GB2540437A

GB2540437A - Cell tray

Info

Publication number: GB2540437A
Application number: GB1512624.6A
Authority: GB
Inventors: Blair Dobson Simon
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2015-07-17
Filing date: 2015-07-17
Publication date: 2017-01-18
Also published as: GB201512624D0

Abstract

A cell tray 1 for a battery pack comprising a plurality of cell holders for holding a plurality of cells, each holder comprising a plurality of first flexible wall portions 2 tangential to a first circle having a first diameter 4 and a plurality of second rigid wall portions 3 tangential to a second circle having a second diameter 5. The first and second circles are concentric and the first diameter 4 is smaller than the second diameter 5. The first flexible wall portions 2 are arranged alternatively with the second wall portions 3 around the perimeter of the cell holder. The cell tray comprises 12 wall portions with an internal angle of 150º between adjacent wall portions. Also claimed is a battery pack comprising a cell tray.

Description

CELL TRAY

Field of the Invention

The present invention relates to cell trays for use in battery packs and battery packs comprising such cell trays.

Background

Battery packs are used in a wide variety of applications including portable electronics and electric vehicles. A typical battery pack includes a plurality of individual cells connected in series or parallel and a battery management system (BMS) which can monitor parameters of the cells and pack and control the charging and discharging of the cells. The individual cells are typically retained in cell holders on a cell tray. The cell holders are often arranged so that the cells are packed closely together, for example in a square or hexagonal ‘honeycomb’ arrangement. The latter offers the greatest packing density within a given volume. The cells are electrically connected at either end to cell connectors. Typical cell types include prismatic, pouch and cylindrical cells. The cell holders described above may often be used for cylindrical cells, such as 18650 cells for example.

It is advantageous for the cells to be firmly retained in the cell holders as this reduces stresses on welds, cell connectors and other features within the pack, and thus increases the expected mechanical life, and robustness of the battery when subject to the loading and vibration environment of the application for which the battery is designed. The battery pack may also be required to pass UN transportation testing for lithium batteries, which includes vibration and physical shock tests.

Existing cell holders may be rigid, with crush ribs, or flexible. Flexible designs are typically formed of a TPE material with a hardness of approximately 80 ShA, and are designed to provide an interference fit to the cells. Rigid designs usually comprise a series of interference “crush ribs” which typically damage the shrink-wrap on the cells, or are themselves damaged or distorted by insertion of the cells. Other rigid designs use a nest of sufficient dimension to contain the required cells, along with a suitable adhesive to retain the cells in place. However, the use of adhesive is an extra manufacturing step, which may increase manufacturing time and cost.

Flexible cell supports have so far been used in relatively compact battery designs. However the detail of the design is critical and this approach is unlikely to be suitable for a larger battery comprising many cells. In a larger battery errors introduced by the manufacturing tolerances will build-up over a large pack and inevitably lead to variation in overall core-pack dimensions. That is likely to affect the retention of cells in the outer regions of the pack and the tolerances of the location of the cell connectors relative to the cells during assembly.

Many crush-rib designs are in current production and may also provide satisfactory results in small and medium packs where stable production using the same cell type, rating and manufacturer is expected. However, while cylindrical cells are manufactured to tight tolerances, it is known that the diameters of the cells differs between positive and negative ends of the same cell, and significant variability is also observed between cells from different manufacturers. For example, in a recent check, cell diameters of nominally 18 mm diameter cells were observed to range from 17.98 to 18.39mm for different cell manufacturers. That variability of around 0.4mm, or 2.3% of the nominal 18mm diameter, means a change in cell type from one manufacturer to another may cause a cell holder design that previously provided a good fit to not retain the new cell type sufficiently firmly or may make it impossible, at least without causing damage to the cells or cell holder during assembly, to install the new cell type due to interference between the cell and cell holder. That can lead to considerable inconvenience due to certification test failure, necessitating design changes and repeat testing, with accompanying cost and delay.

Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages of the prior art. In particular, preferred embodiments of the present invention seek to provide an improved cell holder for retaining cells in a cell tray.

