The present invention relates to protection and reinforcement elements in the car industry, and more specifically relates to the protection of a battery pack of an electric or hybrid vehicle.

Electrical vehicles or hybrid vehicles have to embed at least one heavy and bulky battery pack. This battery pack is made of a plurality of battery cells, that must be very well protected both against physical intrusion that may occur during a car accident and against mechanical shock while the battery pack is moved during assembly to the considered vehicle.

It is known from US patent application 13/940,735 to design a battery pack comprising a plurality of cells inserted in a tray, or tub, comprising a plate bottom and a wall which is upward bent from the peripheral edge of the bottom. The wall is reinforced with an inner frame and an outer frame for better cells protection. However, the clearance angle of the tray forms a lost zone which is a problem in the optimization of the space when inserting the battery cells in the tray.

Moreover, the battery pack structure of the prior art is not used to improve the overall performance of the vehicle in terms of crash management. It is only designed to host and protect the battery cells, without a view to bringing further improvements to the vehicle.

It is known from DE102016115037A1 to form the inner and outer frame structures using four different elements which are welded together after having been formed into three-dimensional parts.

The aim of the present invention is to remedy the drawbacks of the prior art by providing a way to efficiently protect battery cells of the battery pack while optimizing battery cells arrangement of said pack and contributing to an overall improvement of the vehicle crash management. As will be seen through the following figures and description, the present invention offers further advantages in terms of product design flexibility and also in terms of flexibility of the assembly sequence.

For this purpose, a first subject of the present invention consists of a reinforcement frame for a battery pack of an electric or hybrid vehicle, said battery pack comprising a plurality of battery cells lying on and secured to a shield element, said reinforcement frame comprising at least:

- a reinforcement frame fastening portion provided to be secured to both the battery pack and the body of the vehicle, and

- a reinforcement frame hollow portion provided to surround at least the battery cells.

The reinforcement frame according to the invention may also have the optional features listed below, considered individually or in combination:

- The reinforcement frame comprises an inner part and an outer part both having a fastening section and a reinforcement section, wherein the fastening sections are secured to each other in order to form the reinforcement frame fastening portion and wherein the reinforcement sections delimits the reinforcement frame hollow portion.

- The reinforcement sections of the inner and outer parts are both L-shaped and arranged symmetrically with respect to a Y-axis of the reinforcement frame hollow portion then forming a square or rectangular reinforcement frame hollow portion.

- The reinforcement frame comprises a covering frame extending from the reinforcement frame hollow portion, said covering frame being provided to be secured to a top cover of the battery cells.

- The inner and outer parts both have a covering portion extending from the considered reinforcement section, said covering portions being secured to each other in order to form the covering frame.

- The inner and outer parts are made of press hardenable steel.

- The inner and outer parts are made of laser welded blanks. - For any given cross section of the reinforcement frame, the product of the minimum tensile strength by the sheet thickness of the inner part is equal to or higher than the product of the minimum tensile strength by the sheet thickness of the outer part.

- The reinforcement frame has a general square or rectangular shape, and having corners are chamfered corners.

- The reinforcement frame comprises at least a longitudinal reinforcement member which is located inside the reinforcement frame hollow portion and secured to said reinforcement frame hollow portion.

- The longitudinal reinforcement member has an omega-shaped cross- section.

A second object of the invention consists of a reinforced battery pack for an electric or hybrid vehicle comprising a plurality of battery cells and a reinforcement frame, and further comprising the optional features listed below considered individually or in combination:

- a lower protective element named shield element provided to avoid intrusion into the battery pack,

- cooling means lying on the shield element provided to cool down the battery cells,

- a mesh lying on the cooling means and comprising a plurality of housing crossmembers forming a plurality of housing members, each battery cell being housed in a considered housing member,

- the reinforcement frame described above, having a reinforcement frame fastening portion secured to the shield element and a reinforcement frame hollow portion surrounding at least the battery cells, and

- a top cover secured to the reinforcement frame.

