Influence of state of charge inconsistency on electrical performance and thermal runaway characteristics in 2 serials 1 parallel lithium-ion battery pack

Lithium-ion batteries (LIBs) are connected in series during use. State of charge (SOC) difference between two batteries will exist after charging/discharging owing to the capacity or resistance difference between them. This is also seen as SOC inconsistency. SOC inconsistency may cause battery electrical abuse. In this manuscript, a battery testing system, scanning electron microscope, heat furnace, and so on were employed to investigate the influence of SOC inconsistency on electrical performance and thermal runaway (TR) characteristics in 2 serials 1 parallel (2S1P) LIB pack after cycling. The results indicated that the cell with higher SOC in an inconsistent 2S1P pack was overcharged to 4.63 V during first charging, and the cell with lower SOC was over-discharged to 1.90 V during first discharging. The cell with higher SOC became both overcharged and over-discharged gradually with cycles. The cell with lower SOC became neither overcharged nor over-discharged gradually. The electrical performance and TR characteristics of the cell with higher SOC became worse, while that of the cell with lower SOC did not change obviously. The state of health for cells with higher SOC decreased from 100% to 58% after 80 cycles. The time interval between current interrupt device activation and onset temperature of TR for cells with higher SOC was reduced by 183 s, and the maximum surface temperature increased by 34°C.