LFP Cell Capacity Estimation for Overcharge-Safe Bipolar Batteries
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Solution Overview
Problem
Bipolar batteries with lithium iron phosphate (LFP) cells face challenges in estimating full charge capacity efficiently, leading to potential overcharging and short circuits due to variations in cell capacity during manufacturing, which can result in unused capacity and potential defects.
Innovation Solution
A method to estimate full charge capacity by calculating the increase in charge amount when the battery voltage increases from a first voltage to a second voltage within a predetermined range, utilizing the positive linear correlation between charge amount increase and full charge capacity, without charging the cell to full state, using CC or CCCV charging at a rate of 0.1 C or less, and employing a control device with a capacity estimation unit to equalize cell charges.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If charging ends when the cell with small full charge capacity is fully charged, then overcharging of each cell is prevented, but the cell with large full charge capacity is not fully charged
Solution Approach 1:
The patent performs preliminary classification of cells into groups based on their full charge capacity before charging. By estimating the full charge capacity of each cell in advance and grouping them accordingly, the system can charge each group to its appropriate full charge state without overcharging or undercharging, thus resolving the contradiction between preventing overcharging and achieving full charging efficiency
Solution Approach 2:
The patent segments the battery pack into multiple groups based on cell capacity characteristics. Each group is charged independently according to its specific full charge capacity, allowing cells with different capacities to be fully charged simultaneously without causing overcharging issues. This segmentation enables both reliability and productivity improvements
2Reliability
If all cells are charged to fully charged state to dissolve foreign substances, then potential short circuit defects are prevented, but cells with small full charge capacity may be overcharged
Solution Approach 1:
The patent performs preliminary capacity estimation and classification before the charging process. By knowing each cell's full charge capacity in advance, the system can determine the appropriate charging termination voltage for each cell, ensuring that all cells reach their full charge state for foreign substance dissolution without any cell being overcharged
Solution Approach 2:
The patent applies different charging parameters (termination voltages, charge amounts) to different cells or groups based on their individual full charge capacity characteristics. This localized charging approach ensures that each cell receives the precise charging needed to reach full charge state for defect prevention without exposing it to overcharging conditions
3Measurement precision
If full charge capacity of each cell is estimated accurately, then charging can be optimized, but estimation process takes time and resources
Solution Approach 1:
The patent performs full charge capacity estimation on a sample of cells rather than all cells, or performs estimation at selected charging stages. This partial estimation approach provides sufficient accuracy for effective charging optimization while significantly reducing the time and computational resources required compared to estimating every cell's capacity
Solution Approach 2:
The patent uses electrical characteristics (voltage, current, temperature) measured during normal charging operations to estimate full charge capacity, rather than requiring separate dedicated estimation tests. This copying of measurement data from routine operations enables accurate estimation without additional time loss
Data Source
AI summary
A bipolar LFP battery includes a plurality of cells that are stacked together. A positive electrode active material layer of each of the cells includes lithium iron phosphate. A ΔQ calculation unit calculates an increase ΔQ of a charge amount, during charging of the cell of the bipolar LFP battery. The increase ΔQ is an increase in the charge amount of the cell when the voltage of the cell increases from a first voltage to a second voltage. A full charge capacity estimation unit uses a full charge capacity map to estimate the full charge capacity of the cell, using the increase ΔQ of the charge amount as a parameter.