Laser welding systems and methods

Abstract

In the laser welding of busbar connection tabs to electrical battery cells, a first laser 122 can weld busbar connection tabs for the positive terminals of the cells; while a second laser 124 can weld

Description

TECHNICAL FIELD The present disclosure relates to laser welding systems and methods. Aspects of the 5 invention relate to a laser welding system, a laser welding method, a controller, a computer program, a non-transitory computer readable storage medium and a signal. BACKGROUND When welding busbar connection tabs to terminals at opposite ends of electrical cells, it is 10 typically necessary to rotate the electrical cell array and busbar assembly (e.g. through 180°) between welding operations on the different terminals in order to facilitate welding to occur to both ends of the electrical cells by a welding laser. Nonetheless, the rotation process takes time, adds complexity to the system and increases opportunities for uncertainties in the absolute positioning of the cell array and / or busbar assemblies. 15 It is an object of embodiments of the invention to at least mitigate one or more of the CO problems of the prior art. CM CO SUMMARY OF THE INVENTION 20 Aspects and embodiments of the invention provide a laser welding system, a laser welding £\J method, a controller, a computer program, a non-transitory computer readable storage CM medium and a signal as claimed in the appended claims. According to an aspect of the invention there is provided a laser welding system comprising 25 a first welding laser and a second welding laser, the system being arranged to: i) perform a first welding operation at a first process site using a first welding laser beam from the first welding laser, where the first process site comprises a first surface of an article towards which the first laser beam is incident in a first direction; 30 ii) perform a second welding operation at a second process site using a second welding laser beam from the second welding laser, where the second process site comprises a second surface of the article towards which the second laser beam is incident in a second direction, wherein the first and second directions are substantially opposite and substantially 35 horizontal. According to another aspect of the invention there is provided a laser welding system comprising a first welding laser and a second welding laser, the system being arranged to: perform a first welding operation at a first process site using a first welding laser beam from the first welding laser and perform a second welding operation at a 5 second process site using a second welding laser beam from the second welding laser. According to yet another aspect of the invention there is provided a laser welding system comprising a first welding laser and a second welding laser, the system being arranged to: 10 perform a first welding operation at a first process site using a first welding laser beam from the first welding laser and perform a second welding operation at a second process site using a second welding laser beam from the second welding laser, where the first process site comprises a connection tab of a first busbar and a positive terminal of an electrical cell at a first end of the electrical cell and the first 15 welding laser beam is directed towards the first process site in a first direction, and where further the second process site comprises a connection tab of a second CO busbar and a negative terminal of the electrical cell at a second end of the electrical CM cell and the second welding laser beam is directed towards the second process site CQ in a second direction, the first and second directions being substantially opposite one 20 another. CM CM Rotation of an electrical cell (e.g. through 180 degrees) to facilitate welding of a connection tab to each terminal may be undesirable. Specifically, it may require additional time to perform both welding operations and / or may be more likely to lead to 25 inaccuracies / uncertainty in the positioning of the electrical cell and therefore welding errors / failures. Equally, providing a single welding laser capable of repositioning and / or articulation sufficient to allow it to perform both welding operations without adjusting the electrical cell may increase complexity and potential failure modes of the welding laser. Consequently, the system described may increase welding rate and / or reliability. 30 The laser welding system is arranged to perform the first welding operation and the second welding operation simultaneously. By performing the welding operations simultaneously (be that starting and completing the welding operations at substantially the same times or there simply being some point in time when both welding operations are in progress) the overall 35 time required to complete both welding operations may be reduced. Simultaneous welding may be facilitated by the provision of the two appropriately arranged welding lasers. Further, it may be considered that laser welding lends itself to simultaneous welding on an electrical cell in a way that electrical resistance welding does not (the latter giving rise to the risk of creating a voltage across the electrical cell). In some embodiments the first and second ends face in substantially opposite directions. 5 In some embodiments the first and second directions are substantially horizontal. Especially where it is desired to weld both terminals located on opposite ends simultaneously, substantially horizontally propagating welding laser beams may represent a suitable compromise when considering the incidence of welding sputter onto the welding lasers. 10 Were for instance one welding laser beam to be incident from below, with the corresponding welding laser correspondingly positioned, and the other from above, the welding laser positioned below might be rapidly rendered inoperable in view of the effects of gravity in directing welding sputter at that welding laser. As will be appreciated, the electrical cell might also be oriented substantially horizontally to complement the directions of the welding laser 15 beams as described. In some embodiments the electrical cell is arranged such that respective surfaces of the positive and negative terminals towards which the first and second welding laser beams are respectively directed are arranged substantially vertically. Thus, it may be for instance that the first welding laser is arranged to deliver the first laser welding beam incident in a direction substantially normal to the surface to be welded of the positive terminal and that the second welding laser is arranged to deliver the second laser welding beam incident in a direction substantially normal to the surface to be welded of the negative terminal. 25 In some embodiments the connection tab of the first busbar overlays the positive terminal of the electrical cell such that the first welding laser beam is incident on the connection tab of the first busbar prior to the positive terminal and the connection tab of the second busbar overlays the negative terminal of the electrical cell such that the second welding laser beam is incident on the connection tab of the second busbar prior to the negative terminal. Thus, it 30 may be that a surface of each connection tab opposed to a further surface thereof on which the relevant welding laser beam is first incident, faces and / or is in contact with the relevant terminal of the electrical cell. Further, it may be that each connection tab at least partially conceals a line of sight from the relevant welding laser to the relevant terminal of the electrical cell. 35 In some embodiments the laser welding system comprises a jig or platform arranged to support the electrical cell for the first and second welding operations. The jig or platform may at least contribute to securing the electrical cell in a fixed location and / or orientation for performance of the first and second welding operations. The fixed location and / or orientation may be predetermined or may be determined by measurement once the electrical cell is installed in / on the jig or platform. In some embodiments the laser welding system comprises 5 a clamping assembly comprising a first clamping sub-assembly arranged to clamp the connection tab of the first busbar to the positive terminal and a second clamping subassembly arranged to clamp the connection tab of the second busbar to the negative terminal. The clamping may tend to mean that there is adequate contact between the relevant connection tab and its corresponding terminal to facilitate welding or at least 10 welding of improved quality. The clamping may also contribute to locating and / or orienting the electrical cell and / or connection tabs to position for the first and / or second welding operations. The jig or platform may also support the connection tab of the first busbar and / or the connection tab of the second busbar at least prior to clamping. 