Heterogeneous behavior modeling of a LiFePO4-graphite cell using an equivalent electrical circuit

Abstract

The increasing demand for cost-effective and high-performance batteries makes accurate models essential to improve and control them over their entire life time. Recent studies have pointed out that the heterogeneity of a cell electrical behavior plays an important role in its overall performances. In this paper, an equivalent electrical circuit is built thanks to a physical approach to model the heterogeneous behavior of a commercial LiFePO4-graphite cell. Unlike classical homogeneous models, the parameters of this “multibunch model” do not require to be functions of the state of charge (SoC) to bring accurate results. The equivalent electrical parameters are assumed to follow a distribution law and the latter is determined experimentally for the studied cell. The multibunch model is able to reproduce many features of a cell electrical dynamic: disappearance of the open-circuit voltage (OCV) shape at high current, performances decrease, longer relaxation time, electrical losses during relaxation and high local stress. Thanks to the removal of the parameters dependencies to SoC, the duration of the electrical parameters measurements on the whole operating-range of the battery can be shortened. We estimate that it can be reduced from more than 30 days to about 3 days. Moreover, the determination of the internal resistance distribution could be the basis of a non-invasive tool to characterize the quality of a cell or electrode during its entire lifespan.