The reinforced battery pack according to the invention may also have the optional features listed below, considered individually or in combination:

- The covering member is secured to the top cover frame of the reinforcement frame. - The reinforced battery pack comprises regularly spaced anti-intrusion crossmembers located between the shield element and the cooling means.

- The shield element is made of steel having a tensile strength greater than 1300 MPa.

A third object of the invention consists of a process for assembling a reinforced battery pack according to the invention and attaching it to the body of an electric or hybrid vehicle, said body comprising a floor, at least a pair of rear members and a pair of front members, said pairs being opposite to each other and being provided to absorb rear and frontal shocks, and two side sills opposite to each other secured to the floor and provided to absorb lateral shocks, the process comprising at least the steps of:

-providing battery cells

-providing an inner and outer part

-positioning the inner part around the battery cells

-attaching the fastening section of the outer part to the pair of side sills in such a way that the corners of the outer part located towards the front of the vehicle are adjacent to the rear end of the front members and the corners of the outer part located towards the rear of the vehicle are adjacent to the front end of the rear members

-attaching the fastening section of the inner part to the fastening section of the outer part so as to form a reinforcement frame having a fastening portion attached to the body of the hybrid or electric vehicle and a hollow portion surrounding the battery cells.

A fourth object of the invention consists of a process for assembling a reinforced battery pack according to the invention and attaching it to the body of an electric or hybrid vehicle, said body comprising a floor, at least a pair of rear members and a pair of front members, said pairs being opposite to each other and being provided to absorb rear and frontal shocks, and two side sills opposite to each other secured to the floor and provided to absorb lateral shocks, the process comprising at least the steps of:

-providing battery cells -providing an inner and outer part

-attaching the fastening sections of the inner and outer parts to one another so as to form a reinforcement frame having a fastening portion and a hollow portion

-positioning the reinforcement frame around the battery cells -attaching the fastening portion to the pair of side sills in such a way that the corners of the reinforcement frame located towards the front of the vehicle are adjacent to the rear end of the front members and the corners of the reinforcement frame located towards the rear of the vehicle are adjacent to the front end of the rear members.

Other characteristics and advantages of the invention will be described in greater detail in the following description.

The invention will be better understood by reading the following description, which is provided purely for purposes of explanation and is in no way intended to be restrictive, with reference to:

- Figure 1 , which is a perspective view of a body of a vehicle comprising a reinforced battery pack according to the invention;

- Figure 2, which is a perspective view of a part of the reinforced battery pack which includes the reinforcement frame inner;

- Figure 3, which is a top view of the reinforcement frame according to the invention;

- Figure 4, which is a cross section view of figure 3 following arrow IV;

- Figure 5, which is a perspective view of the reinforcement frame inner of figure 3;

- Figure 6, which is a perspective view of the reinforcement frame outer of figure 3; - Figure 7, which is an exploded view of a part of a specific embodiment of the reinforced battery pack of the invention;

- Figure 8, which represents a process of assembling a specific embodiment of the reinforced battery pack of the invention;

- Figure 9, which is the bottom view of the front part of the body of a vehicle;

- Figure 10, which is the bottom view of the rear part of the body of the vehicle of figure 9.

It should be noted that the terms “lower”, “upper”, “above”, “below”, “lowest”, “highest”, “top”, “bottom”, “left”, “right” as used in this application refer to the positions and orientations of the different parts of the reinforcement frame, of the battery pack and of the vehicle when they are positioned vertically on the ground. Furthermore, the terms “front”, “forward”, “rear”, “back”, “backwards” are defined according to the normal driving direction of a vehicle. The terms “substantially perpendicular” define an angle of 90° +/- 15° and the terms “substantially parallel” define an angle of 0° +/- 15°.

A first object of the present invention is the reinforcement frame 1 , which will now be described with the support of figures 1 to 6.

