Lithium Iron-Phosphate Battery Capacity Estimation via Resistance
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Solution Overview
Problem
Lithium iron-phosphate battery packs present challenges in accurately determining state of charge and charge capacity due to their low slope SOC-OCV curve and the impracticality of measuring charging and discharging currents, leading to inaccuracies in estimating available energy for electric vehicles.
Innovation Solution
A method using laboratory-measured battery pack resistance to determine open circuit voltage during charging, integrating charging current over time intervals with known SOC-OCV curve points to calculate battery pack capacity, thereby overcoming the inaccuracies in state of charge and capacity determination.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If open circuit voltage is used to determine state of charge in lithium iron-phosphate batteries, then state of charge can be determined using a known SOC-OCV curve, but the extremely low slope characteristic of the SOC-OCV curve makes it very difficult to accurately determine state of charge
Solution Approach 1:
The patent introduces battery pack resistance as an intermediary parameter to bridge the gap between terminal voltage and open circuit voltage. By measuring terminal voltage during charging and using the known resistance value to calculate the voltage drop, the system can determine open circuit voltage even when the SOC-OCV curve has extremely low slope, thereby resolving the contradiction between ease of operation and measurement precision
Solution Approach 2:
The patent changes the approach from directly using open circuit voltage to determining state of charge to using terminal voltage combined with resistance information. This parameter transformation allows the system to overcome the low slope characteristic of the SOC-OCV curve by calculating open circuit voltage from terminal voltage measurements taken during charging operations
2Duration of action of moving object
If charging and discharging current measurement is used to continuously determine state of charge, then continuous monitoring is achieved, but small errors would accumulate over time to cause a large inaccuracy in measurement
Solution Approach 1:
The patent replaces the mechanical integration of current measurements (which accumulates errors over time) with an electrical measurement approach using terminal voltage and resistance. By substituting the current integration method with a voltage-based calculation method, the system eliminates error accumulation while maintaining continuous monitoring capability
Solution Approach 2:
The patent changes the fundamental measurement parameter from current integration to terminal voltage measurement combined with resistance-based open circuit voltage calculation. This parameter substitution fundamentally eliminates the error accumulation problem inherent in continuous current measurement while preserving the ability to continuously determine state of charge
3Ease of operation
If battery pack capacity is determined without accurate state of charge information, then operational simplicity is maintained, but inaccurate driving range information is provided to the driver
Solution Approach 1:
The patent uses battery pack resistance as an intermediary to enable accurate capacity determination without complex state of charge monitoring. By incorporating resistance measurements into the calculation, the system can determine both state of charge and capacity accurately while maintaining operational simplicity, thus preventing loss of driving range information
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 estimation of lithium iron-phosphate battery pack capacity, enhancing the reliability of state of charge and available energy, ensuring optimal vehicle range and extending battery life by preventing over-charging and over-discharging.
Implementation Method 1
A laboratory-measured battery pack resistance, which is known to remain constant over a battery pack's life, can be used to determine open circuit voltage from terminal voltage during charging
Data Source
AI summary
A method for estimating charge capacity of a lithium iron-phosphate battery pack using data from a plug-in charge event. A laboratory-measured battery pack resistance, which is known to remain constant over a battery pack's life, can be used to determine open circuit voltage from terminal voltage during charging. Actual open circuit voltage after charging can be measured later, after the battery pack has rested for a sufficient amount of time. The two values of open circuit voltage, if taken at points on the battery pack's SOC-OCV curve which have great enough slope, provide two values of battery pack state of charge. By integrating charging current over the time interval between the two open circuit voltage readings, and using the two state of charge values, the battery pack capacity can be determined from the plug-in charge data.