Analysis of parallel connected lithium-ion cells imbalanced performance based on electrothermal modelling environment

Typical battery management systems (BMS) do not provide the opportunity of monitoring each parallel string cell individually, rather considering cells operate at common voltage and current levels. In reality, manufacturing induced characteristics imbalances, pack thermal gradients and interconnection resistances impose uneven states among cells. As such, this paper intends to take advantage of common Equivalent Circuit Modelling (ECM) approaches to investigate the incurring interrelationships among 15 cells connected in parallel. The module is discharged under real-world drive cycle conditions in different scenarios, aligning model parameters variation to literature based evidence to accurately replicate their impact on imbalanced performance. The simulation results indicate a 1% ratio between interconnection resistance and internal resistance has the largest influence on cell-to-cell load current imbalance and thermal gradient among the investigated factors. Second, manufacturing related 25% internal resistance variability and poor cooling system design induced thermal gradient caused non-negligible imbalances in cells' performance. A 9% cell-to-cell variation of capacity, contrarily, emerged as having a limited impact on pack behaviour, although constant current loads could imply different conclusions. Last, between investigated Z and Ladder pack configurations, the first choice guaranteed a more uniform performance. This work draws attention to the fact that the presence of imbalances not captured by the BMS can have a detrimental impact on the performance of different pack areas. Hence, the developed Activate® ECM pack model will be coupled in future work with HyperStudy® optimization tool to perform sensitivity analysis. This is expected to improve and accelerate the pack design process ensuring diagnostics and prevention of protracting imbalances, lowering pack safety and degradation concerns.