This reinforcement frame 1 is designed to protect battery cells 29 of a battery pack of a hybrid or electric vehicle 37 from mechanical shocks and physical intrusions. The reinforcement frame 1 is thus provided to be secured both to the battery pack and to the body 30, 31 of the vehicle 37, as depicted in figure 1 . While protecting the battery cells 29, the reinforcement frame 1 also provides active crash energy management.

The battery pack is a well-known component of electric and hybrid vehicles, and essentially comprises a plurality of cells 29.

In a particular embodiment, the battery cells 29 lie on a shield element 15. This shield element 15 is for example made of fully martensitic steel comprising between 0,15% and 0,5% of carbon in weight. This martensitic steel has a tensile strength greater than 1800 MPa, making the shield element 15 particularly resistant against physical intrusion coming from the bottom.

The reinforcement frame 1 of the invention is made of steel and comprises a reinforcement frame fastening portion 3 which is provided to secure the battery pack to the body 30, 31 of the vehicle 37, and a reinforcement frame hollow portion 4 secured to the reinforcement frame fastening portion 3 and provided to surround at least the battery cells 29.

According to the invention, the reinforcement frame 1 is made of two ring- shaped elements 10, 11 secured to each other. These two elements are further called the inner part 10 and the outer part 11 of the reinforcement frame 1. The inner part 10 is provided to surround at least the battery cells 29, while the outer part 11 is provided to be in contact with a part of the body 30, 31 of the vehicle 37.

As reported in figure 3, the reinforcement frame has a substantially rectangular shape extending along a longitudinal axis X, with two longitudinal sections 33, 34, two transverse sections 31 , 32 and four corners 24.

Each part 10, 11 of the reinforcement frame 1 is manufactured by stamping a steel blank. In a particular embodiment, the stamping operation is a hot stamping operation in which the blank is heated and subsequently quenched in the stamping tool. It is thus possible to obtain the complex shapes needed for the part while ensuring very high mechanical properties to have very good resistance in case of shock.

In a particular embodiment, each part 10, 11 of the reinforcement frame 1 is manufactured by stamping a tailor welded blank, comprising several sub-blanks. Said sub-blanks can have different thicknesses and / or different compositions, in order to optimize the mechanical performance and the weight of the part. In areas where higher mechanical resistance is needed to provide adequate protection in case of shock a higher thickness and / or a more resistant steel grade will be used. On the other hand, in areas where the mechanical resistance can be lower, a lower thickness can be used in order to reduce the weight of the part. An example of such a tailor welded blank design is depicted in figures 2, 3, 5 and 6 where welding lines 35 are made apparent. In this embodiment, both inner and outer parts 10, 11 are made of six sub-blanks or pieces joined together using butt laser welding to form two blanks: one piece for each longitudinal member 33, 34, one piece for two corners 24 and a transverse member 32, and three pieces for the remaining corners 24 and the remaining transverse member 31. Such a laser welded blank conception is very flexible and allows for multiple variants in order to meet the safety, regulatory and optimized weight requirements imposed to the battery pack. In the present example, a laser welded blank having six sub-blanks was presented. However, one can choose as little or as many blanks as necessary taking into account the engineering and cost constraints of the particular battery pack to be designed.

In a particular embodiment, the laser welded blank used to manufacture the inner and outer parts 10, 11 will have higher thicknesses on the portions of the reinforcement frame 1 which are located parallel to the side of the vehicle 37 and on the portions which correspond to the corners of the reinforcement frame 1. Indeed, in case of a lateral impact to the vehicle 37, such as for example a pole impact to the side of the vehicle 37, the portions of the reinforcement frame 1 which are located parallel to the side of the vehicle 37 will be submitted to very high local loads, and therefore need to be reinforced. Also, in the case of frontal or rear crash, the load coming from the impact will be transmitted to the rest of the reinforcement frame 1 through the corner portions, as will be detailed below. The corner portions therefore also need to be reinforced.

