LFP Cell Degradation Detection Using Discharge OCV Capacity Tracking

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Solution Overview

Problem

Conventional methods for detecting the full charge capacity of LFP cells, such as the OCV method, face challenges due to an SOC region where OCV variation is minimal, making it difficult to accurately determine the degradation of LFP cells.

Innovation Solution

A degradation determination device and method that involves charging and discharging the LFP cell to a fully charged state, acquiring an integrated current value during discharge until a predetermined OCV is reached, calculating the full charge capacity by adding a variable battery capacity to a common battery capacity, and determining degradation based on the initial full charge capacity.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If the OCV method is used to detect full charge capacity, then the detection process is simple, but the measurement precision deteriorates in LFP cells due to minimal OCV variation in certain SOC regions

Engineering Contradiction:

Improvedetection process complexityVSAvoidfull charge capacity detection precision

Core Design Contradiction:

Device complexityVSMeasurement precision

Solution Approach 1:

The patent changes the detection parameter from OCV (open circuit voltage) to ODP (open discharge potential). By detecting the ODP during discharge instead of OCV during charge, the method avoids the flat voltage region problem in LFP cells, achieving both simplicity and precision

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

Instead of detecting capacity during charge (OCV method), the patent inverts the approach by detecting capacity during discharge (ODP method). This inversion allows accurate detection in LFP cells where charge voltage remains flat

Inventive Principle:

Principle #13The other way round (Inversion)

2Ease of operation

If conventional OCV detection is used for LFP cells, then the detection method is straightforward, but the reliability of degradation determination deteriorates due to inability to accurately detect full charge capacity

Engineering Contradiction:

Improvedetection method simplicityVSAvoiddegradation determination reliability

Core Design Contradiction:

Ease of operationVSReliability

Solution Approach 1:

The patent changes the detection parameter from OCV to ODP, enabling reliable degradation determination for LFP cells by accurately detecting full charge capacity through discharge potential rather than charge voltage

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent substitutes the OCV detection mechanism with an ODP detection mechanism, replacing a method that fails for LFP cells with one that is specifically suited to their electrochemical characteristics

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Data Source

PatentUS20240272234A1Degradation determination method for LFP cell, degradation determination device, and vehicle including the device

Publication Date: 2024.08.15 TOYOTA JIDOSHA KK

AI summary

The controllers perform a series of charge/discharge processes in which LFP cell is charged to a full state of charge by the charger/discharger, LFP cell in a full state of charge is discharged by the charger/discharger until OCV of LFP cell reaches a predetermined OCV of LFP cell that uniquely represents the common capacity before and after the degradation of LFP cell, and the current integrated value of the discharge current is acquired during the discharge process. The controllers perform the charge/discharge process one or more times to obtain the variation capacity of LFP cell based on the one or more current integrated values obtained thereby. The controllers add the variation capacity to the common capacity, calculate the full charge capacity of LFP cell, and determine the degradation of LFP cell based on the calculated full charge capacity of LFP cell and the initial full charge capacity of LFP cell.