A battery charging method is provided for extending life of batteries. The method includes providing an appropriate charge-off voltage with respect to variation in both of a remaining capacity and an idle time of the battery. Further, the charge-off voltage may vary according to the remaining capacity and the idle time of the battery, so as to increase charging efficiency of the battery. Additionally, the present invention also provides adjusting a charge-off current to a value according to the variation in an actual capacity of the battery.