The inner and outer parts 10, 11 are assembled together. Both inner and outer parts 10, 11 have fastening sections 5, 6 that are secured to each other with securing means 21 , as depicted in figure 8, in order to form the reinforcement frame fastening portion 3 of the reinforcement frame 1. Preferably, the two fastening sections 5, 6 are in contact. In addition, both inner and outer parts 10, 11 have reinforcement sections 7, 8 forming the reinforcement frame hollow portion 4 of the reinforcement frame 1 .

As a preferred example, the fastening sections 5, 6 of the outer and inner parts 10, 11 are bolted together.

As depicted in figure 4 and according to the invention, an example of geometry of each reinforcement section will now be described.

The reinforcement sections 7, 8 of the inner and outer parts 10, 11 are both substantially L-shaped and positioned in symmetrical positions with respect to an axis Y of the reinforcement frame hollow portion 4. More specifically, when looking at figure 4, the reinforcement section 7 of the inner part 10 has a L-shape, whereas the reinforcement section 8 of the outer part 11 has an inverted L-shape. It should be noted that while the reinforcement sections 7 and 8 substantially extend along an L-shape, it is possible to introduce some modifications compared to a perfect L- shape, such as notches or others, to take into account for example the packaging constraints induced by the environing elements of the vehicle 37. It should be also noted that the inner and outer parts 10, 11 can have slightly different shapes so they are not fully positioned in symmetrical positions, provided said inner and outer parts 10, 11 define the reinforcement frame hollow portion 4.

The reinforcement frame hollow portion 4 thus has a substantially square or rectangular cross section. The reinforcement frame hollow portion 4 thus has four main walls: a bottom wall 40 from which the fastening section 5 of the inner part 10 extends and an opposite upper wall 42, an inside wall 41 facing the battery cells 29 and an opposite outer wall 43 from which the fastening section 6 of the outer part 11 protrudes. In addition, the reinforcement frame 1 has a substantially square or rectangular shape.

Thus, the bottom wall 40 continues with the fastening section 5 of the inner part 10 extending from the bottom wall 40 in a parallel manner and the outer wall 43 is substantially perpendicular to the fastening section 6 of the outer part 11 .

In the case of an impact to the vehicle, the reinforcement frame 1 is designed to absorb part of the energy of the impact through the deformation of its outer part 11 while protecting the battery cells 29 thanks to the anti-intrusion behavior of the inner part 10. Because the reinforcement frame 1 is located in the middle of the vehicle, it is expected to play an active role in crash management in the case of a front impact, a rear impact or a lateral impact. The outer part 11 is designed to be able to sustain some amount of deformation in the case of a crash while the inner part 10 defines a “no-go” zone inside which the battery cells 29 are fully protected from intrusion in the case of an impact.

One way to apply this double concept of energy absorption through the outer part 11 and anti-intrusion through the inner part 10 is to provide parts wherein for any given cross section the product of the minimum tensile strength by the sheet thickness of the inner part 10 is equal to or higher than the product of the minimum tensile strength by the sheet thickness of the outer part 11 . As an example, both inner and outer frame 10, 11 is made of press-hardening steel having a tensile strength greater than 1300 MPa. The composition of this steel is for example in weight percentage: In this embodiment, the thickness of the inner part is for example comprised between 1 ,2 and 1 ,6 mm.

In another embodiment, both inner and outer part 10, 11 can be made with a harder steel having a tensile strength greater than 1800 MPa. The composition of this steel is for example in weight percentage:

In this embodiment, the thickness of the inner and outer part 10, 11 are for example comprised between 1 , and 1 ,4 mm, and the part can have a lower weight while keeping the same mechanical resistance as a part made with the lower strength steel described in the previous embodiment.

These two examples of steel alloys have a great rigidity, so that the inner part 10 of the reinforcement frame 1 offers a good protection against any deformation or against any physical intrusion through the battery cells 29.

