Lithium-Ion Batteries: Thermal Behaviour Investigation of Unbalanced Modules

In this paper, the thermal behaviour of an unbalanced battery module made of large lithium iron phosphate cylindrical cells of 18 Ah nominal capacity is investigated during its discharge with 18 A, 54 A and 90 A currents. For this study, several cells were assigned in the module to 5%, 10% and 20% initial depth of discharge (DoD). The thermal management of the cells in the module is achieved based on a forced air cooling. The computations of the temperature distribution inside the cells and the module are performed with a two-dimensional modelling approach. During the discharges, the cells with a non-zero initial DoD show a more pronounced temperature increase up to 5% compared to the reference cells. In the end of the three discharges, the highest temperatures 26.5 °C (18 A), 29.6 °C (54 A) and 32.3 °C (90 A), respectively, are reached for the cells with the highest initial DoD. For these cells, with the increase in the current rate, the highest amount of total heat exceeds 25 W. The reactive and the active heats are found to be the main contributors to the total heat generated by the cells. This study provides the effect of the initial DoD of the cells on the temperature development occurring in an unbalanced battery module during its discharge.