Casing for accommodating at least one battery unit, comprising a modular assembly for the inlet, circulation and outlet of a heat transfer fluid, in particular a cooling fluid, in at least one battery unit for a hybrid or electric motor vehicle having at least one such unit, said casing having a floor in which mutually parallel and rectilinear channels are arranged that receive said fluid, and said casing having: - a module serving as inlet and an identical module serving as outlet for said heat transfer fluid to and from, respectively, the circuit of channels via in each case a base module, each base module comprising a system of substantially rectilinear and parallel channels facing the ends of the channels in said floor, such that the channels of the base module in each case butt in a leaktight manner against said ends, said base modules being fastened to said floor, - a plurality of base modules distributed along each of the two faces of the floor and facing the ends of the channels in said floor, such that the channels of the base module in each case butt in a leaktight manner against said ends, said base modules being fastened to said floor, and - a plurality of bridges fluidically connecting two adjacent base modules situated on one and the same side of said floor together and two base modules situated on the opposite side of the casing together with an offset so as to form a U-shaped or serpentine heat transfer fluid circuit between the two opposite faces of said casing, if necessary along the entire length of the latter, - each base module being made in the form of an elongate bar having at least two, preferably three and even more preferably four channels in leaktight engagement with a corresponding number of channels situated in the floor of said casing, characterized in that each bridge is made in the form of an elongate spacer for fluidically connecting two adjacent base modules and having at least two pivoting levers which are provided, at their lower ends, with hooks that engage at their bottoms with corresponding protrusions disposed on the base modules to be connected and which are provided, at their opposite ends, with fastening points that face corresponding fastening points situated on the base modules, said fastening points being able to take a screw for screwing them together when said pivoting levers are lowered and engaged with the protrusions of the base modules to be connected. - a module serving as inlet and an identical module serving as outlet for said heat transfer fluid to and from, respectively, the circuit of channels via in each case a base module, each base module comprising a system of substantially rectilinear and parallel channels facing the ends of the channels in said floor, such that the channels of the base module in each case butt in a leaktight manner against said ends, said base modules being fastened to said floor, - a plurality of base modules distributed along each of the two faces of the floor and facing the ends of the channels in said floor, such that the channels of the base module in each case butt in a leaktight manner against said ends, said base modules being fastened to said floor, and - a plurality of bridges fluidically connecting two adjacent base modules situated on one and the same side of said floor together and two base modules situated on the opposite side of the casing together with an offset so as to form a U-shaped or serpentine heat transfer fluid circuit between the two opposite faces of said casing, if necessary along the entire length of the latter, - each base module being made in the form of an elongate bar having at least two, preferably three and even more preferably four channels in leaktight engagement with a corresponding number of channels situated in the floor of said casing,