A nonlinear equivalent circuit model is developed for lithium ion cells using variable resistors that are functions of the cell temperature. The voltage and thermal characteristics of a commercial cell with a LiFePO4 positive and graphite negative electrode are modeled using a lumped energy balance coupled with the equivalent circuit representation. The cell voltage and temperature during full depth charge–discharge operations can be represented accurately by a one pair RC model. The model is able to represent cell voltage and temperature over a wide range of powers with a global set of parameters. The parameters established, model predictions of the heat generation rates under various conditions are examined. Due to the unique discharge behavior of LiFePO4 positive electrode, the rate of heat generation is constant over most of the state of charge (SOC) window and steeply rises near the end of discharge. A higher heat transfer coefficient of the cooling medium, in addition to lowering the operating voltage, results in higher heat generation. Thus battery cooling systems should be designed to operate at an optimal rate of heat removal considering the operating efficiency, in addition to battery life.