NOVELTY - The compartment has a set of side modules (4) and a central module (20) with electrochemical cells i.e. lithium-ion cells. A heat exchanger (10) has an embossing (12) for exchanging calories between the modules and the ambient conditions. A set of cylindrical heat pipes (24) is arranged with coolant e.g. glycol, for exchanging calories between a cylindrical upper face and a lower face. A heat diffuser (22) has a thermal conductivity material and includes a surface adjusted to the lower face of the heat pipes, and another surface comprising curve adjusted to embossing of the heat exchanger.    USE - Battery compartment for electric or hybrid vehicle (all claimed).    ADVANTAGE - The heat diffuser has the thermal conductivity material and includes the surface adjusted to the lower face of the heat pipes, and another surface comprising curve adjusted to embossing of the heat exchanger, thus allowing to use heat pipe of simple geometrical forms by ensuring good conduction of calories towards the embossing of the heat exchanger. The diffuser is manufactured by economic methods by molding or machining, and the surface is adjusted on the embossing. The heat diffuser and the heat exchanger improve heat exchange. The conduction and diffusion of heat between the heat exchanger and surface of the modules is ensured without introducing too much thermal load or adding too much mass. The heat pipe with a tubular enclosure of cylindrical form is manufactured easily, and has reduced costs. The volume of materials of the enclosure and the volume of coolant are optimized. The load capacity of the battery compartment and on-board power are improved using all space available in the compartment, thus increasing lifetime of the batteries, while ensuring better cooling and thermal management. The number of curves of exhaust line that generates back pressures slowing down exit of gases is reduced.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for an electric or hybrid vehicle.    DESCRIPTION OF DRAWING(S) - The drawing shows a partial cross-sectional view of a battery compartment levelled with an embossing.    Side modules (4)    Heat exchanger (10)    Embossing (12)    Central module (20)    Heat diffuser (22)    Cylindrical heat pipes (24)