“IMPROVED CROSS-MEMBER FOR A HEAVY VEHICLE SUSPENSION SYSTEM” CROSS-REFERENCE TO RELATED APPLICATIONS This patent application claims priority from Italian patent application no. 102020000025015 filed on 22/10/2020, the entire disclosure of which is incorporated herein by reference. TECHNICAL FIELD The present invention concerns a cross-member for a suspension system, in particular a cross-member for a heavy vehicle suspension system provided with an electronic traction. BACKGROUND OF THE INVENTION Heavy vehicles such road transport vehicles, e.g. trucks, are more and more electrified for reducing their polluting emissions. In particular, such vehicles may be hybrid vehicles or purely electric vehicles. In particular, the electrified vehicles comprises at least an electric axle, i.e. an axle provided with electric machines such as motors/generators configured to impart a torque on the axle to allow its motion or to recharge battery modules of the vehicle when this latter is braking. Such battery modules are usually placed in the proximity of the electric machines of the electric axle in order to provide energy exchange with electric machines in an efficient way. However, as known, an axle of a vehicle is connected to the chassis of this latter via a suspension system configured to dampen the forces acting between the road and the chassis. If a vehicle is provided with an electric axle as described above, the presence of the electric batteries increases the encumbrance around the chassis. Moreover, the presence of peculiar fixing means for stably connecting the electric batteries to the chassis reduces the useful space on the vehicle for the elements of the suspension. Therefore, the need is felt to provide a compact and economic way to allow the fixation of the suspension elements of a heavy vehicle and the electronic batteries in the proximity of an electric axle of the such heavy vehicle. An aim of the present invention is to satisfy the above mentioned needs in a compact, optimized and cost effective way. SUMMARY OF THE INVENTION The aforementioned aim is reached by a cross-member, a suspension system and a vehicle as claimed in the appended independent claims. Preferred embodiments of the invention are realized according to the claims dependent on or related to the above independent claim. BRIEF DESCRIPTION OF DRAWINGS For a better understanding of the present invention, a preferred embodiment is described in the following, by way of a non-limiting example, with reference to the attached drawings wherein: ● Figure 1 is a perspective view showing a portion of a suspension system for an electric axle of a heavy vehicle comprising a cross-member according to the invention; ● Figure 2 is a perspective enlarged view of a portion of the suspension system of figure 1; ● Figure 3 is perspective enlarged view of a cross-member according to the invention; and ● Figures 4 and 5 are respectively a bottom and top view of the portion of suspension system of figure 2. DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows a portion of a chassis 1 of a heavy vehicle such as truck, comprising as known a pair of side members 2, 3 extending longitudinal along a longitudinal axis A of the vehicle. The chassis 1 is connected via a suspension system 4, partially shown, to an electric axle 5. As per se known, the suspension system 4 dampens the relative movements between the chassis 1 and the electric axle 5 that is configured to provide torque to wheels, not shown, carried by the electric axle 5 itself. The chassis 1 is furthermore configured to carry at least one battery module 6 configured to be electrically connected to the electric axle 5 to exchange electric energy. In particular, as known, the electric axle 5 is provided with electric machines that are fed by the at least one battery module 6 to provide torque to the wheels carried by the axle 5 or to receive electric energy generated by the electric machines when the vehicle is braking. In the exemplarily shown embodiment, the chassis 1 carries a single battery modules 6 housed between the side members 2, 3 and in a rear position with respect to electric axle 5. The suspension system 4 comprises, as known, a plurality of elements 7, 8 configured to connect the electric axle 5 to respective side members 2, 3 in order to dampen/limit a relative movement along a direction along axis A or a different axis. In particular, the proposed suspension system 4 comprises longitudinal right and left rods 7 configured to longitudinally limit, i.e. along axis A, a movement of electric axle 5 and a V-rod 8 provided with two inclined arms 8', 8'' with respect to axis A and configured to limit a movement on a transversal axis perpendicular to axis A. The longitudinal right and left rods 7 are connected to the respective side member 2, 3 via a respective front hanger 9 further not described in detail for sake of brevity. Clearly, the suspension system comprises other elements such as air bellow or dampers (not shown) to limit a vertical motion of the electric axle 5 and here not shown and described for simplicity. The V-rod 8 is connected to side members via a cross- member 10 as shown in detail in figures 2 and 3. According to the invention, the cross-member 10 comprises a pair of lateral elements 11 configured to allow the connection of the side members 2, 3 of chassis 1 and to allow the connection of the at least one battery module 6. In greater particular, the lateral elements 11 are advantageously equal, i.e. they have a shape symmetrical with respect to longitudinal axis A. In particular, referring to only one of the lateral elements 11, being the other equal, it comprises a fixation portion 11' configured to allow the fixation of the lateral element 11 to the respective side member 2, 3. In particular, the fixation portion 11' is a plate-like element and defines a plurality of holes 12 configured to allow