PRISMATIC BATTERY CELL LID ASSEMBLY

Abstract

A cover assembly (1) for a battery cell is described, configured to seal, in a fluid-tight manner, an internal compartment (3) of the battery cell and comprising a base plate (4) configured to close an opening (3a) of the internal compartment (3); the base plate (4) is provided with an injection hole (12) to allow the injection of an electrolytic material into the internal compartment (3); the cover assembly (1) further comprises a closure device (8) for sealing the injection hole (12) in a fluid-tight manner; the closure device (8) comprises a closure element (10) coupled to the base plate (4) corresponding to the injection hole (12); the closure element (10) has a substantially flattened configuration and has at least two raised portions (18) arranged on opposite sides of the closure element (10) itself. Main Figure: 3

Description

Title of the invention: COVER ASSEMBLY FOR A PRISMATIC BATTERY CELL technical field

[0001] The present invention relates to a cover assembly for a prismatic battery cell, in particular a cover assembly for closing and sealing in a fluid-tight manner an internal compartment of a prismatic battery cell housing electrodes and an electrolytic material. State of the art

[0002] Battery cells are known, which are used as electrical energy accumulators.

[0003] In particular, the said main batteries are known, which cannot be easily recharged and are therefore intended for single use, or disposable.

[0004] On the other hand, we know of said secondary batteries, which can be recharged several times after each cycle of use.

[0005] Secondary type batteries are known to be used for various small portable electronic devices, such as cell phones, watches, smartwatches, smartbands, etc.

[0006] In addition, batteries comprising a plurality of cells connected in series or in parallel are known, used only in the automotive industry.

[0007] The present invention will refer to a battery cell intended for use in a battery of an electric or hybrid vehicle, without however losing generality.

[0008] Secondary battery cells can be manufactured according to different types and shapes, such as cylindrical or prismatic cells.

[0009] The present invention will refer to prismatic battery cells, without however losing generality.

[0010] A prismatic battery cell typically comprises a hollow prismatic casing (or frame), for example in the shape of a parallelepiped or usually made of a metallic material, and two electrodes (cathode and anode) housed inside the casing, which defines an internal compartment of the cell.

[0011] The internal compartment also houses an electrically interposed separating material between the two electrodes.

[0012] Inside the internal compartment, the electrodes and the separator are immersed in an electrolytic solution, usually liquid or gelatinous.

[0013] The battery cell also includes at least one cover assembly which is coupled to the casing to seal the internal compartment in a fluid-tight manner.

[0014] More specifically, the casing usually has at least one (rectangular) opening, normally located at its apex, through which the electrodes and the separator are inserted into the internal compartment. The lid assembly is coupled to the casing to close this opening, thus sealing the internal compartment.

[0015] The cover assembly typically comprises: a base plate, typically metallic, positioned to coincide with and engage the opening of the envelope, in order to seal the internal compartment; a current collector for each electrode, which is electrically connected to the relevant electrode and configured to collect electrical energy from that electrode and supply it from the internal compartment to the external environment; and a terminal element for each electrode, which is electrically connected to the respective current collector to receive electrical energy from the latter and which is configured to receive a connector from a device intended to be powered by the battery cell.

[0016] For each electrode, the terminal member and the current collector are usually coupled (electrically and mechanically) to each other by means of a pin or rivet, which engages a respective through hole obtained in the base plate.

[0017] In other words, the terminal member is disposed on a first external side of the base plate, while the current collector is disposed on a second internal side of the base plate, opposite the first side.

[0018] In light of the foregoing, the terminal member, the pin, and the current collector together define a terminal group of the cover assembly. The terminal group has a longitudinal axis, and the pin is usually coaxial with such an axis.

[0019] The cover assembly may include two or more such groups of terminals.

[0020] A typical cover assembly also includes a first insulating element for each group of terminals, which is interposed between the relevant terminal element and the base plate to electrically isolate the terminal element from the base plate.

[0021] A typical cover assembly further includes a second insulating element for each group of terminals, which is interposed between the relative current collector and the base plate to electrically isolate the current collector from the base plate.

[0022] During the assembly of the battery cell, the various components mentioned above are stacked axially and compressed together, with respect to the longitudinal axis of the terminal group.

