Cell testing

Abstract

The invention relates to a cell test jig (200) arranged to place a cell (202) on test therein under compressive force. The cell test jig (200) comprises a first plate (204), a second plate (214) and a

Description

TECHNICAL FIELD The present disclosure relates to cell testing. An aspect of the invention relates to a cell test jig. BACKGROUND Certain types of cells (e.g. prismatic Li-ion cells) are known to swell and contract during charging / discharging and to gradually and progressively swell as a result of ageing. These processes may be the result of internal chemistry, e.g. electrolyte gases which build up as the cell is charged and dissipate as it discharges. Plating on an anode and cathode may also accumulate. This swelling can affect performance of the cell. With a view to combating performance degradation, a compressive force can be applied to cells in a cell stack (e.g. 40 cells) when installed in a given system (e.g. a vehicle). This may for instance be achieved using a hydraulic ram. The compressive force applied may be very large e.g. of the order of 21 kN. Nonetheless, the application of compression must not be so great as to risk damaging the cell as it expands. It is therefore desirable to be able to test the performance impacts of cell compression on a cell. Existing approaches to testing do not allow for desirable testing flexibility or the collection of data that is revealing and / or of desirable quality / reliability. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a cell test jig as claimed in the appended claims. According to an aspect of the present invention there is provided a cell test jig arranged to place a cell on test therein under compressive force, the cell test jig comprising a first plate, a second plate and a clamp, wherein the cell test jig is arranged to receive the cell between the first and second plates and the clamp is arranged to retain the first and second plates in a manner applying a compressive force to the cell and where further the cell test jig comprises multiple sensors comprising at least one of: i) multiple displacement sensors at discrete locations each arranged to output a signal indicative of the separation between the first and second plates at the corresponding location; and ii) multiple force sensors at discrete locations each arranged to output a signal indicative of the compressive force applied to the cell by the cell test jig at the corresponding location. The cell test jig may allow for more realistic and / or adaptable testing of a cell for various possible operational installation scenarios. In particular, the compressive force applied may more accurately simulate operational conditions for the cell, which when installed (for instance in a stack of such cells in a vehicle) may have a compressive force applied to it. The operational application of compressive force may be desired with a view to aiding performance. This is because if the cell is allowed to expand sufficiently over the course of operation, charge capacity performance tends to degrade. When in use, the first and second plates and the cell may be substantially aligned in a layered structure. The displacement sensor or sensors may be used to monitor for variations in the separation caused by cell swelling during the testing of the cell. Data collected by the displacement sensor(s) may be used in isolation or in conjunction with other data to analyse cell performance given a particular simulated configuration. Such other data might for instance comprise the compressive force applied by the cell test jig prior to testing and / or as the test progresses, how the displacement sensor data coincides with the cell being charged or discharged during the test and charge capacity variation over time. The displacement sensor(s) may be a linear transducer. This may be preferable to a load cell because it may mean that the orientation of the cell test jig is not significant / restricted. Further, use of a linear transducer may allow improved packaging. The linear transducer(s) may be mounted on one of the first and second plates and have a spring loaded probe measuring the distance to the other of the first and second plates. The force sensor or sensors may allow monitoring / confirmation of an initial compressive force applied to the cell at set-up. This may verify that the planned initial test conditions are attained. The force sensor(s) may allow monitoring of compressive force applied to the cell during the test. This data may be of interest in terms of understanding cell behaviour and performance given particular compressive force application. The force sensor(s) may for instance be a strain gauge. Multiple force and / or displacement sensors can help to build a more complete / detailed assessment of physical changes to the cell rather than changes at a single point location. Multiple force sensors and / or displacement sensors may, for instance, offer greater control and monitoring potential in terms of force applied to / by different parts of the cell, and / or better opportunity to detect uneven swelling / contraction, differential expansion / contraction extents and / or rates, bending and / or twisting. Additionally, improved resolution may be provided. According to another aspect of the present invention there is provided a cell test jig arranged to place a cell on test therein under compressive force, the cell test jig comprising a first plate, a second plate and a clamp, wherein the cell test jig is arranged to receive the cell between the first and second plates and the clamp is arranged to apply a compressive force to the cell and where further the cell test jig comprises multiple sensors comprising at least one of: i) multiple displacement sensors each arranged to output a signal indicative of the separation between the first and second plates; and ii) multiple force sensors each arranged to output a signal indicative of the compressive force applied to the cell by the cell test jig. In some embodiments the