An energy storage device includes a battery with at least one battery cell and two poles, two connection points each connected to battery pole, for connecting to an external current circuit for charging and discharging the battery, a battery charge state monitoring device, an additional energy storage element different than the battery cell, a connection circuit for connecting the additional energy storage element to at least one battery pole, and at least one connection point. The connection circuit is designed such that a specified energy storage element current having a specified relationship with the total current flowing through the energy storage device is charged into and/or discharged from the energy storage element. A voltage measuring device contacts the additional energy storage element to measure an energy storage voltage, and the charge state monitoring device determines the charge state of the battery based at least on the energy storage voltage.