A thermal model considering effects of the state of charge (SOC) and temperature on heat generation is developed for lithium-ion (Li-ion) batteries, which models the ohmic resistance, polarization resistance, and entropy change. This model describes a coupling relationship between heat generation and temperature distribution and can be implemented in real time. Then, the thermal model is applied to predict thermal evolutions of an NCR 18650B Li-ion battery at different ambient temperatures and discharge rates. Extensive experiments are conducted and illustrate that the proposed model can accurately capture the thermal behavior at different ambient temperatures and discharge rates, with an average error of 1.18 degrees C. In addition, comparative studies show that this model significantly outperforms its two counterparts, namely a constant resistance-based thermal model and a model whose heat generation depends on the temperature but not the SOC.