NOVELTY - The element comprises a set of electrochemical cells identical with each other and connected electrically in series and/or parallel for powering an electrical motor drive. The individual cells are mechanically and electrically connected to an inside of a self-supporting electrically insulating container (121) with respect to its adjacent cells to maintain a determined air thickness that exceeds a specified value, around each cell to prevent arc formation in contact with each other at an existing maximum voltage in a battery tray. The container includes a casing presenting walls (1210, 1219).    USE - The element such as batteries tray is useful for electric storage batteries unit of an electric vehicle (all claimed) such as a race car.    ADVANTAGE - The safe element improves the performances of the electric vehicles by improving a ratio of energy/mass of the components, and has resistance to breakdown, heat dissipation property, reduced manufacturing costs and high reliability.    DETAILED DESCRIPTION - The element comprises a set of electrochemical cells identical with each other and connected electrically in series and/or parallel for powering an electrical motor drive. The individual cells are mechanically and electrically connected to an inside of a self-supporting electrically insulating container (121) with respect to its adjacent cells to maintain a determined air thickness that exceeds a specified value, around each cell to prevent arc formation in contact with each other at an existing maximum voltage in a battery tray. The container includes a casing presenting walls (1210, 1219) that include a composite structure provided with mechanical layers of carbon fibers impregnated with resin and coated on an inside by a layer of dielectric material. The dielectric material layer is carried out by a layer of glass fibers impregnated with resin and/or a layer of a varnish and/or a dielectric resin. The composite structure further comprises a layer of Kevlar (RTM: para-aramid synthetic fiber) impregnated with resin. The composite structure of the vertical walls and/or a bottom of the container and/or a fixing point (1215) further comprises an insert device made of carbon fiber impregnated with resin or an inner spacer layer forming a core (C6) that is in a shape of honeycomb oriented transversely to a plane of the composite structure made of aramid material type. One of the walls of the container presents the fixing point to accommodate machine fixing parts to the wall of the container. The fixing point presents an opening emerging only towards outside and entirely isolated from the interior side, and is realized by a mechanical insert included between two of the mechanical layers or by a crossing hole leading towards an interior to a volume closed by a shell including a dielectric composite providing a dielectric continuity with the dielectric material layer. The individual cells have a shape elongated along a longitudinal axis defining two opposite ends and are arranged parallel to each other in a same direction between two parallel plates (123, 124), where each of the cells is attached to the two respective plates by its respective opposite ends. The plates are fixed on the walls of the container surrounding the cells across the plates in different heights so as to provide: a bottom plate on a side of one end of the cells carrying a degassing vent, where a normally closed vent space is adapted to communicate with the degassing vent in the event of a degasification of the cell; a cooling chamber communicating with a ventilation openings formed in the walls between the two plates; and a high plate on the side of one end of cells bearing connection terminals, where an accessible space includes a connection circuit between the cells. The container comprises the ventilation openings arranged to allow a circulation of air between the plates and through the set of individual cells. The composite structure includes a layer of a material forming water tightness towards the interior. The cells are formed with electrically and thermically conducting metal walls having an insufficient electric insulation to avoid the formation of electric arc in the event of contact between the cells with the maximum existing voltage within the battery tray. An INDEPENDENT CLAIM is included for an assembly of batteries.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of walls of a container of a batteries tray.    Self-supporting electrically insulating container (121)    Parallel plates (123, 124)    Walls (1210, 1219)    Fixing point (1215)    Core. (C6)