According to the invention, the outer part 11 is made of a material that has same or greater ductility than the material of the inner part 10. As an example, the outer part 11 can be made with a steel having a tensile strength around 1000 MPa. The composition of this steel is for example in weight percentage:

Since the ductility of the outer part 11 is greater than that of the inner part 10, the outer part 11 may be deformed by any mechanical shock. The deformations that occur in this case result in a mechanical energy absorption, reducing the remaining energy absorbed by the walls 40, 41 of the inner part. As a result, the risk of deformation or deterioration of the inner part 10 is significantly reduced. Preferentially, the corners 24 of the reinforcement frame, and more specifically the corners of the outer part 10 of the reinforcement frame 1 , are chamfered as depicted in figure 6. This reduces thinning of the material in the corners 24, resulting in better resistance of the reinforcement frame 1 and better energy transmission through the longitudinal 33, 34 and transverse members 31 , 32.

According to a particular embodiment as depicted in figure 4, the reinforcement frame 1 comprises a covering portion 9 protruding from the reinforcement frame hollow portion 4. Thanks to this covering portion 9, the top plate 19 (figure 8) provided to seal the top of the battery pack and to protect the battery cells 29 can be secured to the covering portion 9 of the reinforcement frame 1.

In order to form said covering portion 9, both inner and outer parts 10, 11 have a covering section 12, 13 extending from the considered reinforcement section 7, 8. The two covering sections 12, 13 are secured to each other to form the covering portion 9. Preferentially, the two covering section 12, 13 are in contact.

Finally, in order to strengthen the reinforcement frame 1 , several reinforcement members 14 (figure 7) can be inserted inside the hollow body 4, preferably one reinforcement member 14 for each longitudinal and transverse member 31 - 34 and also in the corners if required. Each reinforcement member 14 has preferably an omega shape to offer both good energy absorption and good securing reinforcement members 14 are preferably secured to the inner wall 41 of the reinforcement frame hollow portion 4.

A second object of the present invention is a reinforced battery pack 2 which comprises the above described reinforcement frame 1 surrounding the battery cells 29.

A part of a specific embodiment of the reinforced battery pack 2 is depicted in perspective view in figure 2 and as an exploded view in figure 7. It should also be noted that battery cells 29 are depicted in figure 2 but not in figure 7. The reinforced battery pack 2 comprises the reinforcement frame 1 and the battery cells 29 along with other elements listed below, which can optionally be included in the make-up of the reinforced battery pack 2, taken alone or in any possible combinations of each other. It should be noted that the following list is in no way meant to be comprehensive or limiting the scope of the invention, but is rather given as means of examples to illustrate possible applications of the current invention:

• the shield element 15 described above;

• cooling means 16 lying on the shield element 15 and provided to cool down the battery cells 29. As an example, the cooling means 26 comprises two thermal conducting elements named cladding elements secured to each other and a cooling system (not represented) inserted between the two cladding elements 160, 161 ;

• regularly spaced anti-intrusion crossmembers 20 secured to the shield element 15 and located between said shield element 15 and the cooling means 16;

• a mesh 17 comprising a plurality of housing crossmembers 23 lying on the cooling means 16. The housing crossmembers 23 forms a plurality of regularly dispatched housing members 18.

• Preferably, the transverse housing cross members 23 are aligned with the anti-intrusion crossmembers 20, so that in case of an intrusion coming from the bottom of the vehicle 37, said anti-intrusion crossmembers 20 and transverse housing cross members 23 collaborate together to provide optimal resistance.

• a plurality of battery cells 29 (figures 2, 9 and 10). Each battery cell 29 is housed in the considered housing member 18 and is in contact with the cooling means 16.

According to the invention, the battery pack is reinforced with the reinforcement frame 1 of the invention to form a reinforced battery pack 2. The reinforcement frame fastening portion 3 of the reinforcement frame 1 is for example secured to the shield element 15. The reinforcement frame hollow portion 4 surrounds the battery cells 29, the mesh 17 and the cooling means 16. Only the inner part 10 of the reinforcement frame is depicted in figures 2 and 7.