Summary of Invention

According to a first aspect of the invention, there is provided a cell tray for a battery pack, the cell tray comprising a plurality of cell holders for holding a plurality of cells, each cell holder comprising a plurality of first wall portions tangential to a first circle having a first diameter and a plurality of second wall portions tangential to a second circle having a second diameter, the first and second circles being concentric, the first diameter being smaller than then second diameter and the first wall portions being flexible wall portions.

By providing cell holders having the plurality of first wall portions and the plurality of second wall portions a cell tray may be provided that can accommodate a range of different cell diameters.

Such a cell tray may be advantageous in that a change in cell manufacturer or type can take place without needing to redesign the cell tray. Also the same cell tray design can be used at both ends of the cells, even if the diameters of the positive and negative ends of the cells differ slightly, which reduces the number of unique components to be manufactured and hence reduces cost. The range of cell diameters that can be accommodated depends on the first and second diameters. Any cell that is larger than the first diameter and smaller than the second diameter will be retained in the cell holder as it will cause the first wall portions to flex outwards, thus gripping the cell. For example, the second diameter may be between 0.5% and 15%, preferably between 0.5% and 10% and more preferably between 0.5% and 5% larger than the second diameter. That may provide a sufficient range of variability in the diameter of cells that can be accommodated whilst retaining sufficient lengths of the first wall portions to grip smaller cells firmly.

Preferably the second wall portions are rigid wall portions. In that way the plurality of second wall portions may provide rigidity to the cell tray and assist in maintaining the plurality of cell holders in alignment with each other so that any welded electrical connections between the cells are not strained due to flexing of the cell tray. The rigidity and location of the second wall portions advantageously also defines the spacing of the cells. The first wall portions being flexible and the second wall portions being rigid may be particularly advantageous in that the plurality of second wall portions define the upper limit on cell diameter and provide rigidity to the cell tray and the plurality of first wall portions define the lower limit on cell diameter and grip the cells. Any cells that fit inside the rigid wall portions whilst causing the flexible wall portions to flex outwards will be firmly retained in a rigid arrangement by the cell tray. It will be appreciated that flexible wall portions may be flexible in that they can deform or deflect to permit insertion of a cell into the cell holder, while rigid wall portions may be rigid in that they will not significantly deform or deflect to permit insertion into the cell holder of a cell that would require deformation or deflection of the rigid wall portions for insertion. That may be achieved using different materials, but may also be achieved by using different thicknesses of the same material for the flexible and rigid wall portions.

Preferably the first and second wall portions are straight wall portions. It will be appreciated that there may not necessarily be a sharp corner between adjacent wall portions and that there may instead be a curved section of wall between adjacent wall portions to smooth the transition from one wall portion to the next. In some embodiments the curved sections may be equal or even greater in length than the first and second wall portions and in one embodiment the first and second wall portions have near-zero length so that the holder wall is continuously curved and transitions back and forth around the cell holder between having a radius corresponding to the first wall portions and having a radius corresponding to the second wall portions.