cell test jig comprises four displacement sensors and / or four force sensors. Data from four displacement sensors may be better indicative of separation across an area bounded by the displacement sensors, as well as providing four point sources of separation information. This is not however intended to be limiting and alternative numbers of displacement sensors may be used e.g. 2, 3 or 6. Data from four force sensors may be better indicative of force applied within an area bounded by the force sensors, as well as providing four point sources of force information. This is not however intended to be limiting and alternative numbers offeree sensors may be used e.g. 2, 3 or 6. In some embodiments one, some or all of the sensors are located adjacent respective perimeters of the first and second plates. Consequently one, some or all of the plurality of displacement sensors may be located adjacent respective perimeters of the first and second plates and / or one, some or all of the plurality offeree sensors may be located adjacent respective perimeters of the first and second plates. The perimeter regions of the first and second plates may substantially correspond to the perimeter regions of the cell when installed in the cell test jig and under test. The perimeter regions of the cell might often experience an extreme e.g. minimum or maximum of displacement (e.g. arising from bending, twist or swelling) and / or force, and so measurement taken in this region may be particularly informative. Additionally, perimeter locations might naturally create an interior area between them about which the relevant sensor data may indicate or suggest conclusions regarding separation and / or force application as appropriate. In some embodiments some or all of the sensors are located at their own respective and corresponding corners of the first and second plates. Whether this applies to the displacement sensors and / or the force sensors, positioning substantially in their own corners may increase the interior area between them and / or give rise to a relatively high probability of the relevant sensor experiencing a minimum or maximum in terms of displacement and / or force. As will be appreciated, an instance of the displacement sensors and an instance of the force sensors may be paired in their location substantially in a corner. Such an arrangement may be provided in one, some or all of the corners. In some embodiments the clamp is arranged to retain the first and second plates at any of a range of separations and to be selectively and progressively actuated to adjust the separation and thereby permit varying of the compressive force applied to the cell. The clamp may be an adjustable clamp. The clamp may be arranged to engage the first and second plates. The clamp may for instance be a threaded bolt having a head. The threads of the bolt may engage the second plate. Actuation of the clamp to adjust the separation may be by turning of the head to adjust the extent of the screw thread engaged with the second plate. In some embodiments the cell test jig comprises a biasing assembly arranged to act between the clamp and one of the first plate and the second plate to limit the force with which the clamp retains the first and second plates at a separation set to thereby accommodate a degree of swelling of the cell when under test. The biasing assembly may be provided and / or act between the head and the first plate, biasing them away from each other. The biasing assembly of the cell test jig may allow for some expansion whilst the cell is under test. This may again more accurately simulate operational conditions. Specifically, operational cell stacks may be designed to allow for some cell expansion to mitigate the risk of cell burst. There may for instance be an air gap and / or a compressible thermal isolation material between cells. Consequently, by allowing a degree of expansion, more realistic results indicating cell performance, charge capacity, swelling and / or force experienced overtime may be achieved. The cell test jig may allow expansion in accordance with a pre-determined spring rate, allowing the effects of such a spring rate (e.g. on swelling, charging, discharging and / or capacity performance) to be analysed. Additionally, the spring rate can be set to simulate swelling expected for cells at different positions (e.g. closer to the centre or to an edge) within an operational cell stack. In some embodiments the cell test jig is arranged to accommodate variation in the configuration of the biasing assembly used. The cell test jig may for instance be arranged to accommodate biassing assemblies having different spring rates and / or biasing assemblies which vary in number of biasing elements and / or vary in biasing element arrangement. The scope for variation may allow better tailoring of the spring rate to simulate different cell positions within a cell stack and / or different compressive forces applied. Additionally, the cell test jig may be arranged to accommodate interchanging of the biasing assembly, allowing a single instance of the cell test jig to offer this adaptability. In some embodiments the biasing assembly comprises a disc spring. Disc springs (or Belleville spring washers) may offer a higher force to displacement ratio than alternatives such as coil springs. This in turn may allow for more compact packaging of the cell test jig. The biasing assembly may comprise one or multiple such spring washers. The cell test jig may be arranged to accommodate varying numbers of disc springs, arranged in series (i.e. adjacent washers having reversed orientations - convex facing concave) and / or parallel (i.e. adjacent washers having the same orientations - convex facing convex or concave facing concave). Adding washers in series may increase the sprung stroke whilst adding washers in parallel may increase the spring rate. Allowing for these adaptations may allow better tailoring to a particular simulation scenario. In some embodiments the cell test jig comprises a plurality