15 In some embodiments the clamping assembly is arranged to be passively repositionable to allow it to adopt a location at which clamping forces applied in opposite directions CO respectively by the first clamping sub-assembly and the second clamping sub-assembly to CM the electrical cell supported by the welding jig are substantially in equilibrium as experienced CQ by the electrical cell regardless of the position of the electrical cell as supported by the 20 welding jig within a range of possible positions. It may be challenging to reliably present the CM electrical cell and / or the connection tabs of the first and second busbars and / or the first and CM second clamping sub-assemblies in precisely predetermined positions prior to clamping. Further, it may be challenging to reliably reproduce a precisely predetermined clamping action and strength for both of the first and second clamping sub-assemblies (e.g. due to 25 play caused by wear). Such difficulties may be exacerbated where, as here, welding operations are to be performed at both ends of the electrical cell. Specifically, it may be challenging to provide a support structure fixed in space which can provide a reaction force to match the clamping forces applied by the clamping assembly to clamp the respective connection tabs of the first and second busbars to the respective positive and negative 30 terminals of the electrical cell. That is, access is required to both ends of the electrical cell in order to perform the welding operations and so there may be a tendency for a support structure providing a fixed datum to one end or other of the electrical cell to cause an impediment. Where precise positioning and / or clamping actions cannot be guaranteed, there may be a risk that the forces applied to the respective connection tabs of the first and 35 second busbars may be inconsistent, potentially leading to welding quality inconsistencies and potentially, weld failure. By allowing the first and second clamping sub-assemblies to float as a unit in the axis in which they apply their clamping forces as they are moved towards each other to provide clamping, it may be possible to ensure consistent clamping force applied to each of the connection tabs. In some embodiments the clamping assembly comprises a mobile support on which the first 5 clamping sub-assembly is mounted via a first translating mount and the second subassembly is mounted via a second translating mount, each of the first clamping subassembly and the second clamping sub-assembly being selectively repositionable towards the other via the respective one of the first and second translating mounts to clamp the electrical cell and the connection tabs of the first and second busbars therebetween, and 10 where further the mobile mount is mounted via a third translating mount to a fixed support, the clamping assembly being passively repositionable with respect to the fixed mount via the third translating mount to a location where clamping forces applied in opposite directions respectively by the first clamping sub-assembly and the second clamping sub-assembly to the electrical cell supported by the welding jig are substantially in equilibrium as experienced 15 by the electrical cell regardless of the position of the electrical cell as supported by the welding jig within a range of possible positions. The translation permitted by each of the first, CO second and third translating mounts may be back and forth in a common direction. CM CQ In some embodiments the laser welding system is arranged to deliver an air displacing fluid 20 around the first process site whilst performing the first welding operation and around the C\j second process site whilst performing the second welding operation. This may for instance CM be delivered via the clamping assembly. The air displacing fluid may for instance be an inert gas such as argon. Displacing the air at the welding site may be desirable as the presence of air may lead to oxidation at the weld site. 25 In some embodiments the air displacing fluid is delivered at the first process site substantially throughout the first welding operation, and at the second process site substantially throughout the second welding operation. Especially where the electrical cell is positioned horizontally, it may be desirable to deliver the air displacing fluid throughout the 30 respective welding operations. This is because the air displacing fluid may tend not to pool and remain resident for an extended period at the process site as it might tend to do if the electrical cell were positioned vertically and the air displacing fluid were delivered from above to a connection tab surface facing upwards. Rather, the air displacing fluid may tend to flow away under the influence of gravity and so constant replacement may be desirable. 35 In some embodiments the delivery of air displacing fluid for each of the first and second welding operations is performed in a continuous and uniform manner. Such management of the delivery may be desirable. It may be for instance that discontinuous flow, non-uniform flow or too faster flow may create turbulence tending to mix air back in around the process site. On the other hand, sufficient rate of flow may be required to force air away from the process site. In some embodiments at least one of the first and second welding lasers comprises a cover glass through which the relevant first or second welding laser beam is delivered. In some embodiments the laser welding system is arranged to deliver an air blade across 10 and in front of the cover glass. The distance between the cover glass and relevant process site may be between approximately 330-350mm. The cover glass may therefore be susceptible to being obscured by welding sputter. The quantity of welding sputter incident on the glass may also be increased where for instance the electrical cell is oriented horizontally and the relevant welding laser is positioned to deliver the relevant welding laser beam in a 15 horizontal direction. Provision of the air blade may serve to deflect at least a proportion of welding sputter which would otherwise fall on the cover glass. CO CM in some embodiments the laser welding system is arranged to deliver the air blade such that CQ it's substantially parallel to the cover glass. ^^20 CM In some embodiments the air blade is delivered at between substantially 4 and 6bar and CM from a slot outlet of between substantially 600 and 900microns across its smaller dimension. More specifically the air blade may be delivered at substantially 5 bar and from a slot outlet of substantially 750microns across its smaller dimension. Such pressures and dimensions 25 may be desirable in terms of creating sufficient flow rate to deflect a significant proportion of welding sputter without inducing significant turbulence in the flow which might diminish its effectiveness. The