Heat generation due to electrochemical side-reaction during high-rate pulse cycling has been studied. In order to measure the value, calorimetry was carried out. The excess heat was observed comparing with the electric energy loss for each cycle, which was thought to be caused by side-reactions. Some side-reactions affect the surface characteristics of the electrodes and enlarge the impedance of the battery resulting in more heat generation. (c) 2005 Elsevier B.V. All rights reserved.