of the clamps. There may for instance be two or four clamps but these examples are not intended to be limiting. Four may be desirable in terms of consistency of compressive force applied and in terms of generating a representative force pattern. Any feature previously discussed concerning the clamps may be applied to each of the clamps mutatis mutandis. Thus for instance, there may be one clamp located at each of four corners of the cell test jig (i.e. each at respective and corresponding corners of the first and second plates). Similarly, relationships concerning the clamps previously discussed may be applied to each of the clamps mutatis mutandis. Thus for instance, more than one and optionally all of the clamps may be associated with a corresponding instance of the biasing assembly. By way of further example instances of the displacement sensor(s) and / or of the force sensor(s) may each be located with respect to their relevant clamp in accordance with the relationship previously discussed. Multiple clamps, especially when so arranged, may create a more consistent and / or more realistic compressive pressure distribution on the cell when under test. In some embodiments at least one of the clamps is located adjacent respective perimeters of the first and second plates. This may better facilitate accommodation of the cell and / or cell carrier plate. Additionally or alternatively it may improve the evenness of the compressive force applied and / or the stability of the cell and / or carrier plate when the compressive force is applied. In some embodiments at least one of the clamps is located at respective and corresponding corners of the first and second plates. This may better facilitate accommodation of the cell and / or cell carrier plate. Additionally or alternatively it may improve the evenness of the compressive force applied and / or the stability of the cell and / or carrier plate when the compressive force is applied. In some embodiments some or all of the displacement sensors and / or some or all of the force sensors are each located at a location substantially corresponding to a respective instance of the clamp. The displacement sensor(s) may for instance be located outwards of the footprints of the first and second plates at a perimetrical location aligned with the relevant clamp. Where the location of an instance of the displacement sensor corresponds substantially to that of the clamps, more accurate / useful swelling data may be collected given the proximity to the clamp (and therefore the proximity of the application of compressive force). The force sensor(s) may for instance be arranged to be positioned between the cell and one / or other of the first and second plates. The force sensor(s) may for instance be located within the footprints of the first and second plates at a perimetrical location aligned with the relevant clamp. Proximity to the clamp may result in greater accuracy in terms of monitoring compressive force applied. This may be particularly helpful at set-up in terms of monitoring to better achieve desired compressive force e.g. through accurate application of torque to the clamp where for instance the clamp comprises a bolt. In some embodiments, the cell test jig comprises a cell carrier plate between the first and second plates and arranged to directly support the cell. The cell carrier plate may help to support and prevent damage to the cell as well as provide a more even compressive force. The cell carrier plate may be substantially aligned with the first and second plates in the layered structure. Similarly, the cell may be substantially aligned with the first and second plates in the layered structure when the cell is present (e.g. for a test). In some embodiments the cell test jig has a support formation arranged to support the cell carrier plate in a manner allowing adjustment of the position of the cell carrier plate with respect to the first and second plates when the compressive force is applied, the positional adjustment being in a plane substantially perpendicular to the direction of the compressive force. This positional adjustment may therefore be considered to be in the plane of the cell carrier plate and may be permitted in multiple or all directions in the plane of the cell carrier plate. The cell carrier plate may therefore be considered a floating plate. The combination of the cell carrier plate and support formation may reduce the likelihood of displacement and / or force measurements collected by the respective sensors being adversely affected in terms of accuracy due to shear force experienced by the cell. Significant shear forces might otherwise occur through ‘locking up’ of the cell between the first and second plates, especially given the potentially high compressive forces applied. The cell carrier plate and support formation may allow the cell to find a natural position. Further, the cell carrier plate and support formation combination may be desirable to an alternative whereby a cell carrier plate is provided with guides passing there through. This alternative may be susceptible to lock-up occurring as a result of twisting of the cell carrier plate and it consequently engaging with the guides. It may be that the cell carrier plate is not engaged / interfaced with the clamp. There may be multiple instances of the support formation. There may for instance be two or four instances of the support formation but these examples are not intended to be limiting. Four may be desirable in terms of stability of the cell carrier plate. The support formations may be located at each corner of the cell carrier plate. Each instance of the support formation might correspond to a corresponding instance of the force sensor (each force sensor providing part of the relevant support formation). In some embodiments the support formation comprises a protrusion and a cooperating cupped seat, with the protrusion being received in the cupped seat and a pinnacle of the protrusion contacting a floor of the cupped seat to provide the