In the charged condition, batteries are in a state of high energy relative to that of the system in the discharged state. Hence, there is a 'driving force', corresponding to the free energy of discharge, which tends to spontaneously diminish the charge if some mechanisms for self-discharge exist. We determined the self-discharge of Li/LiCoO2 cells from the decline of their open-circuit voltage and the rate of loss of the discharge capacity. In addition, we studied ac impedance of Li/LiCoO2 cells. Prevention of self-discharge is especially important for munitions that have remained in a state of disuse. In the case of nanocrystalline LiCoO2, cycle performance is superior to coarse-grained LiCoO2. However, self-discharge performance is inferior to coarse-grained LiCoO2. A better cycle performance of up to 200 cycles might be due to a well-formed SEI layer and smaller particle size in nanocrystalline LiCoO2. (C) 2004 Elsevier B.V. All rights reserved.