the insertion of a fixation element, not shown, such as a threaded element or a dowel to fix the fixation portion 11 to the side member 2, 3. Preferably, such plurality of holes 12 is defined by a first plurality of holes 12 grouped in a rear portion of the fixation portion 11' and a second plurality of holes 12 grouped in a front portion of the fixation portion 11'. The lateral element 11 further comprises a bracket 13 extending in a cantilevered way substantially perpendicular along a traversal axis B that is preferably perpendicular to longitudinal axis A towards the opposite side member 2, 3, i.e. in the space comprised between these latter. The bracket 13 advantageously extends from fixation portion 11' between the two pluralities of holes 12 defined by this latter and, more preferably, is placed substantially in the middle between the extremities of the fixation portion 11' along longitudinal axis A. The bracket 13 has a transversal cross-section that is ζ-shaped and essentially comprises a terminal portion 13' opposed to fixation portion 11' that is substantially a parallelepiped. The bracket 13 further defines a seat 13'' that is laterally delimited by a pair of protrusion 14', 14'' and that is opened from one side of bracket 13 with respect to axis A, in particular towards the electric axle 5. The seat 13'' is configured to accommodate a portion of a respective arm 8', 8'' of the V-rod 8 of the suspension system 4 and the protrusion 14', 14'' are configured to allow the connection of such arm 8', 8'' via a hinge connection 15. The seat 13'' is comprised in a transversal direction, i.e. along axis B, between the terminal portion 13' and the fixation portion 11'. The cross-member 10 can advantageously comprise a central element 17 suitable to be connected to the lateral element 11via their respective terminal portions 13'. In particular the terminal portion 13' defines a plurality of holes 16, i.e. at least two holes 16, configured to allow the connection of the central element 12 via a fixation element 18 such as a threaded element, e.g. a bolt as shown, or any other equivalent fixation element such as a dowel. In the disclosed example of the central element 17 comprises a pair of connection elements 19, namely an upper connection element 19 and a lower connection element 19 spaced apart about a vertical axis perpendicular to axis A and B. More in detail, each connection element 19 essentially comprises a plate-like element realized in a material that is different with respect to the material of lateral elements 11. Preferably, the lateral elements 11 are made of cast iron and they are made of a single piece and each connection element 19 is made of steel. The presence itself of central element 17 and, when present, the thickness of each connection element 19 or their number or their overall shape may be varied to vary the stiffness of the cross-member 10, i.e. to vary the mechanical properties of the connection between the side members 2,3. Each lateral member 11 further define at least a fixation point 20 for allowing the fixation of the battery module 6 to the fixation portion 11', i.e. to the respective side member 2, 3. In particular, the fixation point 20 comprise a protuberance 21 that extends getting away from fixation portion 11 and defining a hole 22 (see figure 5) to allow the insertion of a fixation element such as a threaded element or a dowel. In the described example, the fixation point 20 is realized laterally with respect to bracket 13, in particular in a rear portion with respect this latter along the direction of axis A and in the middle of the rear plurality of holes 12. Furthermore, the bracket 13 and the fixation point 20 are realized on the same side of the lateral element 11, i.e. both directed together towards the inner space defined by side members 2, 3. In this way, the opposite side of the lateral element 11 can be used to allow the fixation of this latter to the respective side member 2, 3. The operation of the described cross-member 10 is the following. When fixed via holes 12 to the respective side member 2, 3, the cross-member 10 can be used to securely fix the respective battery module 6 to the chassis 1 in a simple and quick way. In view of the foregoing, the advantages of a cross- member, a suspension system and a vehicle comprising such cross-member according to the invention are apparent. The proposed cross-member 10 allows at the same time the connection in a transversal direction with respect to axis A of side members 2,3 and the securing of the battery module 6. Furthermore, the mounting of the battery module 6 is simplified because of the possibility of mounting separately the lateral elements 11 and, if present, the central element 17. Moreover, the central element 17 may be designed in order to vary the overall transversal stiffness of chassis 1, i.e. between the side members 2, 3. Indeed, a thicker central element 17 or a central element 17 comprising multiple connection elements 19 would provide a more stiff cross-member 10 and vice-versa. The above advantage is particularly useful in electric traction vehicles wherein the dimensions and weight of battery modules 6 may vary and therefore the stiffness required to the chassis 1 may vary accordingly. It is clear that modifications can be made to the described cross-member, suspension system and vehicle comprising such front cross-member, which do not extend beyond the scope of protection defined by the claims. For example, the fixation point 20 may have different shapes, allowing a plurality of fixation holes according to the specific fixation way of the battery module. Similarly, the material and number of elements 19 of the central element 17 may be varied according to the design request of the heavy vehicle. Moreover, the overall described portions of the cross- member 10 may be varied according to the typology of suspension system.