Based on the porous electrode and concentrated solution theory, a thermal model is developed for lithium ion battery pack. The accuracy of predicted battery temperatures is validated by charge–discharge cycling experiments under natural and forced convection conditions. The heat generation and dissipation rates of battery under different conditions are simulated by the proposed model and the results indicate that: (1) the SOC change has a significant effect on the reversible heat generation rate but has almost no influence on the irreversible heat generation rate; (2) the generation rates of reversible and irreversible heat during charge are almost equal to that during discharge with the same SOC and current rate, but the effect of reversible heat on battery temperature is opposite; (3) for enhancing heat dissipation with a given input power of cooling fan, there always exists an optimum value for the resistance coefficient of battery pack, and the optimal coefficient is increased when the input power of fan increases. In addition, the comparisons between the predicted and measured battery temperature indicate that, the reversible heat has significant influence on battery temperature during continuous charge and discharge, especially under low current rate, but the influence can't be observed during charge–discharge cycles.