Study of non-uniform temperature and discharging distribution for lithium ion battery modules in series and parallel connection

Uneven behavior of temperature is always observed among battery modules during charge and discharge. In this paper, an electrochemical-thermal model is established to simulate temperature and discharging distribution in 3 x 3 square lithium-ion battery modules with both series and parallel connection. For series battery arrangement, the final voltage of each cell exhibits small difference after discharge. The final voltage of center battery shows the highest value, while that of battery at the corner shows the lowest values. As discharge rate increases, difference of final voltage between cells increases linearly. For parallel module, energy release is slightly higher than in series, but average temperature and maximum temperature difference are lower than in parallel. The diffusion polarization in electrolyte is found to be the most important factor for unbalanced discharging within modules. Furthermore, as the separation between cells increases in series module, final voltage of center battery gradually increases, and voltages of batteries on the corner continue to decrease. However, the rate of final voltage change of all cells rises more slowly. Moreover, the final temperature of parallel module is always lower than in series, and current difference between cells are almost same when separation between cells is greater than 1.5 mm.