Finally, the reinforced battery back 2 optionally comprises a top plate also named top cover 19 (figure 8) that is secured to the reinforcement frame 1. For example, the top plate 19 is bolted to the covering frame 9 of the reinforcement frame 1. Advantageously, by bolting the top plate 19 to the covering frame 9, it is possible to remove it in case maintenance of the battery cells 29 or other elements is necessary.

According to a particular embodiment, a process for assembling the reinforced battery pack 2 depicted in figure 8 will now be described.

In a first step, a first assembly is provided by securing the anti-intrusion crossmembers 20 to the shield element 15.

In a second step, a second assembly is provided following the sub-steps of:

- assembling the cladding elements 160, 161 and the cooling system to form the cooling means 16;

- securing the mesh 17 to the cooling means 16;

- providing the battery cells 29 inside the housings 18 of the mesh 17;

- positioning the reinforcement frame 1 around the cooling means 16, the mesh 17 and the battery cells 29.

In a third step, the reinforcement frame fastening portion 3 of the reinforcement frame 1 is bolted to the shield element 15 with securing means 21. Preferentially, the inner part 10 and the outer part 11 of the reinforcement frame 1 and the shield element 15 are bolted together in the same step using said securing means 21.

In a particular embodiment, the securing means 21 are Self Pierce and Clinch nuts, such as the SPAC® nuts commercialized by RB&W.

In a fourth and final step, the top plate 19 is secured to the reinforcement frame 1. Advantageously, the top plate 19 is bolted to the covering frame 9 of the reinforcement frame 1.

The reinforced battery pack 2 is then protected against any physical shock and against any physical intrusion and can be securely moved during any further assembling process of the reinforced battery pack 2.

According to another process of the invention, the reinforced battery pack 2 may be assembled to the body 30, 31 of the vehicle 37.

In a first step, the reinforcement battery pack 2 is positioned in the body 30, 31 of the vehicle 37 so that the longitudinal axis X of the reinforcement frame 1 is parallel to the longitudinal axis X’ of the vehicle. Once positioned, the corners 24 of the reinforcement frame 1 are respectively in contact with an extremity of the rear members 25 and with an extremity of the front members 26.

In a second and last step, the reinforcement frame 1 of the reinforced battery pack 2 is secured to the side sills 27 of the body 30, 31 of the vehicle 37.

Thanks to this configuration, the energy of any longitudinal shock against the vehicle 37 will be transmitted through the corresponding members 25, 26 and will be transmitted via the corners 24 through the longitudinal and transverse members 31 - 34 of the reinforcement frame 1 , avoiding any deformation or deterioration of the reinforced battery pack 2. Similarly, any lateral shocks against the side sills 27 of the body of the vehicle 37 will be transmitted through the longitudinal members 33, 34 of the reinforcement frame 1 .

The reinforcement frame 1 of the invention is of great interest to protect battery packs of any electric or hybrid vehicle.

The embodiments described hereinabove are entirely nonlimiting and amendments can be made thereto without departing from the scope of the invention. By way of example, both inner and outer parts may be manufactured with the same high tensile strength steel, like Usibor ^® 1500 or Usibor ^® 2000. Finally, the reinforced battery pack 2 may only comprise the inner part 10 of the reinforcement frame 1 , while the outer part 11 of the reinforcement frame 1 is secured to the body 30, 31 of the vehicle 37. In this case, the assembling of the reinforced battery pack 2 to the vehicle is implemented by bolting the fastening section 5 of the inner part 10 to the fastening section 6 of the outer part 11 .

A third and fourth object of the current invention are assembly processes of the reinforced battery pack 2 attached to the vehicle 37.

The body of the vehicle 37, also known as “body in white” refers to car body's components that have been joined together, using one or a combination of different techniques: welding, riveting, clinching, bonding, laser brazing...

