EV Battery Pack OCV Verification for Faster SOC Estimation
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Solution Overview
Problem
Existing battery management systems in electrified vehicles face challenges in accurately estimating open circuit voltage (OCV) due to varying equilibrium times of battery cells, especially at colder temperatures, which affects state of charge (SOC) estimation and computational demands, particularly in high-voltage systems with numerous cells.
Innovation Solution
A vehicle system estimates OCV by grouping battery cells with similar voltage measurements, using a group OCV to determine cell OCV, and verifies the accuracy of the estimation by comparing pre- and post-activation voltage measurements within a threshold, reducing computational load and improving SOC estimation accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If battery OCV is estimated by measuring voltages for a long duration to ensure equilibrium, then measurement precision improves, but loss of time increases
Solution Approach 1:
The system performs preliminary voltage measurements during vehicle operation before full equilibrium is reached. These preliminary measurements are used to predict OCV and SOC values in advance, allowing the system to have estimation data ready before the vehicle is activated, thus reducing the waiting time for accurate OCV determination.
Solution Approach 2:
The system uses partial voltage measurements taken during non-equilibrium periods combined with correction factors. Instead of waiting for complete equilibrium, it uses available partial data with algorithms that compensate for the non-equilibrium state, achieving sufficient accuracy without the full equilibrium duration.
2Measurement precision
If individual cell OCV is measured for each battery cell to improve SOC estimation accuracy, then measurement precision improves, but device complexity increases
Solution Approach 1:
The battery pack is divided into multiple cell groups, with representative cells selected from each group. Instead of processing all individual cell measurements, the system segments the battery pack into manageable groups and uses measurements from key representative cells to infer the state of entire groups, significantly reducing computational complexity while maintaining accuracy.
Solution Approach 2:
The system merges measurements from multiple cells within each group to determine a representative OCV value for that group. By combining data from representative cells and using group-level analysis, the system reduces the number of individual cell calculations needed while preserving the essential information for accurate SOC estimation.
3Reliability
If OCV estimation uses voltages measured after vehicle activation, then reliability improves, but loss of time increases due to contactor closing delay
Solution Approach 1:
The system performs OCV estimation using voltage measurements taken before vehicle activation (during vehicle off state). By completing the estimation process in advance while the vehicle is off, the system has reliable OCV data ready when the vehicle starts, eliminating the delay that would otherwise be required to wait for post-activation measurements.
Data Source
AI summary
A system for an electrified vehicle (EV) having a battery pack includes a vehicle controller configured to charge and discharge the battery pack according to power limits defined at activation of the EV by a first open circuit voltage (OCV) estimated after a last deactivation of the EV based on voltages measured for a first duration after the last deactivation in response to the first OCV being within an OCV estimation threshold of a second OCV estimated after activation of the EV based on at least a portion of the voltage measured for the first duration and voltages measured prior to a contactor closing to activate the EV.