An electrical energy storage device includes a casing, an input terminal set, an output terminal set, a protection circuit board, and an electricity conducting mechanism. At least one electrical energy storage unit is disposed inside the casing. The input terminal set is disposed on a side of the casing, and the output terminal set is disposed on the other side of the casing to connect with the input terminal set of another electrical energy storage device. The protection circuit board is electrically connected to the power storage unit. Two ends of the electricity conducting mechanism are respectively connected with the input terminal set and the output terminal set, and the input terminal set and the output terminal set are connected with the electrical energy storage unit via the electricity conducting mechanism and the protection circuit board.