blade may also be delivered in a continuous manner. In some embodiments the laser welding system comprises a shrouding plate arranged in 30 front of the cover glass and having an orifice therethrough to allow delivery of the relevant first or second welding laser beam beyond the shrouding plate. The shrouding plate may deflect at least a proportion of welding sputter which would otherwise fall on the cover glass. In some embodiments the shrouding plate is arranged to shroud, from a direction towards 35 and substantially normal to the cover glass, peripheral areas of the cover glass surrounding a region through which the relevant first or second welding laser beam is delivered. Thus, the shrouding plate may be considered to reduce the area of the cover glass exposed to welding sputter travelling in a direction normal to the cover glass to an area including a working area of the cover glass, or to the working area itself. Specifically, it may be only, or little more than the area of the cover glass through which the relevant welding laser beam passes which is not shrouded by the shrouding plate from a direction towards and 5 substantially normal to the cover glass. The shrouding plate may for instance be approximately 30cm from the cover glass. In some embodiments the shrouding plate is attached to the relevant first or second welding laser by at least one side plate creating a substantially enclosed volume between the 10 shrouding plate and the cover glass. The one or more side plates may further reduce the incidence of welding sputter reaching the cover glass and / or may support the shrouding plate. In some embodiments the relevant first and / or second welding laser each comprise a 15 selectively removable and replaceable head portion comprising the cover glass and shrouding plate. Where the cover glass and / or shrouding plate have excessive welding CO sputter build-up, they may be replaced as one as part of the head portion. In some embodiments the laser welding system is arranged to deliver the air blade between the cover glass and the shrouding plate. In this case, the shrouding plate may be considered to protect the air blade and cover glass unless the welding sputter passes through the area of the orifice in the shrouding plate facilitating transmission of the relevant welding laser beam therethrough. Where welding sputter passes the shrouding plate in this way, the air blade may be considered to provide a final line of defence for the cover glass. 25 In some embodiments the first and / or second welding laser is arranged to decrease welding sputter generation by delivering the relevant first or second welding laser beam so that it is partially defocused at the relevant first or second process site and to adjust the relevant first or second welding laser beam power to compensate for the defocusing. 30 In some embodiments the electrical cell is part of an array of similar electrical cells similarly oriented and the laser welding system is arranged to perform instances of the first welding operation on the respective one of the positive and negative terminals of each electrical cell in the array and instances of the second welding operation on the respective other of the 35 positive and negative terminals of each electrical cell in the array. It may be for instance that the first welding laser may perform a sequence of first welding operations on successive positive terminals of the electrical cells in the array and the second welding laser may perform a sequence of second welding operation on successive negative terminals of the electrical cells in the array. The respective connection tabs welded by the first welding operations to the positive terminals of the electrical cells in the array may all be of the first busbar and the respective connection tabs welded by the second welding operations to the 5 negative terminals of the electrical cells of the array may all be of the second busbar. The electrical cells of the array may be secured relative to each other prior to the performance of the first and second welding operations. Each may for instance be provided in a common support structure and / or or may be directly attached to one or more other of the electrical cells in the array (e.g. with adhesive). The jig or platform may support, position and / or 10 orientate the array as a whole and the clamping assembly may clamp all of the connection tabs to their respective terminals simultaneously for all of the electrical cells in the array, e.g. with an array of clamping formations on each clamping sub-assembly. The array may comprise a battery assembly or supercell (e.g. for an electric vehicle). Positioning the array for welding and then performing the first and second welding operations for all of the 15 electrical cells in the array may be efficient in terms of processing time and convenient in terms of producing a battery assembly or supercell. Anything discussed previously in respect CO of an individual electrical cell may be applied to the other electrical cells of the array and / or CM to the array as a whole mutatis mutandis. ¢^20 In some embodiments the laser welding system is arranged to perform instances of the first welding operation and the second welding operation on the array of similar electrical cells with at least some overlap in time of performance of the first welding operations and performance of the second welding operations. It may be for instance that the welding operations are performed on each cell at the same time, or instances of the first and second 25 welding operations are performed simultaneously but on different electrical cells or simply that at least one first welding operation is at least started within the array before all second welding operations on the array are completed or vice versa. According to a further aspect of the invention there is provided a laser welding method 30 comprising: performing a first welding operation at a first process site using a first welding laser beam from a first welding laser and performing a second welding operation at a second process site using a second welding laser, where the first process site comprises a connection tab of a first busbar and a positive terminal of an electrical 35 cell at a first end of the electrical cell and the method comprises directing the first welding laser beam towards the first process site in a first direction, and where further the second process site comprises a connection tab of a second busbar and a negative terminal of the electrical cell at a second end of the electrical cell and the method comprises directing the second welding laser beam towards the second process site in a second direction, the first and second directions being substantially opposite one another. 5 The method comprises performing the first welding operation and the second welding operation simultaneously. In some embodiments the method comprises performing the first welding operation and the 10 second welding operation such that the first and second directions are substantially horizontal. In some embodiments the method comprises arranging the electrical cell such that respective surfaces of the positive and negative terminals towards which the first and second 15 welding laser beams are respectively directed are arranged substantially vertically. In some embodiments the method comprises supporting the electrical cell for the first and second welding operations and clamping the connection tab of the first busbar to the positive terminal and clamping the connection tab of the second busbar to the negative terminal. In some embodiments the method comprises delivering an air displacing fluid around the first process site whilst performing the first welding operation and around the second process site whilst performing the second welding operation. 