Thermochemical properties of a lithium secondary battery are calorimetrically studied. A twin-type heat conduction calorimeter was used and its accuracy was determined by the specific heat capacity measurement of a standard material, synthetic sapphire. The inaccuracy was smaller than 1.1% in the range of 303-343 K. The thermal transitions corresponding to crystal phase transitions of the positive electrode material, Li1-xCoO2. between hexagonal and monoclinic symmetry were observed. An attempt was made to measure the apparent specific heat capacity of the battery and to study its dependence on the state of charge. The influences of the diffusion of lithium ions in the electrode materials and the self-discharge to the apparent specific heat capacity are discussed.