Battery Unit Indicative Cells for Accurate SOC Prediction
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Solution Overview
Problem
Secondary batteries, such as lithium iron phosphate systems, face challenges in maintaining accurate state of charge (SOC) prediction due to the large plateau area in their SOC-OCV curve, leading to errors in battery management systems and reduced service life, especially when not in use for long periods.
Innovation Solution
Incorporating an indicative cell within the battery unit that stops charging when it reaches a specified SOC value, allowing the battery management system to accurately determine full charge and prevent errors, while maintaining a smaller discharge cell balance rate to enhance service life and energy density.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the battery unit uses cells with large plateau area in SOC-OCV curve (such as lithium iron phosphate), then the energy density is improved, but the SOC prediction accuracy deteriorates due to accumulated errors in ampere-hour integration method
Solution Approach 1:
The battery unit is divided into two types of cells: working cells (first cell type) and indicative cells (second cell type). The indicative cells are specifically designed with SOC-OCV curves having small plateau areas to provide accurate SOC reference points, while working cells focus on energy storage. This segmentation allows the system to maintain both high energy density and accurate SOC prediction by using indicative cells to correct accumulated errors from ampere-hour integration.
2Duration of action of stationary object
If the battery unit retains capacity in the high-end SOC area to prolong service life, then the service life is improved, but the BMS prediction accuracy for SOC deteriorates due to long-term non-use of high-end area
Solution Approach 1:
Indicative cells serve as intermediaries between the working cells and the BMS. These indicative cells are charged to high SOC levels (95-100%) regularly to maintain accurate SOC prediction capability, while working cells can retain capacity in the high-end SOC area for service life extension. The indicative cells provide reference SOC values that help the BMS accurately predict the SOC of working cells without requiring the working cells themselves to be frequently cycled through the full SOC range.
3Productivity
If the battery unit uses cells with larger discharge cell balance rate (CB value), then the capacity utilization is improved, but the service life (capacity) compensation is reduced
Solution Approach 1:
Different cell types within the battery unit have different discharge cell balance rates optimized for their specific functions. Working cells can use higher CB values (0.8-1.2) for better capacity utilization and energy density, while indicative cells use lower CB values (0.5-0.8) to prioritize service life compensation and accurate SOC indication. This local optimization allows each cell type to operate at its optimal performance point without compromising the overall battery unit performance.
Data Source
AI summary
A battery unit may comprise a first cell type and a second cell type electrically connected at least in series, wherein the first cell type may include N first cells, the second cell type may include M second cells, and N and M are positive integers; the first cell may have a discharge cell balance rate of CB1, the second cell may have a discharge cell balance rate of CB2, with 0.5≤CB1≤CB2≤1.4, and when the battery unit is charged to 95%-100% of the state of charge, the first cell may have a corresponding open-circuit voltage change rate of not greater than 0.005 V/% SOC, and the second cell type may have a corresponding open-circuit voltage change rate greater than that of the first cell.