The performance of lithium-ion cells, as determined from in-house testing, is primarily a function of cell design/materials, charge/discharge rate, ambient temperature, and the number of charge/discharge cycles. Testing of lithium-ion 18650 cells was performed in order to characterize their behavior and to eventually predict the performance of lithium-ion cells of various sizes. AC impedance spectroscopy was used to determine the interfacial resistance of the lithium-ion cells as a function of temperature, state-of-charge, and cycle number. From these results, a nonisothermal mathematical model was developed and preliminary results are presented.