[0023] According to known processes for manufacturing these battery cells, the internal housing is filled with an electrolytic material after assembling the cover assembly to the casing, and in particular after welding the base plate in correspondence with the opening of the casing.

[0024] To this end, the lid assembly is provided with a relatively small through hole, commonly known as the "injection hole" of the electrolyte.

[0025] The injection hole is arranged through the base plate and serves to inject the electrolytic material inside the internal housing, after the cover assembly has been coupled to the casing and fixed to the latter.

[0026] Therefore, the injection hole defines an opening in the base plate.

[0027] After the electrolyte has been injected, the injection hole must be sealed. For this purpose, the cover assembly includes a closing device, which typically comprises two components: a rubber sealing pin; and a cover or closing element, typically defined by a profiled aluminum plate.

[0028] Typically, the injection hole comprises a through hole arranged through the base plate and a recess or indentation arranged in the base plate and surrounding the through hole.

[0029] The sealing pin is usually mounted with an interference fit into the through hole and so the cover is disposed in a suitable seat defined by the recess and welded to the latter, usually to a perimeter edge of lateral delimitation of the recess.

[0030] The cover is thus located above the sealing pin.

[0031] It is known that the sealing pin can leak slightly in the axial direction from the through hole, due to an elastic return of the sealing pin itself during the assembly process of the cover assembly.

[0032] To this end, the cover can be shaped to obtain a raised central portion corresponding to the sealing pin. A space is thus provided between the cover and the base plate corresponding to the through hole, which can be freely engaged by the portion of the sealing pin that might leak through the through hole during use.

[0033] Consequently, the cover (or injection hole closure element) protrudes from the recess, in order to increase the space mentioned above as much as possible.

[0034] There is a need to reduce the dimensions and limit the protrusions of the base plate, due to interference with other cells or other battery components.

[0035] At the same time, it is necessary to avoid complicating the production and assembly process of the cover assembly, by minimizing additional operations and time losses. Object and summary of the invention

[0036] The object of the present invention is to produce a cover assembly for a prismatic battery cell, which is of high reliability and limited cost and makes it possible to remedy at least some of the disadvantages specified above and related to known type cover assemblies.

[0037] According to the invention, this goal is achieved by a cover assembly.

[0038] for a battery cell, the cover assembly being configured to seal fluid-tightly an internal compartment of the battery cell and comprising a base plate configured to close an opening of the internal compartment;

[0039] the base plate being provided with an injection hole to allow the injection of an electrolytic material into the internal compartment,

[0040] the cover assembly further comprising a closure device for sealing the injection hole in a fluid-tight manner, the closure device comprising a closure element coupled to the base plate corresponding to the injection hole;

[0041] wherein the closing element has a substantially flattened configuration and has at least two raised parts arranged on either side of the closing element itself in an opposite manner. Brief description of the drawings

[0042] For a better understanding of the present invention, a preferred, non-limiting embodiment is described in the following section, solely by way of example and with reference to the accompanying drawings, in which: [Fig.1] Fig.1 is a schematic perspective and exploded view, with parts removed for clarity, of a battery cell comprising a cover assembly according to the present invention; [Fig.2a] The [Fig.2a] is a top view, on a larger scale and with parts removed for clarity, of a closing element of an electrolytic material injection hole of the cover assembly; [Fig. 2b] Fig. 2b is a perspective view of the closing element of Fig. 2a; and [Fig.3] The [Fig.3] is a cross-sectional view, with parts removed for clarity, of an area of ​​the injection hole illustrating the injection hole and an injection hole closure device. Detailed description

[0043] With reference to the attached figures, a cover assembly for a battery cell is indicated, as a whole, by the reference number 1.

[0044] In particular, the battery cell is of the prismatic type and has a hollow prismatic envelope 2 which internally defines an internal compartment 3 and which has an opening 3a for accessing the internal compartment 3.

[0045] According to the illustrated embodiment, the casing 2 has a parallelepiped shape. Alternatively, the casing 2 could have any prismatic shape, for example cubic, triangular-based, etc.

[0046] In one embodiment, the casing 2 could be cylindrical. In such a case, the cover assembly 1 has a circular configuration.

[0047] The present invention will refer to a prismatic battery cell, without however losing generality.

[0048] In particular, the present invention will refer to a battery cell intended for use in the automotive industry, for example in a battery module of an electric or hybrid vehicle, without however losing generality.

