Capacity degradation due to charge/discharge cycles is suppressed in either a non-aqueous electrolyte secondary cell provided with a positive electrode including, as a positive electrode active material, a lithium-transition metal complex oxide having a layered structure and containing at least Ni and Mn as transition metals, and a negative electrode containing a carbon material as a negative electrode active material and having a higher initial charge-discharge efficiency than that of the positive electrode, or an assembled battery having a plurality of cells each of which is the secondary cell. A control circuit incorporated in the secondary cell or the assembled battery, or in an apparatus using the secondary cell or the assembled battery, monitors the voltage of the secondary cell or each of the cells in the assembled battery so that the end-of-discharge voltage of each cell is 2.9 V or higher.