support to the cell carrier plate and where further the cupped seat is dimensioned to give clearance all around a side wall of the protrusion, thereby allowing adjustment of the position of the protrusion within the cupped seat. This arrangement may substantially create a single point of contact at the support formation on which the cell carrier plate is supported, reducing friction and better facilitating relocation of the cell carrier plate within its plane. The cupped seat (or crater) may provide a location guide for the protrusion (or dome) during setup and provide a degree of ultimate limitation to relocation of the cell carrier plate within its plane relative to the first and second plate. One of the protrusion and cupped seat may be part of the cell carrier plate and the other may be part of a feature adjacent to the cell carrier plate on which it is supported (e.g. the second plate or the force sensor where the cell carrier plate is located between the force sensor and the cell). In some embodiments the cell test jig has a jig terminal that is directly supported by the cell test jig, the jig terminal being for connection to a charging cycler via one or more cables and the cell test jig further comprising a connector to connect the jig terminal to a cell terminal of the cell when installed for test in the cell test jig. This may prevent / mitigate weight of connecting cables being placed on the cell terminal, which might bend or tear the cell terminal and / or impact on the accuracy of the test. The cell test jig may have two such jig terminals with corresponding connectors for different polarities. In some embodiments the connector is at least substantially rigid. In this way the connector may be self-supporting of its own weight and so may place reduced / no significant force on the cell terminal. In some embodiments the connector is reorientable and / or removable from the cell test jig for installation and removal of the cell from the cell test jig. This may facilitate easier and / or safer installation / extraction of the cell to / from the cell test jig. 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 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. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a perspective view of a vehicle powered by cells; Figure 2 shows a perspective view of cell test jig; Figure 3 shows an enlarged perspective view of a biasing assembly; Figure 4 shows a cross-sectional view of a biasing assembly; Figure 5a and 5b show schematic views of a threaded bolt and a Belleville spring washer arrangement; Figure 6 shows an enlarged perspective view of part of a cell test jig; Figure 7 shows a cross-sectional view of part of a cell test jig; Figure 8 shows a perspective cross-sectional view of part of a cell test jig; Figure 9 shows a perspective view of a plurality of cell test jigs and a testing shelf; and Figure 10 shows a perspective view of a plurality of cell test jigs and a charging cycler. DETAILED DESCRIPTION Described below, with reference to the Figures, is an embodiment of a cell test jig arranged for use in testing cells, the cells being used at least in part as an energy source to power travelling of a vehicle such as a vehicle 100 shown in Figure 1. Referring now to Figure 2, a cell test jig is generally shown at 200 with a cell 202 under test installed therein. The cell test jig 200 has a series of flat, rectangular plates arranged in a stacked / layered fashion. The plates are similarly (though not identically) sized and are aligned (e.g. their centre points are aligned within the stack). At the top of the stack of plates is a first plate 204. The first plate 204 is made of steel or other hard material and has a series of rebated portions 206 (see Figure 8) with a series of webs 207 between the rebated portions 206. This combination provides resistance to deflection of the first plate 204 whilst also reducing its weight. It will be appreciated however that alternatives are also possible, including for instance a simple flat design (e.g. no rebates or webs) which might reduce cost and / or time in manufacture. The cell 202 on test is positioned next in the stack i.e. adjacent the first plate 204. The face of the plate against the cell is flat. Following that is a cell carrier plate 208 constituting another of the stacked plates and directly supporting the cell 202 within the cell test jig 200. The cell carrier plate 208 is made of steel and has a series of rebated portions 210 with a series of webs 212 between the rebated portions 210 provided again for a combination of deflection resistance and weight saving. It will be appreciated however that alternatives are also possible, including for instance a simple flat design (e.g. no rebates or webs) which might reduce cost and / or time in manufacture. The cell carrier plate 208 is also flat on the surface facing the cell 202 and is similarly sized to the cell 202. It is smaller in length and width than the first plate 204. The final and bottom of the stacked plates is a second plate 214 which supports the cell carrier plate 208 in a manner discussed further below. The second plate is larger in length and width than the first plate 204. The second plate 214 may be made of aluminium in order to reduce weight. The reduced bending resistance of aluminium by comparison for example with steel is not problematic in the case of the second plate 214, because the second plate 214 is supported against deflection by its engagement with a testing shelf (in this case a pair of rigid rails 216 (see Figure 9)) when selectively installed thereon. Between the second plate 214 and the cell carrier plate 208 are four force sensors, in this case strain gauges 218. The strain gauges 218 are separated and measure force at separate locations and are therefore discrete. Each strain gauge 218 has a base portion 224 and extending therefrom a protrusion portion 226 (best seen in Figure 7). The strain gauges 218 are positioned with one substantially at each of four cell carrier plate