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This paper proposes an analytical framework describing how initial capacity and resistance variability in parallel-connected battery cells may inflict additional variability or reduce variability while the cells age. We derive closed-form equations for current and SOC imbalance dynamics within a charge or discharge cycle. These dynamics are represented by a first-order equivalent circuit model and validated against experimental data. To demonstrate how current and state of charge (SOC) imbalance leads to cell degradation, we developed a successive update scheme in which the intercycle imbalance dynamics update the intracycle degradation dynamics, and vice versa. Using this framework, we demonstrate that current imbalance can cause convergent degradation trajectories, consistent with previous reports. However, we also demonstrate that different degradation assumptions, such as those associated with SOC imbalance, may cause divergent degradation. We finally highlight the role of different cell chemistries, including different OCV function nonlinearities, on system behavior, and derive analytical bounds on the SOC imbalance using Lyapunov analysis. [ABSTRACT FROM AUTHOR]

In an electric vehicle, a large number of lithium-ion cells are connected in parallel. While cells in parallel increase the reliability of the battery pack, it increases the probability of current imbalance between the parallel branches, thus ageing gradient. The current peak in a cell also can exceed the maximum charge current capability of the cell; leading to lithium plating, therefore a safety issue. The temperature gradient within a battery pack amplifies this issue. This work reports the impact of such temperature gradient on current imbalance within parallel connected cells, their long-term impact on degradation and evaluation of current distribution with degradation. Employing a real-world relevant experimental setup, a total of 1400 cycles were performed on a module with four cells in parallel. A temperature gradient of 10 °C was introduced among the cells, and current in individual parallel branches was measured employing Hall-effect sensors. Over the course of the experiment, module capacity decreased by 23.6%. Cells at higher temperature/lower exposure to active cooling experienced higher degradation. However, the cell with the lowest starting capacity, although exposed to nominal module temperature and cooling experienced the highest current amplitude towards the end of discharge/charge and thus the highest resistance degradation. It was found that current in an individual parallel branch was exceeding the maximum rated charge/discharge current by 53%. [ABSTRACT FROM AUTHOR]

• A pack comprising the electro-thermal coupling of individual cells was developed. • Z shape module shows a significant advantage over a ladder configuration. • The correlation between system inhomogeneity and design parameters was quantified. • Contact resistance has the highest impact on the inhomogeneity of a system. • There is not a strong correlation between system inhomogeneity and C-rate. Cell-to-cell variations can originate from manufacturing inconsistency or poor design of the battery pack/thermal management system. The potential impact of such variations may limit the energy capacity of the pack, which for electric vehicle applications leads to reduced range, increased degradation along with state of health dispersion within a pack. The latter is known to reduce the accessible energy and the overcharging/discharging of some of the cells within a system, which may cause safety concerns. This study investigates the short-term impact of such effects, which is highly important for designing of an energy storage system. A generic pack model comprising individual cell models is developed in Simscape and validated for a 1s-15p module architecture. The results highlight that a number of cells and interconnection resistance values between the cells are the dominant factors for cell-to-cell variation. A Z shape module architecture show a significant advantage over a ladder configuration due to the reduced impact of interconnection resistance on differential current flow within the module. Current imbalance is significantly higher for a ladder system and its magnitude is not dependent on the module current. Capacity variation does not have a significant impact on the system. By increasing the capacity variation from 9% to 40% the current inhomogeneity increases from 4% to 13%, whilst 25% resistance variation leads to 22% current dispersion. Further, a linear relationship is observed between the current inhomogeneity and thermal gradient (Δ T). A 30 °C Δ T leads to 24% current variation within the module. [ABSTRACT FROM AUTHOR]

With the continuous improvement in battery life requirements, the modeling, analysis and management of battery pack life become an important topic in the design of electric vehicles. A more realistic and generic modeling method coupling the electrochemical, thermal, SEI formation model of cells, fluid dynamics and the series-parallel circuit model is proposed to accurately describe the coupling relationships and quantify the inconsistencies, including temperature and current. A life model including capacity fade and reliability is established to evaluate the life of lithium-ion battery pack system (LIBPs). And a model implementation method is established to raise the computational accuracy and efficiency, then, it is validated from the perspective of electrochemical, degradation, thermal performance and coupling effect using experiments. Moreover, the multiphysical behavior and life of LIBPs in different situations are analyzed and predicted, followed by the analysis of coupling effect, inconsistency, reliability and economy. The results show the degradation rate of LIBPs first decreases and then accelerates in the whole life. Furthermore, adding series cells is uneconomical to extend the life of LIBPs. Finally, a SoH balanced management method of changing the airflow direction is proposed to extend the life of LIBPs, and the optimal strategy for different SoH inconsistency are obtained. • A more realistic and generic model of battery pack is developed. • More comprehensive physical models are coupled for more accurate analysis and design. • Stochastic uncertainty and reliability are described by the percentile life model. • Quantitative analyses of coupling effects and inconsistencies have been carried out. • A SoH balanced management method and optimal strategy for battery pack is studied. [ABSTRACT FROM AUTHOR]