Numerous of lithium ion battery fires and explosions enhance the need of precise risk assessments on batteries. In the current study, 18650 lithium ion batteries at different states of charge are tested using a cone calorimeter to study the burning behaviors under an incident heat flux of 50 kW m−2. Several parameters are measured, including mass loss rate, time to ignition, time to explosion, heat release rate (HRR), the surface temperature and concentration of toxic gases. Although small quantities of oxygen are released from the lithium ion battery during burning, it is estimated that the energy, consuming oxygen released from the lithium ion battery, accounts for less than 13% of total energy released by a fully charged lithium ion battery. The experimental results show that the peak HRR and concentration of toxic gases rise with the increasing the states of charge, whereas the time to ignition and time to explosion decrease. The test results of the fully charged lithium ion batteries at three different incident heat fluxes show that the peak HRR increases from 6.2 to 9.1 kW and the maximum surface temperature increases from 662 to 934 °C as the incident heat flux increases from 30 to 60 kW m−2.