppage periods to prevent excessive deterioration, rather than reacting after damage has been done.
Solution Approach 2:
The system changes parameters by dynamically adjusting state of charge values based on predicted standing time, temperature, and cell characteristics. It modifies operational parameters (state of charge targets) to optimize between capacity maintenance and deterioration suppression.
2Device complexity
If uniform state of charge control is applied to all cells, then the control system is simple, but variations in deterioration among cells occur due to different positions and characteristics
Solution Approach 1:
The system applies local quality by determining individual state of charge adjustment values for each cell based on its specific characteristics, position, and predicted behavior. Instead of uniform control, each cell receives customized adjustment to account for local variations in deterioration patterns.
Solution Approach 2:
The system segments the battery pack into individual cells with unique control strategies. It divides the uniform control approach into cell-specific control, allowing each cell to be managed according to its particular characteristics and deterioration rate.
3Quantity of substance
If state of charge adjustment is performed frequently to maintain capacity retention, then capacity retention is improved, but energy loss increases due to repeated charging and discharging operations
Solution Approach 1:
The system applies partial action by performing state of charge adjustments only when necessary, based on predicted standing time and estimated capacity retention rates. It avoids excessive adjustments by using threshold-based control and prediction to determine when intervention is needed.
Solution Approach 2:
The system implements periodic action by adjusting states of charge during stoppage periods rather than continuously. It utilizes natural stoppages in operation to perform adjustments, converting idle time into opportunities for maintenance without adding continuous operational burden.
Data Source
AI summary
A cell balancing method includes: setting predicted standing time, based on a statistic value of standing time collected over a predetermined period of time; estimating a capacity retention rate of each of cells of a lithium-ion battery at a next start-up after a lapse of the predicted standing time; on the condition that a difference in the estimated capacity retention rate between a maximum value and a minimum value becomes greater than a predetermined value, setting state-of-charge adjustment values for one or both of a cell having the maximum value and a cell having the minimum value; and allowing, during the stoppage of the electric vehicle, a state-of-charge adjustment device to adjust the states of charge of one or both of the cell having the maximum value and the cell having the minimum value to the state-of-charge adjustment values.