[0049] The battery cell further comprises electrodes (cathode and anode, not illustrated), which are housed in the internal compartment 3 together with a separator material (not illustrated) and an electrolytic material (not illustrated).

[0050] The electrolytic material can be liquid or gelatinous.

[0051] The electrodes and the separator material are inserted into the internal compartment 3 through the opening 3a.

[0052] Therefore, the cover assembly 1 is configured to be coupled to the casing 2 in correspondence with the opening 3a to close, in a fluid-tight manner, the internal compartment 3, as required during normal use of the battery cell.

[0053] Therefore, the battery cell comprises the casing 2, with the electrodes, the separator and the electrolytic solution, and the cover assembly 1.

[0054] The battery cell preferably forms part of a battery module comprising a plurality of battery cells electrically connected to each other in parallel.

[0055] The casing 2 may include another opening 3a opposite the opening 3a mentioned above, as shown in [Fig. 1]. Another cover assembly 1 must be coupled to the casing 2 in order to close, in a fluid-tight manner, the other opening 3a.

[0056] In the following section, for the purpose of synthesis, reference will be made to a single lid assembly 1.

[0057] As can be seen in [Fig.1], the cover assembly 1 includes a base plate 4, made of a metallic material, such as aluminum or stainless steel, configured to be coupled to the casing 2 in correspondence with the opening 3a to close the opening 3a of the internal compartment 3, in other words compatible with the shape of the opening 3a, and to engage with the latter, to close the internal compartment 3.

[0058] The cover assembly 1 further includes at least one group of terminals 50 and in particular two groups of terminals 50, configured to be electrically connected, corresponding to its internal side 5, to a respective electrode inside the internal compartment 3 to collect electrical energy from the latter and configured to be electrically coupled, corresponding to its external side 6, to a connector of a device (not illustrated) intended to be powered by the battery cell.

[0059] In particular, each terminal group 50 has a longitudinal axis X and comprises: a current collector 5 which defines the aforementioned inner side of the terminal group 50 and is configured to be electrically connected to the respective electrode; a terminal member 6 which defines the aforementioned outer side of the terminal group 50 and is configured to be electrically coupled with the aforementioned connector; and a pin 5b which is axially interposed between the current collector 5 and the terminal member 6 and electrically connects the latter to each other.

[0060] More particularly, the pin 5b is welded, in correspondence with its axial side, to the terminal member 6 and in correspondence with its opposite axial side, to the current collector 5.

[0061] Preferably, the pin 5b is substantially cylindrical and is arranged coaxially with respect to the X axis.

[0062] The terminal group 50 extends along the axial direction of the internal compartment 3 to an external environment, external with respect to the internal compartment 3.

[0063] In detail, the terminal member 6 is arranged in correspondence to an external side of the base plate 4 (i.e. the side facing the external environment), in order to be connected to the connector and the collector 5 is arranged in correspondence to an internal side of the base plate 4 (i.e. the side facing the internal compartment 3) in order to be connected to the respective electrode.

[0064] Conveniently, the base plate 4 has a through hole 4a, which is engaged by the pin 5b.

[0065] The pin 5b engages the terminal member 6 to establish the electrical connection between the collector 5 and the terminal member 6.

[0066] In light of what is set forth above, the current collector 5 and the terminal member 6 are arranged on opposite sides of the base plate 4, with respect to the axial direction.

[0067] The cover assembly 1 may comprise two or more groups of terminals 50. Figure 1 illustrates two groups of terminals 50. In the following section, for the sake of brevity, only one group of terminals 50 will be considered.

[0068] Preferably, the cover assembly 1 further includes an insulating element 7 interposed axially between the terminal element 6 and the base plate 4 to electrically isolate the terminal element 6 from the base plate 4. Conveniently, the insulating element 7 is made of an electrically insulating material, such as PPS (polyphenylene sulfide) or PP (polypropylene).

[0069] The cover assembly 1 may include sealing elements, for example an O-ring, interposed between the base plate 4 and the terminal group 50, and configured to seal the internal compartment 3 from the external environment in a fluid-tight manner.

[0070] Preferably, the cover assembly 1 further comprises a stop frame 11 made of an electrically insulating material (such as rigid plastic), known per se and not described in detail.

