Assembled Battery SOC Equalization via Bypass Discharge
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Solution Overview
Problem
In assembled batteries used in plug-in hybrid vehicles, variations in the state of charge (SOC) of individual secondary cells lead to accelerated degradation due to uneven charging and discharging, making it difficult to measure SOC accurately over a wide range, especially when partial charging/discharging is repeated.
Innovation Solution
The implementation of a system that calculates voltage change rates and SOC for each secondary cell, allowing for bypass discharging to equalize cell charges, thereby eliminating SOC variations and suppressing battery performance degradation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If charging and discharging are repeated in the assembled battery over a wide SOC range, then the battery capacity utilization is improved, but variations in the SOCs of the secondary cells increase and battery performance degrades
Solution Approach 1:
The patent introduces a bypass discharge control mechanism that dynamically adjusts discharge parameters based on SOC variations among secondary cells. When SOC difference exceeds a threshold, the control unit activates bypass discharge for cells with higher SOC, modifying the discharge parameters (current, time) to equalize SOCs across all cells, thus preventing performance degradation while maintaining high capacity utilization
Solution Approach 2:
The patent implements a feedback control system that continuously monitors SOC of each secondary cell and adjusts bypass discharge operations accordingly. The control unit receives SOC information, compares it with reference values, and dynamically controls the bypass discharge process to maintain SOC equality, creating a closed-loop control that prevents degradation while maximizing battery utilization
2Measurement precision
If the SOC sensing method based on negative electrode voltage change is used, then the measurement can be performed, but the sensing is difficult in the SOC region where partial charging/discharging is repeated and voltage change is small
Solution Approach 1:
The patent uses the positive electrode as an intermediary for SOC measurement. Instead of directly measuring the difficult-to-detect voltage change in the negative electrode, the system measures the open circuit voltage of the positive electrode, which exhibits clearer voltage-SOC relationship characteristics, especially in the partial charging/discharging region. This intermediary measurement approach overcomes the sensing difficulty
Solution Approach 2:
The patent changes the measurement parameter from negative electrode voltage change to positive electrode open circuit voltage. By utilizing the positive electrode's voltage characteristics, which show more pronounced changes in the partial charging/discharging SOC region, the system achieves accurate SOC measurement where the original method failed
3Measurement precision
If charging is performed on the basis of the lowest SOC among secondary cells, then the battery can be fully charged, but the other secondary cells are overcharged and their SOCs exceed 100%
Solution Approach 1:
The patent changes the charging control parameter from a single threshold (100% SOC) to a differential threshold based on SOC difference among cells. The control unit compares each cell's SOC with the minimum SOC and activates bypass discharge when the difference exceeds a predetermined threshold, preventing overcharging while ensuring complete charging of all cells
Solution Approach 2:
Instead of only controlling charge to prevent overcharging, the patent introduces bypass discharge as a complementary mechanism. When SOC differences arise during charging, the system actively discharges higher-SOC cells through bypass paths, inverting the traditional unidirectional charge control into a bidirectional charge-discharge management system
Data Source
AI summary
An assembled battery includes a plurality of secondary cells, a first arithmetic operator, a second arithmetic operator, and a discharger. The first arithmetic operator calculates a voltage change rate of each of the secondary cells during charging/discharging thereof. The second arithmetic operator determines a discharge condition for each of the secondary cells from the voltage change rate calculated in the first arithmetic operator and a state of charge of each of the secondary cells. The discharger performs discharging of each of the secondary cells on the basis of the discharge condition when the assembled battery is neither being charged nor being discharged. The second arithmetic operator determines the discharge condition of each of the secondary cells such that each of the secondary cells after the discharging has the same state of charge.