corners 220 of the cell carrier plate 208. The strain gauges 218 support the cell carrier plate 208 on the second plate 214. In particular, the base portion 224 of each strain gauge 218 directly contacts the second plate 214 and a pinnacle 228 of each protrusion portion 226 directly contacts the cell carrier plate 208. Each protrusion portion 226 is aligned with a corresponding and cooperating cupped seat 230 formed on a lower surface of the cell 8 carrier plate 208. Each pinnacle 228 contacts a floor 232 of the respective cupped seat 230. Each cupped seat 230 is larger than the protrusion portion 226, so that a degree of movement of cell carrier plate 208 relative to the second plate 214 is accommodated within a plane parallel to that of the cell carrier plate 208. Together, each protrusion portion 226 and cooperating cupped seat 230 form a support formation. Four clamps, generally shown at 234, are provided to retain the stack of plates 204, 208 and 214 and the cell 202 in the layered structure and to apply a compressive force to the cell 202. In this case, each clamp 234 has a threaded bolt 236 each having a head 238 at one end and a thread (not shown) at the other. Engaged with each head 238 and contacting a top surface 240 of the first plate 204 is a biasing assembly 242 arranged to limit the force with which the clamps 234 retain the first 204 and second 214 plates at a separation set by rotating the threaded bolt 236. A biasing assembly 242 which is usable as part of each of the clamps of the Figure 2 embodiment is shown in Figure 4. Referring to Figure 4, the clamp has three Belleville spring washers 244, one of which, between the other two, is inverted by comparison with those other two. In each case, the Belleville spring washers 244 are carried in a biasing assembly housing 246 of the biasing assembly 242. The biasing assembly housing 246 has a housing through bore 248 through which the respective threaded bolt 236 passes. The diameter of the housing through bore 248 varies to provide in order: a head slot 250 which receives the head 238 of the bolt 236; a head bearing surface 252 which contacts an underside of the head 250, preventing it from passing further through the biasing assembly housing 246; a spacer slot 254, which when the threaded bolt 236 is present in the housing through bore 248, forms an annulus having a bump stop end surface 256; a washer bearing surface 258 arranged to react a force generated by the Belleville spring washers 244 biasing the biasing assembly housing 246 towards the underside of the head 250;and a washer slot 260, to accommodate the Belville spring washers 244. Additionally, each biasing assembly 242 has a spacer 262. The spacer 262 has a spacer through bore 264 through which the respective threaded bolt 236 passes. An exterior surface 266 of the spacer 262 has a circular cross section and forms washer locator 268 arranged to extend into the washer slot 260 and define an inner surface of an annulus in the washer slot 260 in which the Belleville spring washers 244 are accommodated. The washer locator 268 assists in retaining the Belleville spring washers 244 in a concentric configuration, thereby reducing the loading and / or binding. The exterior surface 266 also forms a washer bearing ledge 270 arranged to react a force generated by the Belleville spring washers 244 biasing the spacer 262 away from the biasing assembly housing 246. The spacer 262 also has a plate face 272 facing and contacting the first plate 204. In the event of the Belleville spring washers 244 being compressed (e.g. by tightening rotation of the threaded bolt 236 and / or expansion of the cell 202), the washer locator 268 is accommodated inside the spacer slot 254. With sufficient compression of the Belleville spring washers 244, the washer locator 268 contacts the bump stop end surface 256, thereby limiting the spring stroke length and preventing over compression of the Belleville spring washers 244. As will be appreciated, alternative numbers and / or configurations of Belleville spring washers 244 are possible in dependence on the extent and / or character of the limitation desired of the force with which the clamps 234 retain the first 204 and second 214 plates at a separation set. Additional Belleville spring washers arranged in series (i.e. adjacent washers having reversed orientations - convex facing concave) may be used to increase the sprung stroke. Additional Belleville spring washers arranged in parallel (i.e. adjacent washers having the same orientations - convex facing convex or concave facing concave) may be used to increase the spring rate. Figure 5a shows an alternative in the form of use of only two (inverted with respect to each other) Belleville spring washers 274, whilst Figure 5b shows an alternative in the form of use of four (alternating in terms of orientation i.e. each inverted with respect to the next) Belleville spring washers 276. Each biasing assembly 242 biases the first plate 204 and the respective head 238 away from each other. The threads of each threaded bolt 236 engage with a corresponding cooperating threaded bore 278 in the second plate 214. Each clamp 234 is located at a respective first plate corner 280 of the first plate 204 and each threaded bore 278 is at substantially a respective second plate corner 282 of the second plate 214. Four displacement sensors are provided, in this case linear transducers 284, each arranged to output a signal indicative of the separation between the first 204 and second 214 plates. The linear transducers 284 are separated and measure displacement at separate locations and are therefore discrete. Each linear transducer 284 has a spring loaded probe 286 in a cylinder 288. The cylinder 288 of each linear transducer 284 is attached to the first plate 204 by a corresponding securing bracket 290 at a respective one of the first plate corners 280. Each spring loaded probe 286 has a tip 292 which engages an opposed face 294 of a respective reference bracket 296 attached to one of the second