[0071] The base plate 4 has an internal surface 14 configured to be turned towards the compartment 3 when the base plate 4 is coupled to the envelope 2 (i.e. when the cover assembly 1 is assembled with the envelope 2), and an external surface 15 opposite to the internal surface 14 and configured to be turned outwards with respect to the compartment 3.

[0072] The cover assembly 1 further comprises: an injection hole 12 to allow the injection (or insertion) of the electrolytic material into the internal compartment; and a closure device 8 to seal the injection hole in a fluid-tight manner.

[0073] In practice, the injection hole 12 defines an opening through which the electrolytic material is inserted into the internal compartment 3, once the cover assembly 1 is coupled to the envelope 2, according to a known modality and not described in detail.

[0074] The closure device 8 includes a closure element 10, preferably defined by a cover or a plate, preferably made of a metallic material, for example aluminium.

[0075] The closing element 10 is coupled to the base plate in correspondence with the injection hole 12.

[0076] In detail, the closing element 10 covers the injection hole 12, as better explained in the following part.

[0077] Preferably, the closing element 10 is defined by a discoidal body with a substantially circular profile.

[0078] In other words, the closing element 10 is defined, according to this preferred non-limiting embodiment, by a circular discoidal plate.

[0079] In the following section, the closing element 10 could be referred to as "the cover 10".

[0080] Preferably, the closure device 8 further includes a sealing pin 13 which makes the injection hole 12 sealed against the fluid.

[0081] The sealing pin 13 is of the known type and is not described in detail.

[0082] The injection hole 12 has a central axis A and includes, proceeding in the radial direction from the inside out, at least one through hole 16 and a first recess 17 surrounding the through hole 16.

[0083] The through hole 16 defines the fluid communication opening with compartment 3 through which the electrolytic material is inserted.

[0084] The sealing pin 13 engages the through hole 16 in a fluid-tight manner, in order to prevent leakage of the electrolytic material or the entry of foreign bodies into the compartment 3.

[0085] The cover 10 surmounts the through hole 16 and the first recess 17 and is coupled to the base plate in order to completely cover the first recess 17 (and therefore also the through hole 16).

[0086] According to one aspect of the present invention, the cover 10 has a substantially flattened configuration and has at least two raised parts 18 arranged from opposite parts of the cover 10 itself.

[0087] In particular, the cover 10 comprises a main (or central) part 10a and a peripheral part 10b, and the raised parts 18 project orthogonally from the main part 10a, thus defining bosses 18 projecting from the main part 10a.

[0088] More specifically, the cover 10 has a first surface 19a oriented towards the injection hole 12, and in particular towards the first recess 17, and a second surface 19b opposite the first surface 19a. The cover 10 includes a first boss 18 disposed on the first surface 19a and a second boss 18 disposed on the second surface 19b.

[0089] In this way, the cover 10 includes at least one boss 18 projecting from each of the surfaces 19a, 19b.

[0090] The presence of the bosses 18, arranged on opposite parts of the lid 10 and in particular each arranged on a respective axial surface 19a, 19b of the lid 10, makes it possible to improve and simplify considerably the production process of the lid assembly 1: in fact, the lids 10 are produced in series and once made, are often accumulated in a container, for example a hopper, while waiting to be packed with the respective base plates 4.

[0091] The bosses 18 significantly limit, and in particular completely eliminate, the risk of unwanted adhesion between the axial surfaces of the two adjacent lids 10 or which are accidentally in contact with each other in this hopper.

[0092] In other words, the bosses 18 act as anti-sticking elements between the lids 10, during production.

[0093] In this way, the loss of time and cost of detaching two or more covers that are glued together, before their assembly to the respective base plates 4, is avoided.

[0094] Advantageously, each boss 18 projects orthogonally from the main part 10a over a distance corresponding to 20-40%, preferably 25-35%, even more preferably 30% of the thickness of the main part 10a.

[0095] For example, if the main part 10a has a thickness of 0.5 mm, each boss 18 protrudes from the latter by a distance equal to 0.15 mm.

[0096] The applicant observed how these dimensions provide the optimal compromise between the anti-stick effect and the dimensions. In fact, thanks to these reduced dimensions, the lid 10 is positioned almost flush with the external surface 15 of the base plate 4.

[0097] Preferably, each boss 18 has a substantially circular shape.