25 In some embodiments at least one of the first and second welding lasers comprises a cover glass through which the relevant first or second welding laser beam is delivered and the method comprises delivering an air blade across and in front of the cover glass. In some embodiments the method comprises shrouding at least a part of the cover glass 30 other than an area thereof allowing delivery of the relevant first or second welding laser beam. In some embodiments the method comprises delivering the relevant first or second welding beam so that it is partially defocused at the relevant first or second process site in order to 35 decrease welding sputter generation and adjust the relevant first or second welding laser beam power to compensate for the defocusing. In some embodiments the method comprises performing instances of the first welding operation on the respective one of the positive and negative terminals of each electrical cell in an array of similar electrical cells similarly oriented and performing instances of the second welding operation on the respective other of the positive and negative terminals of 5 each electrical cell in the array. In some embodiments the method performing instances of the first welding operation and the second welding operation on the array of similar electrical cells with at least some overlap in time of performance of the first welding operations and performance of the second 10 welding operations. According to a still further aspect of the invention there is provided a controller arranged to perform the method described above. 15 According to a yet further aspect of the invention there is provided a computer program that, when read by a computer, causes performance of the method described above. CO C\j According to a yet further aspect of the invention there is provided a non-transitory computer readable storage medium comprising computer readable instructions that, when read by a ¢^20 computer, cause performance of the method described above. CM According to a yet further aspect of the invention there is provided a signal comprising computer readable instructions that, when read by a computer, cause performance of the method described above. 25 Any controller or controllers described herein may suitably comprise a control unit or computational device having one or more electronic processors. Thus the system may comprise a single control unit or electronic controller or alternatively different functions of the controller may be embodied in, or hosted in, different control units or controllers. As used 30 herein the term “controller” or “control unit” will be understood to include both a single control unit or controller and a plurality of control units or controllers collectively operating to provide any stated control functionality. To configure a controller, a suitable set of instructions may be provided which, when executed, cause said control unit or computational device to implement the control techniques specified herein. The set of instructions may suitably be 35 embedded in said one or more electronic processors. Alternatively, the set of instructions may be provided as software saved on one or more memory associated with said controller to be executed on said computational device. A first controller may be implemented in software run on one or more processors. One or more other controllers may be implemented in software run on one or more processors, optionally the same one or more processors as the first controller. Other suitable arrangements may also be used. 5 Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such 10 features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. 15 BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described by way of example only, CO with reference to the accompanying drawings, in which: CM Figure 1a-1c shows show various views of a cell which may have connector tabs welded CO thereto in accordance with embodiments of the invention; O20 Figure 2 shows a perspective view of a connector tab and cell to be joined by welding in £\J accordance with an embodiment of the invention; CM Figure 3 shows a perspective view of a welding laser in accordance with an embodiment of the invention; Figure 4 shows a side view of a clamping assembly and fixed support according to an 25 embodiment of the invention; Figure 5 shows a side view of a laser welding system according to an embodiment of the invention; Figure 6 shows a controller in accordance with an embodiment of the invention; and Figure 7 shows a perspective view of a cell array according to an embodiment of the 30 invention. DETAILED DESCRIPTION Figures 1A-C show different views of a cylindrical electrical cell 100. Electrical cells 100 are available in a variety of different sizes. For example, in traction batteries for vehicle cells 35 having a diameter D of 21mm and a length L of 70mm are often used. Such electrical cells are typically referred to as 21700 electrical cells (the first two numbers referring to the diameter D, in mm, and the last three numbers referring to the length L, in tenths of mm). However, it will be understood that other sizes of electrical cell may also be used in embodiments of the present invention. As will be well understood, the electrical cell 100 comprises a positive terminal 100P, a 5 negative terminal 100N, and vent means 100V. The positive terminal 100P is provided by a steel end cap 106 in a central region of a first end 104 of the electrical cell 100, and the negative terminal 100N is provided by a steel cylindrical case 108. The steel cylindrical case 108 covers the second end 102, the entire cylindrical surface between the first and second ends, and a peripheral region 100S of the first end 104. The first end 104 and second end 10 102 face in substantially opposite directions. The peripheral region of the first end surface may also be referred to as a “shoulder” region 100S of the first end 104. In commercially-available electrical cells, it is sometimes the case that the end cap that defines the positive terminal 100P on the first end 104 protrudes beyond the shoulder region of the first end 104, although this is not the case in the electrical cell shown in Figure 1. This allows a 15 substantially planar connector to be connected to the positive terminal 100P and not the negative terminal 100N. As will be understood, it is important to avoid direct electrical CO connections between the positive 100P and negative 100S terminals, as such connections CM create a short circuit which may damage the electrical cell 100. 20 As shown in Figure 1, the electrical cell 100 comprises three vent means 100V in the first £\J end 104, between the steel end cap 106 that defines the positive terminal 100P and the CM shoulder region 100S of the steel cylindrical case 108. The vent means 100V are gaps that are covered by a material that will rupture to allow hot gases to escape through the gap between the end cap 106 and steel cylindrical case 108 in the event of excessive pressure 25 occurring inside the electrical cell 100, thereby mitigate against the risk of the electrical cell 100 exploding. It may (as performed in the embodiment described here) be desired to incorporate multiple examples of the electrical cell 100 into a cell array 114 (see Figure 7) which may for instance 30 comprise a battery module. In this case it may be that the positive terminals 100P of the electrical cells 100 are connected in parallel by a first busbar 110 connected to each positive terminal 100P by respective connection tabs 112 of the first busbar 110 (see Figure 2). Similarly the negative terminals 100N of the electrical cells 100 are connected in parallel by a second busbar connected to each negative terminal 100N by respective connection tabs of 35 the second busbar. The connection of each connection tab to its respective terminal 100P, 100N is by laser welding. The laser welding is performed through the connection tab overlaying the electrical cell terminal to join the two while they are