Mixed-Chemistry Battery SOC Correction at Low Temperatures
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Solution Overview
Problem
Accurate state-of-charge (SOC) estimation of mixed chemistry batteries, particularly at very low temperatures, is challenging due to differences in capacity retention rates between nickel manganese cobalt (NCM) and lithium iron phosphate (LFP) batteries, with LFP batteries experiencing greater capacity loss and NCM batteries having distinct SOC variations based on open-circuit voltage (OCV) levels.
Innovation Solution
A method that involves obtaining the SOC of both NCM and LFP battery cells using a combination of coulomb counting and Kalman filter methods, along with open-circuit voltage inverse lookup, and updating the SOC based on capacity retention rates obtained from lookup tables specific to each chemistry and temperature, to accurately estimate the SOC and control charging.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If LFP batteries are used in mixed chemistry batteries, then manufacturing cost is reduced, but state-of-charge estimation accuracy deteriorates at low temperatures due to greater capacity loss
Solution Approach 1:
The patent segments the battery system into distinct chemistry groups (NCM and LFP) and applies separate capacity retention rate correction factors to each cell type. The battery management system individually monitors and corrects SOC for LFP cells based on their specific temperature-dependent capacity characteristics, rather than applying a uniform correction to all cells. This segmentation allows accurate SOC estimation for each chemistry type while maintaining the cost benefits of using LFP batteries.
2Power
If NCM batteries are used, then power rating and energy density are improved, but manufacturing cost increases
Solution Approach 1:
The patent applies local quality by assigning different correction characteristics to different battery cell locations and types within the same battery pack. LFP cells receive temperature-based capacity retention corrections while NCM cells use their own correction factors. This allows the system to maintain the high power and energy density benefits of NCM cells in positions where they are most beneficial, while using cost-effective LFP cells in other positions, with each type optimized through chemistry-specific SOC correction.
3Device complexity
If standard SOC estimation methods are used without temperature correction, then device complexity is reduced, but reliability deteriorates at very low temperatures
Solution Approach 1:
The patent implements preliminary action by pre-calculating and storing capacity retention rates for both LFP and NCM battery chemistries across a range of temperatures. These correction factors are determined in advance through testing and stored in lookup tables within the battery management system. During operation, the system simply retrieves the appropriate pre-computed correction factor based on current temperature and applies it to the SOC estimation, avoiding complex real-time calculations while ensuring reliable low-temperature performance.
4Measurement precision
If capacity retention rate correction is applied to all battery cells, then SOC estimation accuracy is improved, but device complexity and computational load increase
Solution Approach 1:
The patent applies partial action by selectively applying capacity retention rate corrections only to LFP battery cells, which exhibit significant temperature-dependent capacity loss, while using standard estimation methods for NCM cells. The battery management system identifies each cell's chemistry type and applies correction only where needed, rather than uniformly to all cells. This partial application of correction maintains high SOC estimation accuracy for the more problematic LFP cells while minimizing the overall complexity and computational burden of the correction system.
Data Source
AI summary
A mixed chemistry battery having a first battery cell having a first chemistry and a second battery cell having a second chemistry that is different than the first chemistry is provided. The first battery cell is connected to the second battery cell in series. The mixed chemistry battery includes a battery monitoring system configured to obtain a first SOC of the first battery cell and a second SOC of the second battery cell and based on a determination that an absolute value of a difference between the first SOC and the second SOC is greater than a threshold value, obtain a first capacity retention rate for the first battery cell and a second capacity retention rate for the second battery cell; and update the second SOC based on the first SOC, the first capacity retention rate, and the second capacity retention rate.