Battery Management System Pause State Discharge for SOC Estimation
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Solution Overview
Problem
Energy storage devices using lithium iron phosphate with a conductive layer face challenges in accurately estimating their charge state due to hysteresis characteristics, where the correlation between voltage and charge state differs between charge and discharge states, leading to inaccurate SOC estimation.
Innovation Solution
A management system that includes current and voltage measurement units, which defines a pause state based on current values and executes a discharge process followed by post-discharge voltage measurement to estimate the charge state using correlations specific to the pause state after discharge, allowing for accurate SOC estimation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conductive layer such as carbon is formed on the surface of lithium iron phosphate to improve conductivity, then electrical conductivity is improved, but accurate charge state estimation becomes difficult due to hysteresis characteristics
Solution Approach 1:
The system performs a preliminary discharge action before measurement to transition the energy storage device from an unknown state to a known pause state after discharge. This preliminary action eliminates the hysteresis effect by ensuring the device is in a standardized state before charge state estimation is performed, thereby enabling accurate measurement despite the presence of a conductive layer
Solution Approach 2:
The system changes the operational state parameter of the energy storage device by transitioning it to a pause state after discharge. This parameter change (from arbitrary state to standardized pause state) allows the voltage-charge state correlation to become predictable and accurate, resolving the measurement precision issue caused by hysteresis characteristics
2Loss of time
If voltage measurement is performed directly without discharge to estimate charge state, then measurement time is reduced, but estimation accuracy deteriorates due to hysteresis characteristics in devices with conductive layers
Solution Approach 1:
A preliminary discharge action is performed before voltage measurement to eliminate hysteresis effects. This ensures the energy storage device is in a standardized pause state after discharge, making the voltage-charge state correlation accurate and reliable for subsequent measurement
Solution Approach 2:
The preliminary discharge acts as an anti-action to counteract the hysteresis effect that would otherwise prevent accurate measurement. By performing this counter-action first, the system eliminates the source of measurement error before the actual charge state estimation is performed
Data Source
AI summary
Provided is a management device (BMS 40) for an assembled battery 30 which uses a positive electrode active material containing lithium iron phosphate and having an electrically conductive layer formed on a surface thereof, the BMS 40 being provided with: a current sensor 43 which measures a current flowing through the assembled battery 30; a voltage sensor 45 which measures a voltage of the assembled battery 30; and a management unit 42. When a state in which the current value measured by means of the current sensor 43 is less than a reference value is defined as a pause state of the assembled battery 30, the management unit 42, when the assembled battery 30 has entered the pause state, performs: a discharge process (S101) for causing the assembled battery 30 to discharge; a post-discharge measurement process (S102) for measuring the voltage by means of the voltage sensor 45 after the assembled battery 30 has been discharged in the discharge process; and a first estimation process (S103) of estimating the SOC of the assembled battery 30 on the basis of the voltage measured in the post-discharge measurement process and OCV-SOC characteristics 62 after the discharge.