Battery State of Charge Determination via Rest Period Voltage Relaxation
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Solution Overview
Problem
Existing methods for determining the state of charge (SOC) of batteries, particularly lithium-ion batteries with biphasic cathode material, face inaccuracies due to small voltage variations and error accumulation over cycles, making them unsuitable for long-term management.
Innovation Solution
A method involving measuring an electrical parameter during a rest period, calculating an indicator using voltage or current values, and referencing a calibration curve to determine SOC, which is calibrated to provide a monotonic relationship between the indicator and SOC, eliminating the need for integrating current over time and reducing uncertainty.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If voltage measurement method is used to determine SOC, then measurement simplicity is improved, but measurement precision deteriorates due to small voltage variations in lithium-ion batteries with biphasic cathode material
Solution Approach 1:
The patent changes the measurement parameter from simple voltage to a composite indicator that combines voltage measurements taken at different time points during a rest period. This indicator (ind1 or ind2) creates a monotonic relationship with SOC that overcomes the flat voltage characteristic of biphasic lithium-ion batteries, enabling precise SOC determination while maintaining measurement simplicity.
2Measurement precision
If current integration method is used to determine SOC, then initial measurement accuracy is improved, but reliability deteriorates due to error accumulation over charging/discharging cycles
Solution Approach 1:
The patent employs a method that uses only voltage measurements during rest periods to determine SOC, eliminating the need for current integration and its associated error accumulation. The system serves itself by using passive voltage relaxation characteristics rather than active current tracking, thereby maintaining reliability over long-term operation without requiring memory for charge corrections.
3Device complexity
If open circuit voltage measurement is used to determine SOC, then device complexity is reduced, but measurement precision deteriorates when battery is under load
Solution Approach 1:
The patent implements a preliminary rest period before SOC measurement, during which the battery is allowed to relax from its loaded state. Voltage measurements are taken at multiple time points (t1 and t2) during this rest period to create an indicator that reflects the battery's true SOC state, independent of recent load conditions. This preliminary action eliminates the need for complex load compensation algorithms.
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 method provides accurate and reproducible SOC determination with low uncertainty (<10%), suitable for lithium-ion batteries, without requiring memory for charge corrections, and is applicable in various energy storage applications.
Implementation Method 1
placing the battery in open circuit during a rest period, measuring at least first and second values of the voltage across the terminals of the battery at different instants during the rest period
Data Source
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AI summary
The method for determining the state of charge of a battery includes measurement of an electric parameter of the battery during a charging or discharging phase of the battery followed by placing the battery in open circuit during a rest period. During the rest period, at least two values of the voltage at the battery terminals are measured. An indicator is then calculated according to the electric parameter and to values of the voltage measured during the rest period, and the state of charge corresponding to this indicator is then determined by means of a calibration curve representative of the variations of the indicator as a function of the state of charge of the battery.