Abstract: An energy storage device, a method for an energy storage device to determine whether a converter device connected thereto has a neutral line, and a power supply system are provided. The energy storage device includes a connection terminal, a neutral line switch, an energy storage unit and a control unit. The control unit is adapted to: obtain, when the energy storage device is in a shutdown state and the converter device is in a state of charging the energy storage device, two port voltages of the energy storage device when a preset condition is satisfied, and determine whether a change of a voltage difference between the two port voltages is within a preset range when the preset condition is satisfied. If the result is yes, the neutral line switch is controlled to be switched on; otherwise, the neutral line switch is controlled to be switched off.