The ability of Li-ion batteries to deliver high power and energy is accompanied by heat generation and thermal runaway resulting in detrimental effects. While capacity fade and the consequential reduced cycle life are prime issues in Li-ion batteries, the impending need for safety in handling them and need for safe failure in off-nominal conditions are critically important. In this contribution, we report heat generation in Li-ion batteries containing cathodes such as NCA (LiNi0.8Co0.15Al0.05O2), NMCs (LiNi0.2Mn0.2Co0.6O2; LiNi0.6Mn0.2Co0.2O2) against graphite anode. The total heat generation is determined using an Accelerating Rate Calorimeter and the reversible heat generation is determined using entropic coefficient measurement. The internal resistance contributing to the irreversible heat generation is estimated by AC impedance studies. Further, variation in the components of internal resistance across different States of Charge determined by fitting an equivalent circuit model is presented. The cell skin temperature recorded at low C rates in 18650-type full cells is correlated with the entropic coefficient in corresponding half-cells to understand thermodynamics during phase transition in cathodes and structural transformation in graphite. The findings from the experiments are helpful in the electrochemical and thermal modeling of Li-ion batteries which in turn would aid better design of thermal management systems. (C) 2017 The Electrochemical Society. All rights reserved.