The objective of the present invention is to provide a high energy density power storage system in which the deterioration of cycle characteristics has been suppressed. The power storage system is equipped with a battery pack and is characterized in that the energy density is 35 Wh/L or greater, and constant current charging is performed at a low rate current value of 0.2 It or lower, continuously from an initial charging stage to charging completion. Even if the high energy density power storage system has a high heat generation density per unit volume and the temperature of the battery pack or the batteries inside the battery pack is prone to increase, deterioration of cycle characteristics can be suppressed.