Battery Control Device Homogenizing LFP Cell States
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Solution Overview
Problem
In battery assemblies using lithium iron phosphate batteries, the flat region in the SOC-OCV characteristic curve makes it difficult to uniquely specify the power storage amount from the open circuit voltage, leading to inefficient homogenization of battery cell states and potential performance deterioration.
Innovation Solution
A battery control device that determines voltage differences and open circuit voltage positions of battery cells relative to a flat region, executing control actions to raise, lower, or maintain power storage amounts to move cells out of the flat region, thereby enhancing the chances of efficient homogenization and maintaining uniform states of charge.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If battery homogenization is performed using open circuit voltage detection, then homogenization can be executed, but in LFP batteries with flat SOC-OCV characteristics the power storage amount cannot be uniquely specified, reducing homogenization efficiency
Solution Approach 1:
The control device performs preliminary actions by detecting open circuit voltages and identifying cells in the flat region before homogenization, then executes charge/discharge operations to move cells out of the flat region. This preliminary identification and preparation enables subsequent accurate homogenization by ensuring cells are in regions where SOC can be uniquely determined from OCV.
Solution Approach 2:
The control device changes the operating parameters of battery cells by executing charge or discharge operations to alter their open circuit voltages. By actively modifying the OCV parameter, the system moves cells from the flat region (where dOCV/dSOC is small) to regions with steeper characteristics, enabling accurate SOC specification and effective homogenization.
2Loss of information
If battery cells operate in the flat region of SOC-OCV characteristics, then voltage measurements are obtained, but the rate of change of voltage with respect to power storage amount is small, making it difficult to uniquely specify power storage amount
Solution Approach 1:
The system performs preliminary detection of open circuit voltages and identification of cells located in the flat region before homogenization operations. This advance identification allows the control device to selectively apply charge/discharge operations only to cells that would otherwise provide ambiguous SOC information, thereby preventing information loss and ensuring reliable SOC specification.
Solution Approach 2:
The control device actively changes the OCV parameter of cells in the flat region by executing charge or discharge operations. This parameter change moves cells to operating regions where the relationship between OCV and SOC is more distinct, thereby recovering the lost information about power storage amount and improving SOC specification reliability.
3Reliability
If homogenization process chances are reduced, then battery assembly performance deteriorates, but increasing homogenization opportunities requires moving cells out of the flat region
Solution Approach 1:
The control device performs preliminary identification of cells in the flat region and determines which cells require charge or discharge operations. By preparing and identifying target cells in advance, the system maximizes the number of cells that can undergo effective homogenization, thereby increasing homogenization opportunities without compromising assembly performance.
Solution Approach 2:
The system actively modifies the OCV parameters of identified cells through charge/discharge operations, moving them out of the flat region where SOC cannot be uniquely determined. This parameter change strategy increases the number of cells eligible for homogenization, thereby improving both homogenization productivity and overall battery assembly performance.
Data Source
AI summary
A battery control device that controls a battery assembly includes a first determining unit that determines whether a voltage difference between minimum and maximum values of OCVs of battery cells constituting the battery assembly and having an SOC-OCV characteristic curve including a flat region is equal to or larger than a predetermined voltage value, a second determining unit that determines whether the OCV of each battery cell is lower than a lower-limit voltage of the flat region, or is equal to or higher than the lower-limit voltage and lower than an upper-limit voltage, or is higher than the upper-limit voltage, a controller that executes control selected from SOC raising control, SOC lowering control, and SOC keeping control of the battery cells, based on determination results of the first and second determining units, and a processor that homogenizes the SOCs of the battery cells controlled by the controller.