Battery Cell SOC Updating Using Differential Capacity Analysis
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Solution Overview
Problem
Determining the state of charge (SOC) of battery cells in battery packs is challenging due to voltage plateaus in their profiles, which lead to inaccurate SOC estimates and potential safety issues, especially in multi-cell packs.
Innovation Solution
An in-situ method is developed to determine SOC values by changing the external cell current through battery cells according to a test profile, obtaining voltage readings, and calculating differential capacities to precisely estimate SOC, even when cells are not in target zones.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If voltage readings are used to determine SOC during voltage plateaus, then the measurement process is simple, but the SOC estimation accuracy deteriorates
Solution Approach 1:
The patent changes the operating parameter from static voltage measurement to dynamic differential capacity analysis. By applying AC test signals and measuring the resulting voltage responses, the system transforms the SOC determination from a static voltage reading problem to a dynamic impedance-based measurement, which remains accurate even during voltage plateaus where traditional voltage-based methods fail.
2Measurement precision
If Coulomb counters are updated frequently to improve SOC accuracy, then the SOC estimation precision improves, but the system complexity and computational load increase
Solution Approach 1:
The patent replaces the mechanical integration approach of Coulomb counters with an electrical impedance-based measurement system. Instead of continuously integrating current over time, the system uses AC test signals and differential capacity calculations to directly determine SOC, eliminating the need for complex continuous integration algorithms and reducing computational complexity while maintaining or improving accuracy.
3Measurement precision
If battery cells are taken offline for SOC determination, then the measurement accuracy improves, but the productivity and operational continuity deteriorate
Solution Approach 1:
The patent enables continuous SOC determination by performing measurements during normal battery operation. The AC test signals are superimposed on the operating current, allowing SOC measurements to be taken without interrupting charge or discharge cycles. This maintains continuous operational productivity while providing accurate SOC updates through differential capacity analysis.
4Measurement precision
If differential capacity measurements are performed to improve SOC accuracy, then the SOC estimation precision improves, but the measurement time and testing duration increase
Solution Approach 1:
The patent uses periodic AC test signals to perform differential capacity measurements efficiently. By applying sinusoidal test signals at specific frequencies during normal operation, the system obtains differential capacity data through frequency-domain analysis, which provides accurate SOC information much faster than traditional slow voltage relaxation methods while maintaining measurement precision.
Data Source
AI summary
Described herein are methods and systems for updating SOC estimates of individual cells in battery packs. Specifically, SOC estimates are updated in-situ, e.g., while the battery packs remain operational. For example, a cell is charged or discharged, independently from other cells, until the cell OCV is at a set value, corresponding to one of target zones. The target zones have more prominent correlations between the OCV and SOC than other parts of the OCV profile. A new SOC value, corresponding to the cell OCV, is used to update the SOC estimate. In some examples, a set of voltages is obtained while the cell is charged or discharged at a constant current/power, e.g., outside of the target zones. One or more differential capacities are determined from this voltage set, and a new SOC value is obtained based on these differential capacities.