[0098] This shape is the simplest to produce during the production process. Therefore, the production of the bosses 18 does not significantly impact the total production and assembly time of a lid assembly 1.

[0099] The cover 10 has a central axis B, coaxial with the axis A of the injection hole 12, when the cover 10 is mounted on the base plate.

[0100] Advantageously, the first boss 18 and the second boss 18 are arranged eccentrically on either side of the axis B in opposite directions.

[0101] The applicant observed how this “symmetrical” arrangement further promotes the aforementioned non-sticking with the other lids 10, during production.

[0102] In an embodiment not illustrated, the bosses 18 might not be arranged symmetrically with respect to axis B.

[0103] In an embodiment not illustrated, the bosses 18 could be arranged concentrically to axis B.

[0104] As can be seen in Figures 2a, 2b and 3, the cover 10 comprises: a first recess 20 disposed on said first surface 19a and defining, on the second surface 19b, the second boss 18, and a second recess 21 arranged on said second surface 19b and defining, on the first surface 19a, the first boss 18.

[0105] Appropriately, the first boss 18 and the first recess 20 are arranged on the first surface 19a eccentrically and on either side of the central axis B in a symmetrically opposite manner; similarly, the second boss 18 and the second recess 21 are arranged on the second surface 19b eccentrically and on either side of the central axis B in a symmetrically opposite manner.

[0106] Ideally, the injection hole 12 includes, on the other hand, a second recess 22 surrounding the first recess 17.

[0107] The through hole 16, the first recess 17 and the second recess 22 are arranged in the base plate 4.

[0108] As shown in [Fig.3], the first recess 17 penetrates deeper into the base plate 4 compared to the second recess 22.

[0109] Ideally, the cover 10 is coupled to the second recess 22 and engages the second recess 22, which is mounted on top of the first recess 17.

[0110] In this way, the space 24 defined between the surface 17a of the cover 10 and the first recess 17 is increased, so that the sealing pin 13 is free to engage there, in the event that it should partially come out of the through hole 16 during use. This reduces the stresses and strains on the sealing pin 13, ensuring proper fluid sealing during use.

[0111] Advantageously, the peripheral part 10b of the cover 10 is welded to a base surface 23, in particular a lower surface facing the external environment, of the second recess 22.

[0112] The applicant observed how this weld on the base surface 23 of the second recess 22, rather than for example on the lateral surface of the latter, allows greater resistance of the seal and of the weld to the thermal and mechanical stresses to which the cover 10 is subjected during use.

[0113] Advantageously, the cover 10 engages the second recess 22 so as to be substantially flush with the external surface 15 of the base plate 4.

[0114] Therefore, the presence of the bosses 18 makes it possible to obtain and use a flat lid 10, thus reducing the dimensions and unwanted protrusions of the base plate 4, considerably limiting the loss of time during production, in that the lids 10, during production and / or storage, do not stick together.

[0115] According to an examination of the characteristics of the cover assembly 1 made according to the present invention, the advantages which the latter makes it possible to obtain are clearly apparent.

[0116] In particular, the presence of the bosses 18, arranged on either side of the cover 10 in an opposite manner, and in particular arranged each on a respective axial surface 19a, 19b of the cover 10, makes it possible to improve and simplify considerably the production process of the cover assembly 1: in fact, the covers 10 are produced in series and once made, are often accumulated in a container, for example a hopper, while waiting to be assembled to the respective base plates 4.

[0117] The bosses 18 significantly limit, and in particular completely eliminate, the risk of unwanted adhesion between the axial surfaces of two covers 10 which are accidentally in contact with each other in such a hopper.

[0118] In other words, the bosses 18 act as anti-sticking elements between the lids 10, during production.

[0119] In this way, the loss of time and cost of detaching two or more lids from each other before assembly is avoided.

[0120] At the same time, this allows the use of a substantially flat cover 10 to close the injection hole 12, so that the cover 10 is substantially flush with the base plate 4 and thus limiting to a maximum the unwanted protrusions and indentations by the latter.

[0121] Furthermore, this limitation, and in particular the significant elimination of unwanted protrusions or indentations (and thus sharp angles) on the external surface 15 of the base plate 4, makes it possible to achieve another important technical effect: in some cases, this external surface 15 (and often the entire battery cell) is covered (except for the terminals 6) with a film of electrically insulating material; in the case of known type closures or covers, which have one or more significant protrusions or indentations relative to the external surface 15, thus defining sharp angles, this insulating film could be easily torn or damaged by such sharp angles. This has been observed in particular in the case where the battery cells are liquid-cooled, due to pressure oscillations or pulsations of the liquid itself.