in contact. In this case the terminals 100P, 100N are of steel and the connection tabs are of copper. Initially the cell array 114 is formed by securing the electrical cells 100 relative to each other 5 using adhesive. The electrical cells 100 are arranged in the cell array 114 so as to form five columns and twelve rows, to give a total of sixty electrical cells 100 in the cell array 114. The electrical cells 100 are all oriented in the same manner and are arranged side by side. Further, top surfaces of the positive terminals 100P at the first ends 104 of each electrical cell 100 are substantially aligned to a first common surface plane. Similarly, top surfaces of 10 negative terminals 100N at the second ends 102 of each electrical cell 100 are substantially aligned to a second common surface plane. Various methods may be used to laser weld the respective connection tabs to their respective terminals. In the present embodiment a laser welding system 120 is used having 15 two welding lasers, a first welding laser 122 and a second welding laser 124. The first welding laser 122 is used, in respective instances of a first welding operation, to weld the CO connection tabs 112 of the first busbar 110 to their respective positive terminals 100P of the CM electrical cells 100 of the cell array 114. The second welding laser 124 is used, in respective CQ instances of a second welding operation, to weld the connection tabs of the second busbar 20 to their respective negative terminals 100N of the electrical cells 100 of the cell array 114. CM The use of the two welding lasers means that it is possible to perform all welding operations CM for the cell array 114 without re-orienting the cell array 114 to allow access to all of its terminals 100P, 100N and their respective corresponding connection tabs for welding by a single welding laser. It also allows for welding operations for positive 100P and negative 25 100N terminals to be performed simultaneously. Consequently, welding accuracy and / or efficiency may be improved. Nonetheless, several challenges are created by use of respective welding lasers for use in welding at respective ends 104, 102 of the electrical cells 100. Specifically, access may be required for welding to both ends 104, 102 of each electrical cell 100, and this may create challenges in terms of accurately positioning and 30 supporting the cell array 114. Due to the effects of gravity on welding sputter, use of two welding lasers as described above may leave one or other of the welding lasers 122, 124 more susceptible to accumulation of welding sputter. Additionally, there may be challenges surrounding retaining adequate air displacing fluid around the site of at least some of the welding operations for the duration of the respective welding process. Discussed below is an 35 embodiment seeking to address at least some of these challenges. Referring now to Figure 3, the first welding laser 122 is described. The first welding laser 122 comprises a welding laser source (not shown), which produces a first welding laser beam. By means described further below, the first welding laser delivers the first welding laser beam to a first process site 130 defined as an area of the relevant connection tab 112 of the 5 first busbar 110, and through it, the positive cell terminal 100P, upon which the first welding laser beam is incident at the relevant time. At the point of its interception of the first process site 130, the first welding laser beam may be considered to define or generate a spot of the first welding laser beam. 10 The first welding laser 122 comprises an optical system via which the first welding laser beam passes from its source to the first process site 130. The optical system comprises, in sequential order from the source (not shown) to the first process site 130, a diverging lens 132, a converging lens 134, a first mirror 136 and a second mirror 138. Each of the mirrors 136, 138 is rotatable about a single axis under the control of a respective galvanometer 140, 15 142. The axes about which the mirrors 136, 138 are rotated are mutually perpendicular, so that the first mirror 136 controls the location of the first process site 130 with respect to a first CO axis direction (in this case the X-axis direction) and the second mirror 138 controls the CM location of the first process site 130 with respect to a second axis direction (in this case the CQ Y-axis direction). The diverging lens 132 is movable along an axis parallel to the initial 20 direction of the first welding laser beam from the laser source, and may therefore be CM considered an adjustable focussing lens. This allows the position of the focal point of the first CM welding laser beam to be adjusted in a third axis direction (in this case the Z-axis direction). It will be appreciated that in other embodiments the order of the provision of the various components of the optical system may be adjusted (for instance the order of the first 136 25 and second 138 mirrors may be reversed). In the present embodiment the welding laser source is a single-mode infra-red laser operating at substantially 1070nm wavelength. The welding laser source is a continuous wave type which is modulated (e.g. switched on and off) as required. The power of the 30 welding laser beam emitted is substantially 600W. As will be appreciated, the second welding laser 124 may be arranged similarly in order to generate a second welding laser beam for delivery to a second process site defined as an area of the relevant connection tab of the second busbar, and through it the negative cell 35 terminal 100N, upon which the second welding laser beam is incident at the relevant time. For convenience however, the description is not repeated here. Prior to the instances of the first and second welding operations being performed, a positioning step is performed which puts the cell array 114 and connection tabs of the first 110 and second busbars in position for laser welding. The positioning step sets in space, with respect to the first, second and third axes, the position of the cell array 114 and the 5 connection tabs of the first 110 and second busbars. The positioning step is achieved through use of a positioning system comprising a welding jig 146 (into which the cell array 114 and connection tabs 112 are placed) and the action of a clamping assembly 150 (described further below). With the cell array 114 and connection tabs placed therein, the welding jig 146 is arranged to secure the cell array 114 in an orientation such that the 10 longitudinal axis of the electrical cells 100 is arranged horizontally (which in this case is aligned with the third axis direction as discussed above with respect to the first welding laser 122 and second welding laser 124). Consequently, the respective top surfaces of the positive 100P and negative 100N terminals at the respective first 104 and second 102 ends of each electrical cell 100 are arranged vertically. The position of the cell assembly as 15 determined by the welding jig 146 is also substantially between the first 122 and second 124 welding lasers, with the first end 104 of the electrical cells 100 substantially facing the first CO welding laser 122 and the second end 102 of the electrical cells 100 substantially facing the CM second welding laser 124. The welding jig 146 also supports the first 110 and second CQ busbars in such a manner that a common surface plane of the connection tabs 112 of the 20 first busbar 110 is vertical and a common surface plane of the connection tabs of the second c\j busbar is vertical. CM When the welding jig 146 has positioned the cell array 114 and connection tabs of the first 110 and second busbars, the clamping assembly 150 is actuated to apply a force on the 25 connection tabs towards their respective terminals, bringing them, where they were not already, into contact therewith. The clamping assembly 150 comprises opposed first 152 and second 154 clamping sub-assemblies, each comprising an array of cylindrical bodies 156. One cylindrical body 156 is provided on the first clamping sub-assembly 152 for each of the positive terminals 100P of the cell array 114. Similarly, one cylindrical body 156 is provided 30 on the second clamping sub-assembly 154 for each of the negative terminals 100N of the cell array 114. Each cylindrical body 156 has an end surface 158 which is applied to the relevant connection tab, and an open centre through which welding is performed for the connection tab to which it is applied. 