SE543914C2 - Production line station for a production line for producing battery cells or for assembling of battery packs - Google Patents

Abstract

The invention concerns a production line station (7, 8) for a production line (1 , 2) for producing battery cells (10, C) or for assembling of battery packs (100) comprising a plurality of battery cells (10, C), wherein the production line station (7, 8) comprises a measuring device (5, 6) configured to be connected to first and second terminals (11, 12) of a battery cell (10, C) in the production line (1, 2) and to determine at least one electrical property of the battery cell (10, C). The invention is characterized in that the measuring device (5, 6) comprises an impedance measuring arrangement (20, 21) configured to, when connected to the first and second terminals (11, 12), connect the battery cell (10, C) to a measurement circuit (30, 31), wherein the impedance measuring arrangement (20, 21) is further configured to: provide a time-varying test current (Itest) through the measurement circuit (30, 31); measure a voltage (Ucell) over the battery cell (10, C) while the time-varying test current (Itest) is provided; and determine a phase shift and a magnitude ratio between the measured voltage (Ucell) and the time-varying test current (Itest) in the measurement circuit (30, 31). The invention also concerns a production line comprising a production line station of the above type and a method for producing battery cells (10) or for assembling of battery packs (100) comprising a plurality of battery cells (10).

Description

There is an increasing interest for using battery packs made up of a pluralityof battery cells in various applications, such as for powering of electric or hybridvehicles or for use as stationary power sources. Presently, there is a particular focus on cells of the Li-ion type.

Typically, the individual cells in a battery pack have somewhat differentcapacities, for instance due to production variations, and may be at differentlevels of state of charge (SOC) etc. Most battery packs are provided with abattery management system (BMS) or at least some form of balancing circuitto e.g. prevent overcharging of individual cells and to increase power outputand extend life time of the battery pack. lt is generally an advantage if theproperties of individual cells differ from each other as little as possible becauseit makes battery managing and balancing easier and it improves the functionof the battery pack. ln addition, the charge capacity of a battery pack is normally limited to the capacity of the weakest/worst cell.

The desire to decrease quality fluctuations during production of high-energybatteries has been addressed by Schnell and Reinhart who proposes animproved quality management concept where different measures are takenduring the production process (Quality Management for Battery Production: AQuality Gate Concept, Precedia CIRP 57 (2016) 568-573). Exactly whichmeasurements should be made or which deviations should be detected is,however, not specified.

US2014/0212730 discloses a method of producing a battery pack where cellsare classified based on measurement of, in particular, change in internalresistance after application of a pressure, whereby cells having similarproperties are selected and grouped in corresponding battery packs so as toform battery packs ofdifferent quality. This is an interesting concept forformingbattery packs with more uniform performance of the cells making up the pack.However, it is not described how the pressure test should be carried out, andthe concept is less useful for cells for which pressure does not have anyparticular or consistent effect on the internal resistance, and it is also lessuseful in cases where “quality” of a battery cell has a wider or another meaningthan internal resistance after application of a pressure.

The problem related to cells having different capacity or quality is generallysolved by improving the battery managing or balancing system and/or bymeasuring the voltage over each cell, i.e. the voltage between the twoterminals/poles of the cell, prior to assembling the battery pack and sorting outcells with clear quality (voltage) defects.

There is still a desire for improvements in the field of identifying and handlinga variation of quality/performance of the individual cells to be used in battery packs.

SUMMARY OF THE INVENTION An object of this invention is to provide for a production line and method formanufacturing of battery cells or for assembling of battery packs that exhibitan improved quality control compared to conventional production lines. Thisobject is achieved by the production line and method defined by the technicalfeatures contained in the independent claims. The dependent claims containadvantageous embodiments, further developments and variants of the invenfion.

The invention concerns a production line station for a production line forproducing battery cells orfor assembling of battery packs comprising a pluralityof battery cells, wherein the production line station comprises a measuringdevice configured to be connected to first and second terminals of a batterycell in the production line and to determine at least one electrical property of the battery cell.

The invention is characterized in that the measuring device comprises animpedance measuring arrangement configured to, when connected to the firstand second terminals, connect the battery cell to a measurement circuit,wherein the impedance measuring arrangement is further configured to:provide a time-varying test current through the measurement circuit; measurea voltage over the battery cell while the time-varying test current is provided;and determine a phase shift and a magnitude ratio between the measured voltage and the time-varying test current in the measurement circuit.

The phase shift and magnitude ratio between voltage and current form thebasis for calculating the internal impedance of the battery cell. Although a finalcalculation of the impedance is useful in many applications, it is not necessaryto make this final calculation since the “raw data”, i.e. the voltage-current phase shift and magnitude ratio, already reflect the internal impedance of the cell.