plate corners 282. Each opposed face 294 limits the extension of the respective spring loaded probe 286 from its associated cylinder 288 driven by the respective spring. The use of linear transducers 284 means that the cell test jig 200 orientation is not significant for assessing the separation between the first 204 and second 214 plates. This may facilitate different packaging and support options for the cell test jig 200 (see for instance Figure 9, showing cell test jigs 200 mounted to rigid rails 216 in different orientations). The cell test jig 200 has two jig terminals 298 (one positive one negative) best shown in Figures 2 and 6. The jig terminals 298 are directly supported by the cell test jig 200. The jig terminals are therefore retained in a substantially fixed position relative to the cell 202 when in the cell test jig 200. Each jig terminal 298 has a busbar with multiple connection points 300 (in this case four) for connecting electrical cables 302 thereto. These electrical cables 302 connect the respective jig terminal 298 to a corresponding terminal of a charging cycler 304 (see Figure 10). The cell test jig 200 also has a pair of connectors 306, one each to connect the respective jig terminal 298 to a cell terminal 308 of the cell 202 when installed for test in the cell test jig 200. Each connector 306 is rigid, thereby supporting its own weight. In this way, the application of forces to the cell terminals 308 that might damage the cell terminals 308 may be avoided. Additionally, adverse influence on the accuracy of the measurements taken by the strain gauges 218 and / or linear transducers 284 caused by the weight and / or inflexibility of the electrical cables 302 may be avoided. The cell terminals 308 themselves are sufficiently flexible in order to allow for their displacements relative to the relevant connector 306 caused by expansion / contraction of the cell 202. Such expansion / contraction may be caused by ageing and / or ‘breathing’ of the cell 202 with charging / discharging. Additionally, each connector 306 is re-orientable whilst still secured to the relevant jig terminal 298 and removable (if desired) from the relevant jig terminal 298. This functionality may be used to ease the installation and / or removal of the cell 202 from the cell test jig 200. The purpose of the cell test jig 200 is to allow fortesting of the cell 202 whilst simulating installed conditions of the cell 202 in a stack of cells. The testing comprises cycled charging and discharging of the cell 202. The performance of the cell 202 during the testing can be monitored at least in part by the strain gauges 218 and linear transducers 284. Performance monitored might include physical changes (e.g. swelling, contracting, bending and / or twisting) and charge performance (e.g. maximum charge, time to charge and / or time to discharge). The simulation of the installed conditions comprises a compressive force being applied to the cell 202. This is because compressive forces are applied to cell stacks (and indeed therefore the cells within) in installed applications in order to aid performance. If the cell 202 is allowed to expand sufficiently over the course of operation (which it will tend to do naturally), charge capacity performance tends to degrade. The simulation of installed conditions also comprises limiting to some extent the compressive force applied by permitting a degree of cell 202 expansion. This may again simulate real world operational conditions, with cell stacks being designed to allow for some cell expansion to mitigate the risk of cell burst. There may for instance be an air gap and / or a compressible thermal isolation material between installed cells. In use, in a first step, the cell 202 may be installed into the cell test jig 200. The connectors 306, threaded bolts 236, their associated biasing assemblies 242 and the first plate 204 are removed and the cell 202 placed onto the cell carrier plate 208. The number and configuration of Belleville spring washers 244 in each biasing assembly is checked / altered in order to conform with the desired test. In particular the number and configuration of the Belleville spring washers 244 is selected in order to give a desired degree of limitation to the force with which the threaded bolts 236 will retain the first 204 and second 214 plates at a separation set by the adjustment of the threaded bolts 236 and potentially the effect of the biasing assemblies 242 in combination therewith. Consequently, the biasing assemblies 242 may be configured to simulate a specific degree of permitted cell 202 expansion during the test, which may for instance be selected to conform to a particular theoretical or actual installed configuration. In this way, conditions in an operational cell stack may be better simulated, where a degree of cell expansion may be permitted by design in order to mitigate the risk of cell burst. In such operational cell stacks, there may for instance be an air gap and / or a compressible thermal isolation material between cells. Additionally or alternatively, the number and configuration of Belleville spring washers 244 may be selected to simulate permitted expansion at a particular location within an cell stack (which may for instance vary in dependence on the position of the cell within the cell stack). The first plate 204 is then placed on top of the cell 202 and the threaded bolts 236, carrying their associated biasing assemblies 242, reinstalled by inserting them through corresponding through bores in the first plate 204 and screwing them into their corresponding threaded bores 278 in the second plate 214. The threaded bolts 236 are then tightened to adjust the separation between the first 204 and second 214 plates, thereby compressing the cell 202. The tightening is continued until a pre-determined compressive load is applied to the cell 202. Correct application of this force may be at least partly determined in accordance with the application of a pre-determined torque to the threaded bolts 