[0122] The substantially flattened configuration and the minimally limited protrusions and indentations of the lid 10 according to the present invention makes it possible to limit or to avoid damaging the insulating film, thereby reducing the risk of short circuits and increasing the useful life of the battery cell.

[0123] It is clear that modifications and variants can be made to the cover assembly 1 described and illustrated herein, without going out of the scope of protection defined by the claims.

Claims

Demands

1. A cover assembly (1) for a battery cell, the cover assembly (1) being configured to seal fluid-tightly an internal compartment (3) of the battery cell and comprising a base plate (4) configured to close an opening (3a) of the internal compartment (3); the base plate (4) being provided with an injection hole (12) for allowing the injection of an electrolytic material into the internal compartment (3), the cover assembly (1) further comprising a closure device (8) for sealing the injection hole (12) fluid-tightly, the closure device (8) comprising a closure element (10) coupled to the base plate (4) corresponding to the injection hole (12); wherein the closure element (10) has a substantially flattened configuration and has at least two raised portions (18) arranged oppositely on either side of the closure element (10) itself.

2. Lid assembly (1) according to claim 1, wherein the closure element (10) comprises a main part (10a) and wherein said raised parts (18) project orthogonally from the main part (10a), thus defining bosses (18) projecting from the main part (10a).

3. Cover assembly (1) according to claim 2, wherein each boss (18) projects orthogonally from the main part (10a) over a distance corresponding to 20-40%, preferably 25-35%, still preferably 30%, of the thickness of the main part (10a).

4. Cover assembly (1) according to claim 2 or 3, wherein the closure element (10) has a first surface (19a) oriented towards the injection hole (12) and a second surface (19b) opposite the first surface (19a); the closure element (10) comprising a first boss (18) disposed on the first surface (19a) and a second boss (18) disposed on the second surface (19b).

5. Lid assembly (1) according to claim 4, wherein the closing element (10) has a central axis (B); and wherein the first boss (18) and second boss (18) are arranged eccentrically on either side of the central axis (B) in opposite ways.

6. Lid assembly (1) according to claim 4 or 5, wherein the closing element (10) comprises: a first recess (20) disposed on said first surface (19a) and defining, on said second surface (19b), the second boss (18), and a second recess (21) disposed on said second surface (19b) and defining, on said first surface (19a), the first boss (18).

7. Lid assembly (1) according to claims 5 and 6, wherein the first boss (18) and the first recess (20) are arranged on the first surface (19a) in an eccentric and symmetrically opposite manner on either side of the central axis (B); and wherein the second boss (18) and the second recess (21) are arranged on the second surface (19b) in an eccentric and symmetrically opposite manner on either side of the central axis (B).

8. Cover assembly (1) according to any one of the preceding claims, wherein the injection hole (12) has a central axis (A) and comprises, proceeding in the radial direction from the inside out, a through hole (16), a first recess (17) surrounding the through hole (16) and a second recess (22) surrounding the first recess (17); wherein the first recess (17) penetrates axially deeper into the base plate (4) relative to the second recess (22); wherein the closure element (10) has a central part (10a) and a peripheral part (10b); and wherein the peripheral part (10b) is welded to a base surface (23) of the second recess (22).

9. Lid assembly (1) according to claim 8, wherein the base plate (4) has an external surface (15); and wherein the closing element (10) engages the second recess (22) so as to be substantially flush with the external surface (15) of the base plate (4).

10. Lid assembly (1) according to any one of the preceding claims, wherein each raised part (18) has a substantially circular shape, and / or in which the closing element (10) is defined by a discoidal body with a substantially circular profile, and / or wherein the closure device (8) further comprises a sealing pin (13) which engages the injection hole (12) in a fluid-tight manner, and / or wherein the cover assembly (1) further comprises at least one group of terminals (50) configured to be electrically connected, corresponding to its internal side (5), to a respective electrode inside the internal compartment (3) for collecting electrical energy from the latter and configured to be electrically coupled, corresponding to its external side (6), to a connector of a device intended to be powered by the battery cell.