Preferably the plurality of second wall portions are arranged alternately with the plurality of first wall portions around the perimeter of the cell holder. The angle between adjacent wall portions is preferably equal around the cell holder. Alternating the wall portions in this way may advantageously result in a symmetric cell holder that is more readily arranged in a uniform array with the other cell holders on the cell tray. Preferably there are a total (that is, the sum of the number of first and second wall portions) of 6 or more wall portions, more preferably a total of 8 or more wall portions and yet more preferably a total of 12 or more wall portions. That may be a sufficient number of wall portions to provide firm support against components of shock or vibrational forces in all directions in the plane of the cell tray. Preferably there are an equal number of first and second wall portions. For example, in a preferred arrangement of the cell holder there may be 12 wall portions with an internal angle of 150° between adjacent wall portions. Adjacent wall portions may be connected by curved transitions in the cell holder wall. The wall portions alternate between flexible wall portions tangential to the first circle (that is, first wall portions) and rigid wall portions tangential to the second circle (that is, rigid second wall portions). Another way to view that arrangement is that the plurality of first wall portions form part of the perimeter of a hexagon whose sides are tangential to the first circle and the second wall portions form part of the perimeter of a hexagon whose sides are tangential to the second circle. The two hexagons are offset from each other by 30°. Such an arrangement may be particularly advantageous in that the cell holders may then be arranged in a hexagonal close packed arrangement with the rigid wall portions of adjacent cell holders being connected. That may produce a strong ‘honeycomb’ arrangement of the cell holders that can firmly retain cells of varying diameter in the desired packing arrangement. It may be particularly preferable that the second diameter is less than 1.155 times (i.e. 1/cos(30)) the first diameter when such an arrangement is used. If the second diameter is larger than that there is no overlap between the perimeter of the hexagon whose sides are tangential to the first circle and the perimeter of the hexagon whose sides are tangential to the second circle and concave wall portions may be required between the first and second wall portions. The concave portions may form an arch-like structure, thus acting as a stiff spring, and the force required moving them apart when inserting a large cell may be correspondingly large. On the other hand, the cells may then be securely retained by the springing effect once inserted.

Preferably the cell holders are arranged such that points of contact between adjacent cell holders coincide with at least some of the second wall portions. That may be advantageous in that the second wall portions may be rigid wall portions and may therefore provide a more rigid cell tray if they are located at points of contact between adjacent cell holders. However, it may also be advantageous in that the points of contact may lie on the shortest distance between centres of adjacent cell holders and may thus define the interaxial distance of the cell holders. Since the adjacent wall portions at those points may be formed from a single piece of material it may be possible to provide a rigid wall portion for both cell holders at that point without needing twice the thickness of material of a single rigid wall portion. As a result the interaxial distance may be reduced to its minimum feasible value, which increases the efficiency of the packing of the cell holders on the cell tray and thus reduces the overall size of the cell tray for a given number of cells. That may be particularly advantageous since the number of cells is typically determined by the required output of the battery pack but there is often only limited space available in the final product, for example an electric vehicle, into which the battery pack must fit.

Preferably the cell holders are arranged such that the first wall portions are alongside voids between the cell holders. While in some embodiments the flexibility of the first wall portions may be provided by the use of a compressible material, the flexible wall portions are preferably provided by an arrangement in which the flexible wall portions can flex outwards into voids between the cell holders. The thickness of the first wall portions can then be selected so as to provide sufficient resistance to flex that the plurality of first wall portions will grip cells placed into the cell holders, whilst not providing so much resistance that the cells cannot be forced into the cell holders in the first place.

Preferably the cell holders have 12 wall portions with an internal angle of 150° between adjacent wall portions, the wall portions alternating between flexible wall portions tangential to the first circle (that is, first wall portions) and rigid wall portions tangential to the second circle (that is, rigid second wall portions), the cell holders being arranged in a hexagonal close packed arrangement with adjacent cell holders being connected by the rigid wall portions. Preferably there are voids in the interstitial spaces within the hexagonal close packed arrangement and the flexible walls are designed to flex outwards into the voids when a cell is inserted into the cell holder. Thus each cell holder preferably comprises rigid wall portions where it connects with adjacent cell holders in the hexagonal close packed arrangement and flexible wall portions alongside the interstitial voids.

Such an arrangement may result in the optimum use of space in the cell tray and provide for a rigid structural arrangement of the cells whilst being able to accommodate a range of cell diameters.

Preferably the walls have a draft angle. That is, the walls are preferably inclined such that the first and second diameters are slightly larger at the top of the cell holder than at the bottom of the cell holder. It will be appreciated that such an arrangement is typically used in moulded products to permit removal from the mould. It may also be beneficial in that it assists in forcing the cells into the cell holder during assembly of the battery pack. It may be that a chamfer or radius is provided on the upper end of the cell holder walls so as to further assist in inserting the cells into the cell holders during the assembly process.

