26650 ternary lithium-ion batteries with LiNiCoMnO2 (NCM) as the cathode material have been widely adopted in electric vehicles due to their excellent electrochemical performance and low cost. To investigate the effects of ambient temperature and depth of discharge (DOD) on the overdischarge behavior of these batteries, this paper performs discharge tests at 100%, 110%, 120%, and 130% DOD under normal, low, and high ambient temperatures, followed by impedance analysis before and after the tests. The results show that at low ambient temperatures, the onset of internal short circuits is delayed. During discharge at 110%—130% DOD, severe voltage oscillations may occur, accompanied by an extended time to reach peak temperature and an increased overall temperature rise rate. At high ambient temperatures, the occurrence of internal short circuits is also delayed, while the time required for the battery to reach its maximum temperature is prolonged. Impedance test results indicate that the change in internal resistance is significantly reduced after overdischarge at low ambient temperatures. After 110%, 120%, and 130% DOD overdischarge at high ambient temperatures, the rate of change in internal resistance is higher than that at room temperature. This study reveals the failure mechanisms of 26650 NCM lithium-ion batteries under overdischarge conditions at different ambient temperatures and DOD levels, providing critical support for setting early warning parameters for overdischarge faults and developing preventive strategies against thermal runaway induced by over-discharge.
