In a lifetime estimating method for a rechargeable lithium battery, the open circuit voltages of the rechargeable lithium battery after discharging for at least two different charge/discharge cycle numbers are detected while charge/discharge cycles go on. Next, at least the two of the voltage values are plotted for respective cycle numbers, and a circular arc passing the plotted points is drawn. Furthermore, the lifetime of the rechargeable lithium battery is estimated based on a size of the circular arc. The progression of deterioration can be suppressed by controlling the charge and discharge of the rechargeable lithium battery based on the lifetime estimation.