Effect of the number of parallel batteries on thermal runaway evolution in LiFePO4 battery

With the increasing demand for longer drive range, lithium-ion batteries (LIBs) are connected in parallel and in series to meet the power requirement of electric vehicles. In contrast to series connection, the presence of parallel connection may exacerbate thermal runaway (TR) issues of LIBs owing to the possible electricity transfer between batteries. However, the complex electricity and heat interactions between parallel-connected LIBs challenge the in-depth understanding of the effects of parallel connection on TR evolution. In this study, detailed effects of the number of parallel-connected batteries on TR evolution mechanisms are investigated by removing the heat conduction between batteries. Differing from the conventional belief that the electricity transfer is interrupted when the electrochemical system inside battery is damaged in the process of TR, it has been observed that the continuous electricity transfer occurs in the batteries connected in parallel with more than two units. Increasing the number of parallel-connected batteries facilitates the occurrence of continuous electricity transfer. The occurrence of TR is significantly advanced and the corresponding onset temperature decreases from more than 200 °C to less than 180 °C when the number of parallel batteries exceeds two, and the transferred electrical energy between batteries is determined as the dominant cause of the advanced TR. Particularly, parallel-connected batteries with more numbers exhibit a higher risk of fire during TR because of the ignition role of transferred electrical energy. This work reveals the detailed effects of the number of parallel batteries on TR evolution and triggering mechanisms, which contributes to sufficient evidence for reliable early warning and safety design of energy systems containing parallel-connected batteries.