Storage device for a high voltage electrical power including at least two modules connected in series, each module including at least one dc power-source in a container with positive and negative terminals. The device including positive and negative poles, first and second conductors arranged to form a current path between the poles, the first conductor including a plurality of conductor parts connected to the terminals of the modules to provide a series connection, a first module of the series being connected to one of the poles, and the second conductor is connected between a last module in the series and the other pole, wherein the conductors are arranged to pass through the containers in parallel such that a current flows through the first conductor in a first direction and through the second conductor in an opposite second direction.