Current distribution in parallel-connected battery cells

The load in parallel-connected battery cells splits depending on the parameters of the system and cells. A symmetric distribution only occurs when all values are identical, which is never the case in reality. This study investigated the current distribution caused by internal cell parameters by means of simulation and measurement. A model simulated the cell voltages and calculated the current distribution. A measuring setup was used to acquire the cell currents of a parallel connection of two individually tempered cells.

Asymmetric current comes from voltage differences in the cells. Cell voltage consists of the equilibrium voltage and the voltage drop at the internal impedance. Furthermore, the hysteresis of the equilibrium voltage and entropy influence the potential. Temperature differences—and with them impedance differences—are some of the most crucial drivers of asymmetric current distribution. The temperature dependence of the impedance of cells plays a vital role in the current distribution. In cells with flat open-circuit voltage, hysteresis and entropy can lead to differences in the state of charge in a double-digit percentage range. Strong asymmetry appeared especially at steps of the total current. At the end of the charging process of two parallel-connected cells with a temperature difference, the cold cell took the major part of the current. This led to accelerated ageing caused by lithium plating. An ageing experiment was used to confirm this hypothesis.

To prevent asymmetric current distribution, it is a good idea to consider homogeneity in the system and cell design. Critical effects such as those of nonlinear open-circuit voltages can be eliminated by controlling the load profile. Therefore, the results of this work can help to develop intelligent and gentle fast charging strategies for lithium-ion batteries.