Li-Ion Cell SOC Balancing via Differential Capacity Curves
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Solution Overview
Problem
Existing methods for determining and balancing the state of charge in lithium-ion battery cells are unreliable, especially at partial charges, due to the limitations of voltage and temperature measurements, and are not feasible for real-time monitoring and balancing during charging and discharging cycles.
Innovation Solution
The method involves observing changes in differential capacity curves during discharge or charge cycles to determine the state of charge, using specific spikes in the curves to identify the charge level, and employing a shunt circuit or balancing units to balance charge differences between cells, allowing for real-time monitoring and balancing during both charging and discharging.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If voltage measurement is used to determine state of charge, then the method is simple to implement, but the determination is unreliable especially at partial charges
Solution Approach 1:
The patent introduces an intermediary substance (salt solution) and measurement system that indirectly determines state of charge by measuring ionic conductivity, which changes with electrolyte concentration as water is consumed during charging. This mediator provides reliable measurement without directly measuring voltage, resolving the contradiction between simplicity and reliability.
2Quantity of substance
If charging continues until all cells reach maximum voltage, then full capacity is utilized, but cells with different initial states of charge experience deep discharge or overcharge
Solution Approach 1:
The system performs preliminary measurement of ionic conductivity in each cell before charging begins, determining the initial state of charge. Based on these preliminary results, the charging process is adjusted for each cell to prevent deep discharge or overcharge, while still achieving full capacity utilization when all cells are balanced.
Solution Approach 2:
The patent applies different charging strategies to different cells based on their individual states of charge. Each cell receives customized charging control rather than uniform treatment, allowing cells with different initial conditions to be balanced simultaneously while maintaining safety.
3Ease of manufacture
If balancing is performed only towards the end of charging cycle, then passive balancing method can be used, but the power take-up capacity is limited and balancing takes too long
Solution Approach 1:
The system performs preliminary assessment of cell states before charging begins, allowing active balancing measures to be implemented during the charging process itself rather than waiting until the end. This preliminary action enables faster balancing while maintaining system simplicity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables accurate and reliable determination and balancing of state of charge differences between cells, preventing deep discharge or overcharge, even at partial charge levels, thereby optimizing the utilization of battery capacity in hybrid systems.
Implementation Method 1
a control unit (CU) is provided which, during a charge or discharge cycle of the battery pack, observes changes in the differential capacity curves of the battery cells and determines a state of charge of the battery cells on the basis of the observed changes in the differential capacity curves
Data Source
Figure 1A~1C
Figure 2A~2C
Figure 3~4C
AI summary
The state of charge of a lithium-ion cell module comprising several cells is managed by observing changes in a cell's differential capacity curve exceeding or falling short of a preset limit value during a discharging or charging process of the cell. Each of said changes is a change which is specific for this particular cell and consistent with a specific state of charge of the cell, whereby the ordinal number of a differential capacity curve change is used as a basis for determining a state of charge of the cell consistent with said change or a difference in the states of charge between two or more cells. The thus determined state of charge differences between cells are then sought to be balanced by means of a balancing system adapted to balance a charge.