Abstract: Lithium ion batteries have become a hotspot in the field of electric vehicles due to their excellent performance of high specific energy, long lifespan, high output power, and wide range of charge-discharge rates. The thermal behavior of the batteries during the charge-discharge cycle has a large impact on their performance, life cycle and safety. In this work, an electrochemical-thermal model is established based on a 21700 type cylindrical lithium ion battery, and corresponding experiments are conducted to validate the model. The thermal behavior during the charge process is analyzed, including the temperature and heat generation distribution. The results show that temperature rise in the charging process is mainly affected by the constant current charging process, the average temperature and heat generation of the battery both exhibit a trend of ascending first and descending later, and the irreversible heat shares a larger proportion of the total heat generation. The thermal behavior is contrasted between the charge and discharge process, and it is found that compared with the discharge process, the thermal behavior at the charge process has the characteristics of quick temperature change and high heat generation peaks.