35 The clamping assembly 150 also comprises a mobile support 160 which supports the first clamping sub-assembly 152 via a first translating mount 162 and the second clamping subassembly 154 via a second translating mount 164. The arrays of cylindrical bodies 156, first 152 and second 154 clamping sub-assemblies and first 162 and second 164 translating mounts are arranged such that the first 152 and second 154 clamping sub-assemblies are translatable with respect to the mobile support 160 such that the end surfaces 158 of the different arrays of cylindrical bodies 156 are selectively moveable towards and away from 5 each other in an opposed manner. These movements are in a direction normal to the top surfaces of the positive terminals 100P at the first ends 104 of each electrical cell 100 and to the top surfaces of the negative terminals 100N at the second ends 102 of each electrical cell 100. That is, the movements are in the third axis direction. The movements are made to respectively clamp and release the cell array 114 and connection tabs of the first 110 and 10 second busbars positioned by the welding jig 146, via respective first and second actuators (not shown) respectively acting on the first 152 and second 154 clamping sub-assemblies. In addition, the laser welding system 120 comprises a fixed support 166 which supports the mobile support 160 via a third translating mount 168. The third translating mount 168 allows 15 movement of the mobile support 160 (and therefore the first 152 and second 154 clamping sub-assemblies) with respect to the fixed support 166. This movement is in a direction CO normal to the top surfaces of the positive terminals 100P at the first ends 104 of each CM electrical cell 100 and to the top surfaces of the negative terminals 100N at the second ends CQ 102 of each electrical cell 100. That is, the movements are in the third axis direction. A pair 20 of centralising springs 170 are located to act between respective opposite side walls 172 of CM the fixed support 166 and respective opposite side walls (not shown) of the mobile support CM 160, thereby acting to provide equal and opposite forces tending to bias the mobile support 160 to a central location between the opposite side walls 172. As a consequence of the arrangement as described, the clamping assembly 150 is arranged to be passively 25 repositionable to allow it to adopt a location at which clamping forces applied in opposite directions respectively by the first clamping sub-assembly 152 and the second clamping subassembly 154 to the cell array 114 supported by the welding jig 146 are substantially in equilibrium as experienced by the electrical cells 100 of the cell array 114 regardless of the position of the electrical cells 100 as supported by the welding jig 146 within a range of 30 possible third axis positions. Referring now to Figure 5, the laser welding system 120 also comprises a first repositioning assembly generally shown at 176 and a second repositioning assembly generally shown at 178. Each repositioning assembly 176, 178 comprises a rail 180 and a cart 182. On the cart 35 182 of the first repositioning assembly 176 is mounted the first welding laser 122. On the cart 182 of the second repositioning assembly 178 is mounted the second welding laser 124. Each cart 182 comprises running wheels (not shown) which support the cart 182 on its respective rail 180 and at least one of which is selectively driven by a motor (not shown) also provided on the relevant cart 182, to selectively reposition the relevant cart 182 (and so the corresponding first 122 or second 124 welding laser device) along the relevant rail 180. 5 Each rail 180 is aligned with the first axis which corresponds to the direction in which the columns of the cell array 114 run. Each rail 180 is located with respect to the cell array 114 when mounted in the welding jig 146 for the first and second welding operations such that its corresponding first 122 or second 124 welding laser is positionable so as to be substantially aligned in the third axis direction with each of the electrical cells 100 in the cell array 114 and 10 further such that the relevant connection tab for the relevant electrical cell 100 is between the relevant terminal of that electrical cell 100 and that corresponding first 122 or second 124 welding laser. Further, the rails 180 are positioned such that the welding jig 146, cell array 114 and connection tabs of the first 110 and second busbars are substantially between them when is positioned for performance of the first and second welding operations. 15 Following the positioning step, a welding step is performed comprising multiple instances of CO the first welding operation (each performed by the first welding laser 122 to weld one of the CM connection tabs 112 of the first busbar 110 to its corresponding positive terminal 100P) and CQ second welding operation (each performed by the second welding laser 124 to weld one of 20 the connection tabs of the second busbar to its corresponding negative terminal 100N). CM CM In the welding step, each cart 182 is periodically moved at intervals along its rail 180. Between the movements, the respective welding laser 122, 124 performs welding on a proportion of the connection tabs 112 and their corresponding terminals to which it will not 25 be nearer on another occasion. This includes such connection tabs and terminals in each of the columns of the cell array 114. The optical system (and specifically the first 136 and second 138 mirrors) of each of the first 122 and second 124 welding lasers is controlled to steer its respective welding laser beam both in performing welding and for re-siting of the respective welding laser beam to other connection tab and terminal combinations whilst the 30 relevant cart 182 is stationary. The incidence direction of the respective welding laser beam on each connection tab and terminal is substantially aligned with the third axis. Indeed in this embodiment, each first welding laser beam is directed towards the respective first process site 130 in a first direction which is substantially horizontal, substantially normal to the surface of the connection tab on which it is incident and opposite to a second direction. Each 35 second welding laser beam is directed towards the respective second process site in the second direction which is substantially horizontal and substantially normal to the surface of the connection tab on which it is incident. Further, in this embodiment, there is at least some overlap in time of performance of the first welding operations and performance of the second welding operations. Indeed, in some embodiments, each instance of the first welding operation is performed at the same time as a corresponding instance of the second welding operation. Such instances may be with respect to the same electrical cell 100 or different 5 electrical cells 100 within the cell array 114. The optical system (and specifically the diverging lens 132 position) of each of the first 122 and second 124 welding lasers is controlled to focus the welding laser beam it generates at a desired position with respect to the third axis direction. In most cases each welding laser 10 beam will be focussed at or near to the relevant process site at the relevant time and will have a spot size between 30 and 