Abstract: A multifunctional portable energy storage device, particularly, an energy storage device with functions of electric quantity storage, AC and DC charging, electric quantity detection, and DC boost output is provided, which is applicable for AC and DC bidirectional charging. The energy storage device includes an electrical core, a charging interface, and a powering interface. An external power supply charges the electrical core through the charging interface, and the electrical core supplies power to an external device through the powering interface. The energy storage device further includes an AC/DC converter, a control unit, and a charging management unit. The charging interface is connected to an input terminal of the AC/DC converter. An external AC current is converted into a DC current by the AC/DC converter and then input to the charging management unit.