Lithium-ion batteries are currently the most widely technology used for electric mobility. During their service life, batteries can be subjected to high discharge currents, which increase the temperature of the cells. Therefore, it is essential to properly design the battery thermal management system to keep the batteries in the optimal temperature range and to avoid inefficiencies, reduction of life cycles and thermal runaway. These systems require the knowledge of the battery heat generation to be as accurate as possible. The purpose of this work is to suggest a methodology to evaluate the heat generation of batteries during discharge and to compare the thermal behavior of three commercial batteries that are usually adopted in electric vehicles. In particular, LFP, NCA, and NMC batteries were experimentally tested at ambient temperature and under different operating currents, measuring cell voltage and surface temperature. The heat generation was evaluated using a simplified equation and the results were deeply analyzed and discussed. The results show that the NCA cell has the highest heat generation and surface temperature. Also, the ratio between the heat generated and the electrical energy supplied is higher for the NCA cell, while the NMC cell exhibits the lowest value. The NMC cell shows the highest energy efficiency among the batteries under investigation. The mean efficiencies obtained were 0.8, 0.76, and 0.82, respectively, for the LFP, NCA, and NMC cells.