In lithium-ion cells, heat is generated via joules losses and exothermic electro-chemical reactions, causing cells to self-heat and risk premature aging or catastrophic failure. However, heat generation is not constant throughout the cells discharge, due to the complex chemical reactions and changes in the micro structure of the active materials of the electrodes. Nonlinear or complex systems thermodynamic behavior is commonly represented by efficiency maps, generated from empirical data. These empirical models are used to estimate a system heat generation at a given operating state. Although very common in many fields of engineering, efficiency maps are yet to be used for lithium-ion batteries. This paper presents a simple method to achieve accurate estimation of thermodynamic efficiency over the full range of use. In this method, the thermodynamic losses are assumed to be proportional to the measured ohmic resistance of the cell. This empirical method is intended to be a fast and reliable characterization tool that can replace other modeling methods, while capturing the non-linear nature of lithium-ion cells. It can be used for energy management and assessment purposes, for battery design or for thermal management system sizing and control.