Lithium Ion Battery State of Charge Estimation via Voltage Change Rate
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Solution Overview
Problem
It is challenging to accurately estimate the state of charge (SOC) of lithium ion secondary batteries, particularly when the SOC is high, as the open circuit voltage (OCV) remains constant, making it difficult to determine the remaining capacity from the voltage alone.
Innovation Solution
A method and apparatus that measure the voltage of a lithium ion secondary battery at a predetermined high current rate, calculate the rate of increase in voltage, and use reference data to estimate the SOC, with separate data sets for different current rates and temperatures, allowing for accurate SOC estimation based on the voltage change over time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If open circuit voltage (OCV) is used to estimate state of charge (SOC), then the estimation method is simple, but the estimation precision deteriorates when SOC is high because OCV remains constant
Solution Approach 1:
The patent changes the parameter used for SOC estimation from static OCV to dynamic voltage change rate (dV/dt). By measuring how quickly the voltage changes during charging at high current rates, the system obtains a parameter that varies with SOC even in the high SOC range where OCV is constant, thereby resolving the precision problem while maintaining operational simplicity
Solution Approach 2:
The patent transitions from using a static parameter (OCV) to a dynamic parameter (voltage change rate during charging). The dynamic measurement captures the transient voltage behavior during high-rate charging, which provides SOC information that is sensitive to charge state even when the battery is nearly full, thus solving the constant OCV problem
2Measurement precision
If high current rate charging is used to enable voltage change measurement, then SOC estimation precision is improved, but the battery may be subjected to harmful effects from high rate charging
Solution Approach 1:
The patent performs the high current rate charging measurement as a brief preliminary action specifically for SOC estimation purposes, rather than as the normal charging mode. This temporary high-rate pulse provides the necessary voltage dynamics for accurate measurement, after which normal charging resumes, thus obtaining precise SOC data without subjecting the battery to prolonged harmful high-rate stress
Solution Approach 2:
The patent employs periodic high current rate measurement pulses during the charging process to obtain SOC estimates at key intervals. These periodic measurements provide necessary SOC information without requiring continuous high-rate charging, thereby balancing measurement precision with battery protection by limiting the duration and frequency of high-stress events
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 precise SOC estimation by correlating the rate of voltage increase with the battery's state of charge, preventing overcharging and maintaining stable battery control, even in high SOC states where OCV is constant, thus extending battery life and ensuring reliable energy storage.
Implementation Method 1
a positive electrode and a negative electrode, wherein the positive electrode includes a positive electrode active material comprising nickel, manganese, and titanium
Implementation Method 2
charging or discharging is achieved as a result of migration of electric charge together with lithium ions between a positive electrode and a negative electrode
Data Source
AI summary
A state of charge estimation method includes: measuring a voltage of a lithium ion secondary battery that is being charged at a predetermined current rate higher than or equal to a predetermined charging rate; obtaining a rate of increase in the measured voltage; and estimating a state of charge of the lithium ion secondary battery based on the rate of increase in the measured voltage and first reference data. The first reference data are data including a correlation between a state of charge of a reference lithium ion secondary battery and a rate of increase in voltage of the reference lithium ion secondary battery when the reference lithium ion secondary battery is charged at the predetermined current rate.