236. Nonetheless, the application of the desired compressive load may additionally or alternatively be determined with reference to data generated by the strain gauges 218. The compressive force applied is selected / varied, in accordance with the parameters of a particular test. It may be for example that there is a desire to test the effect of the application of greater or lesser compressive force or to simulate for a particular position of the cell 202 within a cell stack, where compressive force applied varies in dependence on position within the cell stack. The use of four threaded bolts 236 at respective corners of the cell test jig 200, may improve the stability of the cell test jig 200 and cell 202 and the consistency of the compressive force distribution applied to the cell 202. As will be appreciated, particularly in view of the adjustability of the separation of the first 204 and second 214 plates, cells of different sizes can be accommodated in the cell test jig 200. Next, the connectors 306 are re-installed, by screwing one end to the respective jig terminal 298 and the other to the respective cell terminal 308. The cell test jig 200 with the cell 202 installed therein is then secured to the pair of rigid rails 216 in a test zone. The electrical cables 302 are installed to connect the jig terminals 298 to respective and opposite polarity terminals of the charging cycler 304. The testing of the cell may proceed, with the charging cycler performing charging and discharging cycles of cell 202. This charge and discharge cycling may be conducted over an extended period (e.g. 9-12 months) and optionally substantially continuously, thereby simulating ageing of the cell. As the cell 202 is charged and discharged it will naturally expand and contract. Additionally, as the cell 202 ages in terms of the total number of charges and discharges, it will tend to expand. The compression applied by the cell test jig 200 may at least partially counteract this effect, to thereby preserve additional charging performance in the cell 202 as it ages. Further, through the effect of the biasing assemblies 242, the expansion / contraction may be accommodated in accordance with test design whilst a substantially consistent compressive force is maintained. Both as compression is initially applied to the cell 202 by the cell test jig 200 and as the cell 202 is affected by the testing (e.g. expansion / contraction / twisting / bending), the protrusion portion 226 and cooperating cupped seat 230 of the support formation mean that the cell carrier plate 208 is better able to adjust its own position and that of the cell 202 in directions perpendicular to the direction of the compressive force. This may help to avoid ‘locking up’ of the cell 202 and / or cell carrier plate 208 which in turn may improve the safety of the testing, reducing the likelihood of damage occurring and / or improving the utility of the data collected by the strain gauges 218 and / or linear transducers 284. As the test is conducted, the timings of charging and discharging cycles and / or the charge that is taken by the cell 202 during charging cycles may be monitored. Additionally / alternatively, the compressive force experienced by the cell 202 and / or different parts of the cell 202 may be monitored in accordance with the data output by the strain gauges 218. This data may additionally be time sequenced in accordance with the monitoring of the timings of the charging and discharging cycles. Because the location of each strain gauge 218 is proximate a respective instance of the threaded bolts 236, greater accuracy may be achieved by the strain gauges 218 in reflecting actual compressive force experienced by the cell 202. Additionally, in view of the use of multiple strain gauges 218, variations in the compressive force experienced by the cell 202 across its body (e.g. as a consequence of the cell experiencing uneven expansion / contraction) may be more likely to be detected (i.e. differences in the compressive forces reported by the strain gauges 218). Additionally / alternatively, the displacement of the cell 202 and / or different parts of the cell 202 (e.g. resulting from expansion / contraction / twisting) may be monitored in accordance with the data output by the linear transducers 284. This data may additionally be time sequenced in accordance with the monitoring of the timings of the charging and discharging cycles. Because the location of each linear transducer 284 is proximate a respective instance of the threaded bolts 236, greater accuracy may be achieved by the linear transducers 284 in reflecting actual displacement experienced by the cell 202. Additionally, in view of the use of multiple linear transducers 284, variations in the displacement experienced by the cell 202 across its body (e.g. as a consequence of the cell experiencing uneven expansion / contraction, bending and / or twisting) may be more likely to be detected (i.e. differences in the displacement reported by the linear transducer 284). By positioning the strain gauges 218 and / or the linear transducers 284 at the extremities (i.e. the corners of the cell test jig 200), it may be more likely that phenomenon such as uneven expansion, bending and twisting are detected. The data collected may be of interest in terms of understanding cell behaviour and performance, given particular compressive force application and / or provision for particular degrees of expansion in accordance with the configuration of the biasing assembly 242. This in turn may inform the expected performance of a cell given a particular installation arrangement and / or suggest an installation arrangement that may improve performance. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application. By way of example, we note that the identity of the first and second plates may be reversed. Thus for instance, a feature described as being between the cell carrier plate and the second plate might alternatively be located between the cell carrier plate and the first plate.

