NOVELTY - The bag (100) has a compartment for retaining batteries (110, 120) that delivers direct current (DC). A battery connection interface (130) connects the batteries, and a conversion unit is connected to the battery interface for converting the DC delivered by the batteries into an alternating current (AC). A user connection interface (150) is fed with the AC by the conversion unit and connected with an electrical appliance for supplying the AC and power of 1500 watts. The battery and user interfaces are arranged in an upper part of the compartment, where the bag comprises straps (160).    USE - Back bag for transporting batteries such as heavy modular batteries i.e. lithium-ion batteries, that are utilized for supplying power to a vehicle (all claimed) e.g. electric car, hybrid car and motor car, and an electrical appliance e.g. lawn mower and power tool.    ADVANTAGE - The bag enables easy transportation of the batteries in the vehicle and easy utilization of the batteries for supplying the electric power to the electrical appliance. The bag avoids utilization of a separate battery for supplying the power to the vehicle. The conversion unit allows charging of the battery, so that the battery is completely managed. The straps in the bag is installed or removed according to a wish of the user. The user connection interface is directly fixed on the battery connection interface, thus reducing complexity of connection between the interfaces. The battery and user interfaces are arranged in the upper part of the compartment, so that the interfaces are prevented from being in contact with water, thus avoiding creation of electric problems. The user connection interface supplies the power of 1500 watts to the electrical appliance, thus enabling transportation of powerful batteries.    DESCRIPTION OF DRAWING(S) - The drawing shows an isometric side view of a back bag.    Back bag (100)    Batteries (110, 120)    Battery connection interface (130)    User connection interface (150)    Straps (160)