Information on the internal impedance of the cell is useful since it reflects thequality of the interior of the cell. For instance, vague or slight electrode defectsmay be identified by comparing the determined internal impedance (or thedetermined “raw data”) with reference values for the same type of cell. Suchinformation is otherwise difficult to obtain; for instance, it cannot be obtainedby measuring only the voltage or resistance of the cell. Cells that exhibit asufficiently deviating impedance may be sorted out from the production line. Ageneral advantage of this is that also cells with slight defects, such as defectsthat may not be immediately noticed during first use of the cell but that mayincrease e.g. aging rate of cell, can be removed from the production line at anearly stage. At a cell production site, such cells can be removed already atproduction and at a site for assembling battery packs it can be avoided toinclude deviating or deficient cells in the pack.

The time-varying test current can include a broad frequency range or a pluralityof frequency ranges to cover various frequencies that might be of interest forthe type of battery cell to be analysed. Different properties of the cell chemistryand material properties respond to certain frequency ranges and if there is aninterest in some specific properties it is possible to focus the energy of the testcurrent to the corresponding frequencies. ln some applications it might,however, be sufficient to focus on a few frequencies or narrow frequencyranges. The term “time-varying” means that the test current oscillates withsome frequency or frequencies, which is in clear contrast to regular DC.

The use of several frequencies or a wide frequency range for determining aninternal impedance of e.g. electric circuits is sometimes denoted impedancespectroscopy and is known as such. Conventional impedance spectroscopy istypically used in research to understand the function or investigate agingeffects and similar of a circuit or component. Normally, a signal with one singlefrequency is applied each time and the analysis is relatively time-consuming ifmany frequencies are to be covered. The frequency range of interest in generalimpedance spectroscopy is typically in the range 10 mHz to 100 kHz, but wider ranges are tested in research while smaller ranges or even sub ranges can give much information about cell quality.

At least in theory, the time-varying test current generated in the measurementcircuit does not necessarily have to be measured but can be determined frominformation about the signal applied to the measurement circuit. For instance,if all electronic components involved in generating the test current worktheoretically perfect or alter a signal only in a fully predictable way, the currentin the measurement circuit will be identical to or form a known function of adesired (and known) control signal, such as a time-varying test stimulus signal.However, transistors, amplifiers and other components are likely to at leastslightly alter signals in an unpredictable way, so to get a reliable determinationof phase shift and magnitude ratio it is suitable to thoroughly measure theactual test current in the measurement circuit. Measurement of the test currentalso allows for the use of randomized test stimulus or other stimulus that are not fully known or controlled.

Voltage and current is determined/measured in a synchronized manner to allow for comparison between phase and magnitude. ln a state-of-the-art cell production or battery pack assembly line, with highthroughput of cells, there is not much time for making measurements on theindividual cells. To still allow for impedance measurements in such productionlines it is proposed in this enclosure an embodiment where a signal thatcontains several frequencies superimposed onto each other is used in the test current provided (and also in the time-varying test stimulus signal). lt is also proposed to combine such a signal with integrating the measuringdevice in an assembly line machine/robot, in particular an assembly line robotused for gripping, lifting and moving the battery cell in the production line (e.g.from the cell production line into a shipping container or out from a shippingcontainer into a battery pack at a line for assembling of battery packs). This way the impedance measurement can be carried out during lifting and movingof the cell, which will not interfere with the manufacturing of cells or packs. Atime period of maybe 20 seconds is then available, but this time window mightbe selected to be longer or shorter depending on the specific need in theapplication. The frequencies of the test current can also be adapted to a givenmaximum time window by leaving out frequencies that are too low to give ameaningful measurement (i.e. the frequencies may have too long periods to give a meaningful measurement).

The term production line station does not only refer to a production or assemblyline machine/robot as exemplified above but also to e.g. an impedancemeasurement station that can be more or less integrated with the productionline and the handling and measurement of the cells can be more or lessautomated. ln some variants, the cell may be transferred manually or by meansof some kind of gripping robot to an impedance measurement station locatedsomewhat aside of the production line. Moreover, the production line stationmay be arranged to measure several cells simultaneously, for instance byhaving a plurality of measuring devices or providing the measuring device witha plurality of impedance measuring arrangements with corresponding cell terminal connections. ln an embodiment the impedance measuring arrangement is configured tocalculate an internal impedance of the battery cell based on the determinedphase shift and magnitude ratio. ln an embodiment the impedance measuring arrangement is configured to measure the test current in the measurement circuit ln an embodiment the time-varying test current provided through themeasurement circuit comprises at least a first component forming a directcurrent component that centres the current through the measurement circuit at a certain current magnitude and a second component that is a time-varying test stimulus signal and that generates the time-variation of the time-varyingtest current when combined with the first component. ln an embodiment the first component of the time-varying test current is drawn from the battery cell. No external power source is thus needed. ln an embodiment the impedance measuring arrangement comprises current stimulus circuitry configured to generate the time-varying test stimulus signal. ln an embodiment the impedance measuring arrangement comprises atransistor configured to receive the time-varying test stimulus signal and tocontrol the first component of the time-varying test current. ln an embodiment the impedance measuring arrangement comprises currentsensing circuitry configured to generate a signal representing the test current in the measurement circuit. ln an embodiment the impedance measuring arrangement comprises voltagesensing circuitry configured to measure a voltage between the first and second terminals. ln an embodiment the impedance measuring arrangement comprises a dataacquisition system connected to the current stimulus circuitry, the currentsensing circuitry and the voltage sensing circuitry, wherein the data acquisitionsystem comprises a computation circuitry configured to calculate, based on the test current and the voltage, an internal impedance of the battery cell. ln an embodiment the impedance measuring arrangement comprises afeedback loop configured to: measure the signal representing the test currentin the measurement circuit; compare this signal with the time-varying teststimulus signal; and adjust an input voltage to the transistor so as to improveagreement between the two signals in the measurement circuit. This feedback arrangement compensates for nonlinear Characteristics inherent in thetransistor and ensures that the test current will follow the test stimulus. ln an embodiment the time-varying test current contains at least one frequencyorfrequency range within the interva| 10 mHz to 100 kHz. Preferably, the time-varying test current contains a plurality of superimposed frequencies orfrequency ranges within the interva| 10 mHz to 100 kHz. ln an embodiment the production line station is a production or assembly linemachine/robot. Preferably, the measuring device is integrated into theproduction or assembly line machine/robot. Preferably, the production orassembly line machine/robot is configured to grip and/or lift the battery cell. ln an embodiment the first and second terminals form a positive pole and a negative pole, respectively, of the battery cell. ln an embodiment the battery cells are of the Li-ion type.