Claims

1. A cell test jig arranged to place a cell on test therein under compressive force, the cell test jig comprising a first plate, a second plate and a clamp, whereinthe cell test jig is arranged to receive the cell between the first and second plates andthe clamp is arranged to retain the first and second plates in a manner applying a compressive force to the cell and where furtherthe cell test jig comprises multiple sensors comprising at least one of:i) multiple displacement sensors at discrete locations each arranged to output a signal indicative of the separation between the first and second plates at the corresponding location; andii) multiple force sensors at discrete locations each arranged to output a signal indicative of the compressive force applied to the cell by the cell test jig at the corresponding location.

2. A cell test jig according to claim 1 comprising four displacement sensors and / or four force sensors.

3. A cell test jig according to any preceding claim where one, some or all of the sensors are locatedadjacent respective perimeters of the first and second plates.

4. A cell test jig according to any preceding claim where one, some or all of the sensors are located at their own respective and corresponding corners of the first and second plates.

5. A cell test jig according to any preceding claim where the clamp is arranged to retain the first and second plates at any of a range of separations and to be selectively and progressively actuated to adjust the separation and thereby permit varying of the compressive force applied to the cell.

6. A cell test jig according to any preceding claim comprising a biasing assembly arranged to act between the clamp and one of the first plate and the second plate to limit the force with which the clamp retains the first and second plates at a separation set to thereby accommodate a degree of swelling of the cell when under test.

7. A cell test jig according to claim 6 arranged to accommodate variation in the configuration of the biasing assembly used.

8. A cell test jig according to claim 6 or claim 7 where the biasing assembly comprises a disc spring.

9. A cell test jig according to any preceding claim comprising a plurality of the clamps.

10. A cell test jig according to claim 9 where at least one of the clamps is located adjacent respectiveperimeters of the first and second plates, and, optionally, where at least one of the clamps is located at respective and corresponding corners of the first and second plates.11, A cell test jig according to claim 9 or 10 where some or all of the displacement sensors and / or some or all of the force sensors are each located at a location substantially corresponding to a respective instance of the clamps.

12. A cell test jig according to any preceding claim comprising a cell carrier plate between the first and second plates and arranged to directly support the cell.

13. A cell test jig according to claim 12 having a support formation arranged to support the cell carrier plate in a manner allowing adjustment of the position of the cell carrier plate with respect to the first and second plates when the compressive force is applied, the positional adjustment being in a plane substantially perpendicular to the direction of the compressive force.

14. A cell test jig according to claim 13 where the support formation comprises a protrusion and a cooperating cupped seat, with the protrusion being received in the cupped seat and a pinnacle of the protrusion contacting a floor of the cupped seat to provide the support to the cell carrier plate and where further the cupped seat is dimensioned to give clearance all around a side wall of the protrusion, thereby allowing adjustment of the position of the protrusion within the cupped seat.

15. A cell test jig (200) according to any preceding claim having a jig terminal (298) that is directly supported by the cell test jig (200), the jig terminal (298) being arranged to be for connectioned to a charging cycler (304) via one or more cables (302) and the cell test jig (200) further comprising a connector (306) arranged to connect the jig terminal (298) to a cell terminal (308) of the cell (202) when installed for test in the cell test jig (200), optionally where the connector (306) is at least substantially rigid and further optionally where the connector (306) is reorientable and / or removable from the cell test jig (200) for installation and removal of the cell (202) from the cell test jig (200).where the connector is reorientable for installation and removal of the cell from the cell test jig and / or the connector is removable.

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