According to figures 1 , 9 and 10, the body 30, 31 of the vehicle 37 extends regarding a longitudinal axis X’ and comprises a floor 28, at least a pair of rear members 25 and a pair of front members 26. The front members 26 are in the front of the vehicle 37 and the rear members 25 are located towards the rear of the vehicle. The two pairs of members 25, 26 are thus opposite to each other and provided to absorb shocks coming from the front and from the rear. In addition, the body 30, 31 of the vehicle 37 comprises two side sills 27, secured to the floor 28 and positioned opposite to each other. Those side sills 27 are provided to absorb lateral shocks.

The above described reinforcement frame 1 can be integrated in the overall vehicle architecture in several different ways, following two main assembly possibilities:

-According to a first possibility, the outer part 11 is attached to the body 30, 31 , while the inner part 10 is positioned around the battery cells 29. The inner part 10 is then attached to the outer part 11 in order to secure the battery pack 2 to the vehicle.

-According to a second possibility, the inner and outer parts 10, 11 are first fastened together to form the reinforcement frame 1 and then positioned around the battery cells 29. The thus assembled battery pack 2 is then attached to the body in white by attaching the reinforcement frame 1 to the body in white. In this second configuration, it can be considered that the full reinforcement frame 1 belongs to the battery pack 2.

Turning to the first possibility, the assembly sequence comprises the following steps:

-providing the battery cells 29

-providing the inner and outer part 10 and 11

-positioning the inner part 10 around the battery cells 29. For example, the battery cells 29 are laid upon the shield element 15 and the fastening portion 5 of the inner part 10 is fastened to the shield element 15.

-attaching the fastening section 6 of the outer part 11 to the pair of side sills 27 in such a way that the corners of the outer part 10 located towards the front of the vehicle 37 are adjacent to the rear end of the front members 26 and the corners of the outer part 10 located towards the rear of the vehicle are adjacent to the front end of the rear members 25

-attaching the fastening section 5 of the inner part 10 to the fastening section 6 of the outer part 11 so as to form a reinforcement frame 1 having a fastening portion 3 attached to the body of the hybrid or electric vehicle 37 and a hollow portion 4 surrounding the battery cells 29.

Turning to the second possibility, the assembly sequence comprises the following steps:

-providing the battery cells 29 -providing an inner and outer part 10 and 11

-attaching the fastening sections 5 and 6 of the inner and outer parts 10 and 11 to one another so as to form the reinforcement frame 1 having a fastening portion 3 and a hollow portion 4

-positioning the reinforcement frame 1 around the battery cells 29. For example, the battery cells 29 are laid upon the shield element 15 and the fastening portion 5 of the inner part 10 is fastened to the shield element 15. -attaching the fastening portion 3 to the pair of side sills 27 in such a way that the corners of the reinforcement frame 1 located towards the front of the vehicle 37 are adjacent to the rear end of the front members 26 and the corners of the reinforcement frame 1 located towards the rear of the vehicle are adjacent to the front end of the rear members 25.

Thanks to this configuration of the reinforcement frame 1 inside the vehicle, the energy of a frontal or rear collision against the vehicle 37 will be transmitted through the corresponding members 25, 26 and will be transmitted via the corners 24 through the longitudinal and transverse members 31 - 34 of the reinforcement frame 1 , avoiding any deformation of the inner part 10 of the reinforcement frame 1 . Furthermore, the reinforcement frame 1 serves to deflect and distribute the impact energy in case of a frontal or rear collision. Indeed through its high strength and central position with the vehicle architecture, the reinforcement frame 1 is placed on the path of the load transmitted by the impact, as materialized by the arrows 36 on figures 9 and 10, which show the path that the impact force will take. The load will first be transmitted to the vehicle structure through the members 25 or 26 respectively in the case of a rear or frontal crash. The load will then be picked up by the rest of the vehicle structure and in particular by the reinforcement frame 1 . The load will be deflected from the center path of the vehicle 37, wherein lays the sensitive battery cells 29, to the sides of the vehicle 37, through the reinforcement frame 1.