On Accessible Capacity of Parallel Assembly of Lithium-Ion Batteries

A rechargeable battery (RB) is a parallel-sequential assembly of cells, the actual capacity and changes of which are specific and determined by the nominal parameters of the cell, the structure of the assembly and the algorithms of the control system (BMS). The article considers the problem of determining the statistical patterns of the actual capacity and its dynamics during cycling for a battery module of the LiFePO₄ chemical system with a 16s structure and a parallel pair of such modules. The operation of the corresponding RBs is modeled depending on the initial load variation of the component cells, the balancing characteristics of the latter and other parameters. The effect of disrupting the balancing of sequential assembly cells at a low voltage threshold for the start of balancing has been discovered, and its nature has been explained. Statistical mode- ling of the charging and discharging cycles of battery modules connected in parallel has been carried out. The values of the imbalance of charging and discharging currents of parallel modules and the dispersion of this imbalance are found. Thus, for the considered cell types, there are areas of charge degree (SOC) values in which the charging and discharging currents are compared regardless of the initial imbalance of the cells in the modules. The general trends of reaching the maximum accessible capacity of such systems at different values of the initial cell imbalance are shown. The work is methodical in its nature, and it demonstrates the influence of the statistical variation of cell parameters on the operational parameters of batteries, primarily the accessible operating capacity and the values of the charging and discharging currents of parallel modules. The quantitative results can be used to better understand the processes of balancing and operation of cells in series-parallel assemblies, to optimize BMS operation, and to predict the accessible capacity of lithium-ion cell assemblies.

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