Abstract: This paper aims at studying thermal safety of cell and battery module under different states of charges (SOC) and different overcharge rates. To study thermal safety of lithium-ion batteries, domestic 5 A·h cells and self-made battery modules were tested. Results show that the maximum temperature, the maximum temperature change rate and heat production increase gradually in pace with the rise of SOC, which result in the lower thermal safety. The temperature of the battery overcharge test can be divided into four stages:the first stage is the temperature stable zone, the second stage is the temperature slow rise area, the third stage is the temperature abrupt zone and the fourth stage is the cooling zone. Moreover, the open circuit voltage of the battery dropped significantly before the battery came into thermal runaway in the overcharge tests, which indicated that the open circuit voltage drop of battery can be regarded as one of the signs for thermal runaway. The time of thermal runaway in the module is later than the cell, and the highest temperature of thermal runaway is lower than that of the cell. However, the temperature of the cell decreases rapidly after the thermal runaway, and the temperature of individual cells in the module rises slowly until the whole module is out of control. Although self-made battery module did not come into thermal runaway in the overcharge test, a little electrolyte inside was ejected through the vent. And it may lead to the external short circuit.