45 micro meters at the relevant process site. Nonetheless, focussing at a point somewhat away from the relevant process site may be advantageous in certain regards e.g. in controlling penetration and / or reducing sputter. In this embodiment therefore, the relevant first and second welding laser beams are delivered so that they are 15 partially defocused at their process sites. Adjustment to the power of the first and second welding laser beams is made to compensate for the defocusing. CO CM As each instance of the first and second welding operations are performed, air displacing CQ fluid (in this case argon gas) is injected by a delivery system of the clamping assembly 150 20 into the relevant cylindrical body 156. This forms a body of argon gas around the relevant CM process site, tending to displace the air at the process site. The argon gas is delivered CM substantially throughout the relevant instance of the first or second welding operation in a continuous and uniform manner. It may also be delivered in a laminar manner. This may mitigate the increased tendency of the argon to flow away from the relevant process site 25 under the influence of gravity given the horizontal nature of the orienting of the cell array 114. The present embodiment also provides additional protection from welding sputter to a cover glass of each of the first 122 and second 124 welding lasers, which in this embodiment are 30 approximately 340mm distant from their respective process sites. This may be desirable given the increased susceptibility of the respective cover glasses to be at least partially obscured by welding sputter more quickly than would otherwise be the case in view of the vertical orientation of the respective process sites and the positioning of the first 122 and second 124 welding lasers in order that they deliver their respective first and second welding 35 laser beams in a horizontal direction. A first degree of protection is provided by a shrouding plate (not shown) arranged in front of each cover glass and having an orifice therethrough to allow delivery of the relevant first or second welding laser beam beyond the shrouding plate. Each shrouding plate is arranged to shroud its respective cover glass from a direction towards and substantially normal to the 5 cover glass. Each shrouding plate may therefore be considered to protect peripheral areas of the cover glass surrounding a region through which the relevant first or second welding laser beam is delivered. Each shrouding plate forms one wall of a head portion of the respective welding laser, the head portion being removable and replaceable. The relevant cover glass forms another wall of the relevant head portion, opposite the relevant shrouding 10 plate. The remaining walls of each head portion are side plates, extending perpendicularly to the relevant shrouding plate and cover glass (which are parallel). Each shrouding plate, cover glass and side walls form a cube or cuboid shaped substantially enclosed volume. In this embodiment, each shrouding plate is separated from its corresponding cover glass by a distance of substantially 30cm and the side walls extend beyond the shrouding plate in a 15 direction away from the cover glass by a distance of approximately 15cm. CO A further degree of protection is provided by delivery of a respective air blade across and in CM front of each cover glass. Each air blade is delivered from a slot outlet provided in one of the CQ side plates of the relevant head portion, formed to deliver the air blade in a direction 20 substantially parallel to the cover glass. As will be appreciated therefore, in this embodiment, CM each air blade is delivered between its respective cover glass and shrouding plate and in CM this case at a distance of approximately 5cm from the respective cover glass. In this embodiment each slot has an outlet which is rectangular in cross-section and has a distance of substantially 750microns across its smallest dimension. Air is delivered through each slot 25 at substantially 5bar pressure. In the present embodiment the various controllable operations discussed above are controlled by a controller 200 (see Figure 5). The controller 200 has an input means 202, a processor means 204 and an output means 206. In this case the controller 200 is operable 30 to independently control the power of the first and second welding laser beams and to selectively and independently switch the first and second welding laser beams on and off via control of the respective welding laser sources. The controller 200 is also operable to independently adjust the focal position of the first and second welding laser beams, and to independently adjust positions of the spots of the first and second welding laser beams by 35 controlling the angular positions of the respective first and second mirrors 136, 138. The cart 182 is also controlled by the controller 200. The controller 200 is also operable to control the positioning step. In particular, the controller 200 controls an automated operation of the positioning system to load the cell array 114 into the welding jig 146 and the clamping assembly 150 to clamp the connection tabs of the first and second busbars against the corresponding cell terminals. Delivery of the air displacing fluid and delivery of the air blades. 5 The processor means 204 performs the control functions. For instance, it determines the order and timing of the positioning step, clamping step and welding operations as well as coordinating and controlling constituent parts of these steps (e.g. welding laser beam on / off, steering, repositioning and focussing, cart repositioning, air blade delivery and air displacing fluid delivery). As will be appreciated, suitable inputs are received by the controller 200 and 10 in particular the processing means 204, via the input means 202 in order to instruct such control and / or in order that such controls may be calculated by the processing means 204. Such inputs may for instance have been programmed by a user or system and may be inputted via the input means 202 from a sensor, a memory, a server, direct user input on a user interface or the like. 15 The output means 206 provides the communication paths via which the controller 200 CO exerts the control as determined by the processing means 204. CM CO It wiH be appreciated that embodiments of the present invention can be realised in the form O20 of hardware, software or a combination of hardware and software. Any such software may £\J be stored in the form of volatile or non-volatile storage such as, for example, a storage CM device like a ROM, whether erasable or rewritable or not, or in the form of memory such as, for example, RAM, memory chips, device or integrated circuits or on an optically or magnetically readable medium such as, for example, a CD, DVD, magnetic disk or magnetic 25 tape. It will be appreciated that the storage devices and storage media are embodiments of machine-readable storage that are suitable for storing a program or programs that, when executed, implement embodiments of the present invention. Accordingly, embodiments provide a program comprising code for implementing a system or method as claimed in any preceding claim and a machine readable storage storing such a program. Still further, 30 embodiments of the present invention may be conveyed electronically via any medium such as a communication signal carried over a wired or wireless connection and embodiments suitably encompass the same. All of the features disclosed in this specification (including any accompanying claims, 35 abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, 5 each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this 10 specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. The claims should not be construed to cover merely the foregoing embodiments, but also any embodiments which fall within the scope of the claims. CO CM CM CM

