Despite of the numerous research on thermal-runaway in valve regulated lead-acid batteries, its exact cause is not well known yet and it is not clear which physical phenomena contribute to thermal rise. To have a better understanding, the main sources of heat generation in lead-acid batteries are studied using the governing equations of battery dynamics derived in Part I. The governing equations including the conservation of energy are applied to different electrochemical reactions that take place during the overcharge of the battery. The reversible and irreversible heat sources of each reaction are determined separately. To investigate the heat generation contribution of each reaction, the governing equations are solved numerically using a two-dimensional thermal-electrochemical model. In this investigation, each term in the energy equation is calculated quantitatively in order to realize its contribution to heat generation. The numerical results show that beside Joule heating, only irreversibility contributes to heat generation which in turn results into temperature rise and the reversible part has no contribution. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.030302jes] All rights reserved.