Battery SOC Estimation via Dynamic OCV Curve Correction
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Solution Overview
Problem
Existing methods for estimating the state of charge of iron phosphate lithium ion batteries, such as those described in Japanese Unexamined Patent Application Publication No. 2017-167163, fail to accurately account for battery aging, leading to significant differences between estimated and actual state of charge as the battery ages.
Innovation Solution
A control device that includes a voltage control unit, an obtaining unit, and a correction unit, which sequentially switches command voltages to measure terminal voltages during charge and discharge cycles, allowing for the correction of the state of charge versus open circuit voltage characteristic curve based on differences in obtained terminal voltages, thereby improving accuracy in estimating the state of charge of aged batteries.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the SOC-OCV characteristic curve is used for state of charge estimation, then the estimation is simple and fast, but the accuracy deteriorates in the flat region where the rate of change in open circuit voltage with respect to state of charge is small
Solution Approach 1:
The patent performs preliminary action by acquiring terminal voltages at multiple state of charge points in advance and storing them. This allows the system to have reference data ready before estimation is needed, enabling accurate state of charge determination even in flat regions where real-time measurement would be inaccurate. The pre-acquired voltage data serves as a reference map for accurate estimation.
2Device complexity
If the existing SOC-OCV characteristic curve is used without correction, then the estimation method remains simple, but the accuracy deteriorates as the battery ages
Solution Approach 1:
The patent implements feedback by comparing newly acquired terminal voltages with previously stored reference voltages and using the differences to correct the SOC-OCV characteristic curve. This feedback mechanism allows the system to adapt to battery aging automatically, maintaining high estimation accuracy for aged batteries while keeping the overall method relatively simple through automated curve correction.
Solution Approach 2:
The patent applies parameter changes by updating the SOC-OCV characteristic curve parameters based on acquired terminal voltage data. As the battery ages, the characteristic curve parameters are adjusted to reflect the changed battery behavior, enabling accurate state of charge estimation for aged batteries without requiring a completely different estimation method.
3Measurement precision
If terminal voltages are acquired at multiple state of charge points, then the correction accuracy is improved, but the measurement time and energy consumption increase
Solution Approach 1:
The patent applies partial action by selecting specific state of charge points for voltage acquisition, particularly focusing on points that are most critical for accurate estimation including the flat region boundaries. This approach achieves sufficient correction accuracy without the need to measure at every possible state of charge point, thereby reducing measurement time and energy consumption while maintaining adequate correction precision.
Data Source
AI summary
A control device includes: a voltage control unit that sequentially switches a command voltage of a direct current-direct current converter between different predetermined measurement voltages to control the command voltage; an obtaining unit that obtains, for each of the measurement voltages, a first terminal voltage of a battery when constant current discharge is performed and a second terminal voltage of the battery when constant current charge is performed; and a correction unit that corrects an SOV-OCV characteristic curve indicating a relationship between the state of charge and an open circuit voltage of the battery, based on a comparison between the first terminal voltages newly obtained by the obtaining unit and the first terminal voltages immediately previously obtained by the obtaining unit and a comparison between the second terminal voltages newly obtained by the obtaining unit and the second terminal voltages immediately previously obtained by the obtaining unit.