The overpotential resistance and the entropy change of two small lithium-ion secondary batteries, which are the heat source terms to increase the battery temperature, have been measured by several methods, changing the battery temperature and the state of charge. The temperature increase and the total heat generation rate of the batteries were calculated during the discharge cycle by using the measured resistance and entropy, being compared with the experimental results of the temperature increase and the total heat generation rate. The overpotential resistance was estimated by four measurement methods, i.e., the battery voltage-current characteristics during the constant current discharge, the difference between the open-circuit voltage and the cell voltage, the voltage change during the intermittent discharge for 60 s, and the ac impedance measurement. The overpotential resistance by the voltage-current characteristics is almost the same as by the difference between the open-circuit voltage and the cell voltage. However, in some cases the resistances by the intermittent discharge and the ac impedance are smaller than the former two resistances. The entropy change DeltaS measured by the temperature change of the open-circuit voltage agrees almost with the DeltaS measured by the heat production difference between the charge and discharge cycle. The temperature increases and the total heat generation rates estimated from the overpotential resistances by the voltage-current characteristics and DeltaS by the temperature change of open-circuit voltage agree well with the measured ones for the two batteries during the constant current discharge. (C) 2003 The Electrochemical Society.