State of Charge Determination via Open Circuit Voltage Rate of Change
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Solution Overview
Problem
Existing methods for determining the state of charge (SOC) of electrical energy storage units, such as batteries, face inaccuracies due to flat open-circuit voltage (OCV) curves, especially in lithium iron phosphate cells, and require lengthy relaxation times for accurate measurements, making it difficult to determine SOC across a wide range or in series-connected cells.
Innovation Solution
A method that determines the state of charge by analyzing the rate of change of the open-circuit voltage during charging and discharging, using predefined rate of change thresholds to identify state of charge values, even with flat voltage characteristics, and calculates current capacity based on these values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If open-circuit voltage measurement is used to determine SOC, then SOC can be determined with sufficient accuracy, but a long relaxation time (at least 20 minutes at room temperature) is required which increases measurement time
Solution Approach 1:
The patent changes the parameter being measured from absolute OCV value to the rate of change of OCV (dOCV/dt). By monitoring how the open-circuit voltage changes over time during charging/discharging, the system can determine SOC without requiring the battery to reach complete equilibrium, thus reducing relaxation time while maintaining measurement accuracy.
2Measurement precision
If OCV measurement is used for lithium iron phosphate battery cells, then SOC can be determined when the cell is almost discharged or fully charged, but the flat OCV curve makes SOC determination impossible in the middle range due to measuring tolerances
Solution Approach 1:
The patent transforms the measurement approach by using the derivative (rate of change) of the OCV curve instead of the absolute OCV value. This transformation converts the flat region problem into a detectable signal, as the rate of change exhibits characteristic patterns even when the absolute voltage remains relatively constant, enabling SOC determination across the entire charge range.
Solution Approach 2:
The patent performs preliminary differentiation of the OCV curve to identify characteristic rate-of-change patterns that correspond to specific SOC levels. By pre-establishing these patterns and their relationships to SOC states, the system can quickly determine SOC without requiring extensive measurement time or multiple measurements.
3Measurement precision
If multiple OCV measurements are required to determine current capacity, then capacity can be accurately determined, but the process can take hours due to relaxation times
Solution Approach 1:
The patent changes the measurement parameter from static OCV values requiring equilibrium to dynamic OCV rate-of-change values that can be measured during active charging/discharging. This enables capacity determination through integration of current over time between two SOC points identified by rate-of-change measurements, dramatically reducing measurement time while maintaining accuracy.
4Measurement precision
If cells are equalized through balancing to enable SOC determination in series-connected cells, then SOC can be determined for all cells, but the process is very time-consuming
Solution Approach 1:
The patent enables independent SOC determination for each cell in a series connection by monitoring the rate of change of each cell's OCV individually. This eliminates the need to equalize cells to the same voltage level, as each cell's SOC can be determined based on its own dOCV/dt characteristics, allowing SOC measurement to proceed in parallel across all cells without sequential balancing.
Data Source
AI summary
A method for determining the state of charge of an electrical energy storage unit. In one example, the method includes approximately determining a state of charge of the electrical energy storage unit, so that the state of charge is known at least with a predefined tolerance; transmitting a first control command for charging and/or discharging the electrical energy storage unit depending on the approximate state of charge determined; and determining a rate of change of the open circuit voltage of the electrical energy storage unit during charging and/or discharging. When the predefined rate of change exceeds or falls below a predefined first rate of change, a first state of charge value is determined depending on the determined rate of change of the open circuit voltage during charging and/or discharging.