Claims

1. A laser welding method comprising:performing a first welding operation at a first process site using a first welding laser5 beam from a first welding laser and simulatenously performing a second weldingoperation at a second process site using a second welding laser, where the first process site comprises a connection tab of a first busbar and a positive terminal of an electrical cell at a first end of the electrical cell and the method comprises directing the first welding laser beam towards the first process site in a first direction, and10 where further the second process site comprises a connection tab of a secondbusbar and a negative terminal of the electrical cell at a second end of the electrical cell and the method comprises directing the second welding laser beam towards the second process site in a second direction, the first and second directions being substantially opposite one another.15252. A laser welding method according to claim 1 where the first and second ends face in substantially opposite directions.

3. A laser welding method according to any preceding claim where the first and second directions are substantially horizontal.

4. A laser welding method according to any preceding claim where the electrical cell is arranged such that respective surfaces of the positive and negative terminals towards which the first and second welding laser beams are respectively directed are arranged substantially vertically.

5. A laser welding method according to any preceding claim where the connection tab of the first busbar overlays the positive terminal of the electrical cell such that the first welding laser beam is incident on the connection tab of the first busbar prior to the 30 positive terminal and the connection tab of the second busbar overlays the negativeterminal of the electrical cell such that the second welding laser beam is incident on the connection tab of the second busbar prior to the negative terminal.

6. A laser welding method according to any preceding claim comprising supporting the 35 electrical cell for the first and second welding operations with a jig, and clamping theelectrical cell via a clamping assembly, wherein the connection tab of the first busbar to the positive terminal is clamped via a first clamping sub-assembly, and theconnection tab of the second busbar to the negative terminal is clamped via a second clamping sub-assembly.

7. A laser welding method according to claim 6 where the clamping assembly is 5 passively repositionable to allow it to adopt a location at which clamping forcesapplied in opposite directions respectively by the first clamping sub-assembly and the second clamping sub-assembly to the electrical cell supported by the welding jig are substantially in equilibrium as experienced by the electrical cell regardless of the position of the electrical cell as supported by the welding jig within a range of 10 possible positions.

8. A laser welding method according to claim 6 or claim 7 where the clamping assembly comprises a mobile support on which the first clamping sub-assembly is mounted via a first translating mount and the second sub-assembly is mounted via a second15 translating mount, each of the first clamping sub-assembly and the second clampingsub-assembly being selectively repositionable towards the other via the respective one of the first and second translating mounts to clamp the electrical cell and the C\j connection tabs of the first and second busbars therebetween, and where further themobile mount is mounted via a third translating mount to a fixed support, the ■^—20 clamping assembly being passively repositionable with respect to the fixed mount viathe third translating mount to a location where clamping forces applied in opposite directions respectively by the first clamping sub-assembly and the second clamping sub-assembly to the electrical cell supported by the welding jig are substantially in equilibrium as experienced by the electrical cell regardless of the position of the25 electrical cell as supported by the welding jig within a range of possible positions.

9. A laser welding method according to any preceding claim further comprising delivering an air displacing fluid around the first process site whilst performing the first welding operation and around the second process site whilst performing the30 second welding operation.

10. A laser welding method according to claim 9 where the delivery of air displacing fluid for each of the first and second welding operations is performed in a continuous and uniform manner.3511. A laser welding method according to any preceding claim where at least one of the first and second welding lasers comprises a cover glass through which the relevantfirst or second welding laser beam is delivered, the method comprising delivering an air blade across and in front of the cover glass.

12. A laser welding method according to claim 11 where the air blade is delivered at 5 between substantially 4 and 6bar and from a slot outlet of between substantially 600and 900microns across its smaller dimension.

13. A laser welding method according to any of claims 11 to 12 where the welding system comprises a shrouding plate arranged in front of the cover glass and having 10 an orifice therethrough to allow delivery of the relevant first or second welding laserbeam beyond the shrouding plate.

14. A laser welding method according to claim 13 further comprising, shrouding via the shrouding plate, from a direction towards and substantially normal to the cover glass, 15 peripheral areas of the cover glass surrounding a region through which the relevantfirst or second welding laser beam is delivered.C\j 15. A laser welding method according to any preceding claim where the first and / orsecond welding laser is arranged to decrease welding sputter generation by 20 delivering the relevant first or second welding beam so that it is partially defocused atthe relevant first or second process site and to adjust the relevant first or second welding laser beam power to compensate for the defocusing.

16. A laser welding method according to any preceding claim where the electrical cell is 25 part of an array of similar electrical cells similarly oriented, and instances of the firstwelding operation is performed on the respective one of the positive and negative terminals of each electrical cell in the array and instances of the second welding operation is performed on the respective other of the positive and negative terminals of each electrical cell in the array.3017. A controller arranged to control a laser welding system to perform the method of claim 1.

18. A computer program that, when read by a computer, causes a laser welding system 35 to perform of the method of claim 1.

19. A non-transitory computer readable storage medium comprising computer readable instructions that, when read by a computer, cause a laser welding system to perform the method of claim 1.5 20. A signal comprising computer readable instructions that, when read by a computer,cause a laser welding system to perform the method of claim 1.09 12 24

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