Preferably the walls of each cell holder, and more preferably the cell tray, are formed from a single moulded piece, for example a moulded piece of plastic material. That may simplify the manufacturing process and reduce the cost of the cell tray.

Preferably the cells are cylindrical cells, such as 18650 cells. Some embodiments may also be suitable for retaining prismatic cells.

According to a second aspect of the invention there is provided a battery pack comprising a cell tray according to the first aspect of the invention and a plurality of cells retained in the plurality of cell holders. Preferably the battery pack comprises two cell trays, with each end of the cells being retained in a cell holder on a respective one of the trays. In that way the cells are firmly sandwiched between the two cell trays with the cells retained at each end in the cell trays.

It will be appreciated that features described in relation to one aspect of the invention may be equally applicable in another aspect of the invention. For example, features described in relation to the battery system of the invention, may be equally applicable to the method of the invention, and vice versa. Some features may not be applicable to, and may be excluded from, particular aspects of the invention.

Description of the Drawings

Embodiments of the present invention will now be described, byway of example, and not in any limitative sense, with reference to the accompanying drawings, of which:

Figure 1 is a plan view of a cell holder according to the invention with construction lines;

Figure 2 is a plan view of part of a cell tray according to the invention;

Figure 3 is a plan view of the cell tray of Figure 2 with cells of a minimum diameter retained in the cell holders;

Figure 4 is a plan view of the cell tray of Figure 2 with cells of a maximum diameter retained in the cell holders;

Figure 5 is a view of a cell tray according to the invention;

Figure 6 is a view of a plurality of cells sandwiched between two cell trays according to the invention; and

Figure 7 is a view of a battery pack according to the invention with the lid open.

Detailed Description

In figure 1 a cell holder 1 has a plurality of first wall portions 2 and a plurality of second wall portions 3. The first wall portions 2 are tangential to a first circle 4. The second wall portions 3 are tangential to a second circle 5. The second circle 5 is concentric with the first circle 4 and has a larger diameter than the first circle 4. The wall portions 2 and 3 alternate around the perimeter of the cell holder 1 with an equal internal angle of around 150° between adjacent wall portions 2 and 3. Thus the plurality of first wall portions 2 form part of the perimeter of a hexagon 6 whose sides are tangential to the first circle 4 and the plurality of second wall portions form part of the perimeter of a hexagon 7 whose sides are tangential to the second circle 5. The hexagon 6 is offset by and angle of 30° to the hexagon 7 and the intersections of the hexagons 6 and 7 define the corners at which the wall portions 2 and 3 join. While the embodiment in figure 1 has sharp corners it will be appreciated that the corners may be rounded, particularly if the product is formed by moulding. The diameters of the first and second circles 4 and 5 are selected based on the cell size range that the cell holder is designed to accommodate. Typically the size range will reflect the variation in cell diameters across different manufacturers and cell types for cells of nominally equal diameters. The diameter of the first circle 4 is selected to be slightly less than the smallest cell diameter to be accommodated and the diameter of the second circle 5 is selected to be slightly larger than the diameter of the largest cell diameter to be accommodated. For example, the diameters of the first and second circles may be determined so that, taking into account manufacturing tolerances in the cell holder 1, cells of the smallest diameter to be accommodated will not be loose in the cell holder I and cells of the largest diameter to be accommodated will fit into the cell holder 1. As an example, for nominally 18 mm diameter cells the diameter of the first circle 4 may be 17.8 mm and the diameter of the second circle 5 may be 18.5 mm.

In figure 2 a part of a cell tray 10 includes four cell holders 11. The cell holders 11 are arranged in a hexagonal close-packed arrangement with voids 18 between the cell holders 11. The cell holders II have a plurality of first wall portions 12 and a plurality of second wall portions 13. The first wall portions 12 are tangential to a first circle (not shown) having a first diameter and the second wall portions 13 are tangential to a second circle (not shown) having a second diameter that is larger than the first diameter. The second wall portions 13 are at the contact points between adjacent cell holders 11 and are rigid. They thus provide strength to the cell tray. The first wall portions 12 are alongside the voids 18 and are flexible. The wall portions 12 and 13 together form a continuous wall to the cell holder 11 and the second wall portions 13 are shared between adjacent cell holders 11.

