Lithium Ion Battery SOC Estimation via Voltage Change Rate
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Solution Overview
Problem
Existing methods for estimating the state of charge (SOC) of lithium ion batteries using olivine-type lithium iron phosphate as a positive electrode and graphite-based carbon as a negative electrode face challenges, particularly in detecting SOC within intermediate levels without charging to 100% or discharging to 0%, and struggle with constant voltage issues that hinder voltage-based measurement.
Innovation Solution
A state-of-charge estimation method that calculates the voltage change rate during charging or discharging and uses pre-stored maps to determine the SOC, allowing estimation without requiring full charge or discharge cycles, by measuring voltage and current changes and comparing candidates from charging and discharging maps.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If open circuit voltage measurement is used for SOC detection, then SOC can be detected with reference to OCV-SOC characteristics, but it requires charging to 100% or discharging to 0% to create the characteristics, and cannot be used for intermediate SOC ranges
Solution Approach 1:
The invention changes the measurement parameter from open circuit voltage (OCV) to voltage change rate during charging/discharging. By measuring how quickly voltage changes during active charging or discharging at intermediate SOC levels, the system can determine SOC without requiring extreme charge states. This parameter transformation enables SOC detection in the previously inaccessible intermediate range while reducing time loss.
2Measurement precision
If voltage-based SOC detection is used for iron-phosphate-based lithium ion batteries, then SOC can be detected from voltage, but the voltage remains constant over a wide SOC range, making detection difficult
Solution Approach 1:
The invention applies preliminary action by pre-storing voltage change rate characteristics for different SOC levels and charging/discharging rates in maps during the battery's operational life. These pre-collected data maps enable real-time SOC determination by simply comparing current voltage change rate measurements against the stored characteristics, eliminating the need for complex real-time analysis and enabling accurate detection even in the constant voltage region.
Solution Approach 2:
The invention introduces the voltage change rate as an intermediary parameter between the constant voltage and SOC. Instead of directly measuring voltage (which is constant), the system measures the rate of voltage change, which varies with SOC even when voltage itself is constant. This intermediary parameter bridges the gap between the unchanging voltage and the varying SOC, enabling detection.
3Measurement precision
If current integration method is used for SOC estimation, then SOC can be estimated by integrating charging/discharging current, but it requires charging to 100% or discharging to 0% as reference points
Solution Approach 1:
The invention changes from integrating current over time to measuring voltage change rate during charging/discharging. By focusing on the relationship between voltage change rate and SOC at various charging/discharging rates, the system can determine SOC at any intermediate point without needing full charge/discharge reference points. This parameter shift from cumulative current to instantaneous voltage rate enables intermediate SOC measurement.
Data Source
AI summary
A method of estimating a state of charge of a lithium ion battery, which uses a lithium phosphate compound having an olivine-type crystal structure for a positive electrode and uses an active material exhibiting no potential fluctuation due to an insertion/desorption reaction of lithium for a negative electrode, based on a voltage change rate. The method includes: storing a map representing a correspondence relationship between the voltage change rate and the state of charge at a plurality of charging/discharging rates; obtaining a candidate for a present state of charge from an actually measured voltage change rate, by referring to the map; and obtaining the present state of charge through a comparison between the candidate for the state of charge obtained during the charging and the candidate for the state of charge obtained during the discharging.