A Study on Parameter Variation of Cells Effects on Battery Groups with Different Topologies and Load Profiles

To satisfy the power and energy requirement of the systems, such as electrical vehicles, the battery packs are constructed with hundreds of single cells connected in series and parallel connection. The most significant difference between a single cell and a battery pack is cell-to-cell variation. Not only does cell-to-cell variation have a big effect on the available energy and power of the battery packs, but also it causes early degradation of battery and potential safety issues. The cell variation effects on battery packs are widely studied because it is of great significance for battery sorting and management scheme. In this paper, battery pack inconsistency is clearly defined and the resulting battery capacity loss and aging acceleration problems are analyzed in detail. A comprehensive LiFePO₄ battery pack model was established, which has taken into account cell-to-cell variation, thermal model, capacity degradation, resistance increasing and different battery topologies. A set of parameters are introduced to study the cell variation and their impacts on battery pack are analyzed through the different working conditions simulation. The battery sorting suggestions are given based on the quantitative result from the simulation. The quantitative results from this research demonstrate that the capacity, self-discharge rate and initial SOC in series battery group, capacity, resistance in parallel connection are the major affecting parameters of cell-to-cell variation, which leads to significant aging acceleration and capacity loss of the battery pack. In the meanwhile, it has been pointed that dynamic load profile caused faster cycling aging and more serious self-balancing compared with the constant load profile.