In figure 3 cells 19 have been inserted into the cell holders 11 of the cell tray 10 of figure 2. The cells 19 are of a minimum size for the cell holders 11. The cells 19 therefore do not contact with the rigid wall portions 13, which are spaced to allow larger cells than the cells 19 to fit into the cell holders 11. However, the cells 19 contact sufficiently with the flexible walls 12 to be gripped by the flexible walls 12 and thus firmly retained in the cell holders 11.

In figure 4 cells 20 have been inserted into the cell holders 11 of the cell tray 10 of figure 2. The cells 20 are of a maximum size for the cell holders 11. The cells 20 therefore contact with the rigid wall portions 13, which are spaced to accommodate them. The cells 20 also contact with the flexible walls 12 and cause the flexible walls 12 to deflect into the voids 18 between the cell holders 11. The flexible walls 12 thus still grip the larger cells 20 and firmly retain them in the cell holders 11.

In figure 5 a cell tray 100 has an array of cell holders 101 with voids 118 between them. In figure 6, the cell tray 100 is filled with cells 121 fitted into the cell holders 101. A second cell tray 100 is provided at the other end of the cells 121 so that the cells 121 are sandwiched between the cell trays 100. In figure 7, the cell trays 100, with the cells 121 sandwiched between them and held firmly in the cell holders 101 are placed in a battery pack 125. The pack is contained within a housing comprising a lid 123 and a base 124 on which one of the cell trays 100 is mounted. On top of the other cell tray 100 circuit boards 122 providing the battery management systems are mounted.

It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only, and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims. For example, cell holders with different numbers of wall portions and arranged in different packing arrangements on the cell tray can be envisaged. For example, the cell holders might have 8 wall portions and be arranged in a square packing arrangement.

Claims

1. A cell tray for a battery pack, the cell tray comprising a plurality of cell holders for holding a plurality of cells, each cell holder comprising a plurality of first wall portions tangential to a first circle having a first diameter and a plurality of second wall portions tangential to a second circle having a second diameter, the first and second circles being concentric, the first diameter being smaller than then second diameter and the first wall portions being flexible wall portions.

2. A cell tray according to claim 1 wherein the second wall portions are rigid wall portions.

3. A cell tray according to any preceding claim wherein the plurality of second wall portions are arranged alternately with the plurality of first wall portions around the perimeter of the cell holder.

4. A cell tray according to any preceding claim wherein there is a curved section of wall between adjacent wall portions.

5. A cell tray according to any preceding claim wherein the angle between adjacent wall portions is equal around the cell holder.

6. A cell tray according to any preceding claim wherein the cell holders comprise a total of 6 or more wall portions.

7. A cell tray according to any preceding claim wherein the cell holders comprise 12 wall portions with an internal angle of 150° between adjacent wall portions.

8. A cell tray according to any preceding claim wherein the cell holders are arranged such that points of contact between adjacent cell holders coincide with at least some of the second wall portions.

9. A cell tray according to any preceding claim wherein the cell holders are arranged such that the first wall portions are alongside voids between the cell holders.

10. A cell tray according to any preceding claim wherein the cell holders have 12 wall portions with an internal angle of 150° between adjacent wall portions, the wall portions alternating between flexible wall portions tangential to the first circle and rigid wall portions tangential to the second circle, the cell holders being arranged in a hexagonal close packed arrangement with adjacent cell holders being connected by the rigid wall portions.

11. A cell tray according to claim 10 wherein there are voids in interstitial spaces within the hexagonal close packed arrangement and the flexible walls are designed to flex outwards into the voids when a cell is inserted into the cell holder.

12. A battery pack comprising a cell tray according to any preceding claim.

13. A cell tray substantially as herein described with reference to the accompanying figures.

14. A battery pack substantially as herein described with reference to the accompanying figures.