Power supply unit of a vessel, the vessel having a battery room, the length of which extends along an X axis and is delimited by a back wall, the width extends along a Y axis and is delimited by two side walls (16, 17), and the height extends along a Z axis and is delimited by a floor and a ceiling; the power supply comprising a plurality of battery packs fixed in a stacked manner on top of one another; each battery pack having a substantially parallelepipedal shape defining two side walls (22-1, 22-2), a proximal wall, a distal wall, an upper wall, and a lower wall, each side wall (22-1, 22-2) defining a plurality of grooves; the power supply unit being designed to be installed in the battery room so that the battery packs are laid in several layers on the floor of the battery room between its side walls (16,17) and against the back wall, the battery packs of the same layer being designed to be disposed next to the adjacent one via their side wall (22-1, 22-2) so that the grooves of these side walls (22-1, 22-2) are disposed to face each other and the distal walls of these packs are adjacent to the wall bottom; the power supply unit further comprising a fastening system for fixing the battery packs in the battery room. the system comprising: - a plurality of braces, each brace defining a lower wall, an upper wall and a shouldered through-orifice defining an internal stop and extending between the lower and upper walls of this brace along an orifice axis (D 1), each brace being designed to be inserted into an orifice formed between two grooves of two facing battery packs disposed next to each other in a same layer of battery packs; - a plurality of fixing screws, each fixing screw being designed to be inserted into the shouldered through-orifice of a brace until it abuts against the internal stop by partially passing through the shouldered through-orifice, and, when this brace is inserted between two grooves facing the corresponding battery packs, being screwed into a tapped hole facing the shouldered through-orifice, thus securing the corresponding battery packs along the X axis. the power supply comprising a plurality of battery packs fixed in a stacked manner on top of one another; each battery pack having a substantially parallelepipedal shape defining two side walls (22-1, 22-2), a proximal wall, a distal wall, an upper wall, and a lower wall, each side wall (22-1, 22-2) defining a plurality of grooves; the power supply unit being designed to be installed in the battery room so that the battery packs are laid in several layers on the floor of the battery room between its side walls (16,17) and against the back wall, the battery packs of the same layer being designed to be disposed next to the adjacent one via their side wall (22-1, 22-2) so that the grooves of these side walls (22-1, 22-2) are disposed to face each other and the distal walls of these packs are adjacent to the wall bottom; the power supply unit further comprising a fastening system for fixing the battery packs in the battery room. - a plurality of braces, each brace defining a lower wall, an upper wall and a shouldered through-orifice defining an internal stop and extending between the lower and upper walls of this brace along an orifice axis (D 1), each brace being designed to be inserted into an orifice formed between two grooves of two facing battery packs disposed next to each other in a same layer of battery packs; - a plurality of fixing screws, each fixing screw being designed to be inserted into the shouldered through-orifice of a brace until it abuts against the internal stop by partially passing through the shouldered through-orifice, and, when this brace is inserted between two grooves facing the corresponding battery packs, being screwed into a tapped hole facing the shouldered through-orifice, thus securing the corresponding battery packs along the X axis.