The invention also concerns a production line for producing battery cells or forassembling of battery packs comprising a plurality of battery cells, wherein theproduction line comprises a production line station of the above type.

The invention also concerns a method for producing battery cells or forassembling of battery packs comprising a plurality of battery cells, wherein themethod comprises the step of: determining at least one electrical property ofthe battery cell by connecting a measuring device to first and second terminals of a battery cell in a production line.

The method is characterized in that it comprises the steps of:- connecting the battery cell to a measurement circuit of an impedancemeasuring arrangement arranged in the measuring device; - providing a time-varying test current through the measurement circuit; - measuring a voltage over the battery cell while the time-varying test currentis provided; and- determining a phase shift and a magnitude ratio between the measured voltage and the time-varying test current in the measurement circuit.

Embodiments of the method may comprise one or several of the followingsteps: - calculating an internal impedance of the battery cell based on thedetermined phase shift and magnitude ratio; - measuring the test current in the measurement circuit; - providing the time-varying test current through the measurement circuit by:providing at least a first component forming a direct current component thatcentres the current through the measurement circuit at a certain currentmagnitude; providing a second component that is a time-varying test stimulussignal and that generates the time-variation of the time-varying test current;and combining the first and second components; - drawing the first component of the time-varying test current from the batterycell; - generating the time-varying test stimulus signal by means of a currentstimulus circuitry; - receiving the time-varying test stimulus signal at a transistor arranged in theimpedance measuring arrangement; - controlling the first component of the time-varying test current by means ofthe transistor; - generating a signal representing the test current in the measurement circuitby means of a current sensing circuitry arranged in the impedance measuringarrangement; and/or - gripping and/or lifting the battery cell by means of a production or assemblyline machine/robot into which the measuring device is integrated while measuring the voltage over the battery cell.

BRIEF DESCRIPTION OF DRAWINGSln the description of the invention given below reference is made to the following figures, in which: Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 shows, in a schematic view, an example of a production line forproducing battery cells and an example of a production line forassembling of battery packs comprising a plurality of battery cells.shows, in a schematic view, a first embodiment of an impedancemeasuring arrangement suitable for use in e.g. the productionlines shown in figure 1. shows, in a schematic view, a second embodiment of animpedance measuring arrangement suitable for use in e.g. theproduction lines shown in figure 1. shows an example of a time-varying test stimulus signal that canbe used to generate a time-varying test current; the upper plotshows frequency contents of the signal and highlights thatdifferent amounts of energies can be used at different frequencyranges; the bottom plot shows the resulting signal that containsthe frequency spectrum from the upper plot. shows examples of Nyquist plot impedance spectra for a first anda second battery cell impedance measurement; each spectrumshows simultaneously the phase and magnitude of the compleximpedance; each dot represents the complex valued impedancefor a single frequency and entails the magnitude and phase by itsposition in the complex plane.

DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION Figure 1 shows a first production line 1 for production of battery cells 10 (denoted “C” in figs. 2 and 3) and a second production line 2 for assembling battery packs 100 containing a plurality of battery cells 10. ln this example each production line 1, 2 comprises a measuring device 5, 6 configured to be connected to first and second terminals 11, 12 of a battery cell 10 in the corresponding production line 1, 2 and to determine, by means of animpedance measuring arrangement 20, 21 (see figs. 2-3), an impedance ofthe battery cell 10. The measuring device 5, 6 is provided with first and secondconnection members (not shown) adapted to be connected to the first and second cell terminals 11, 12, respectively.

As shown in figure 1, a transportation of cells 10 from the first to the secondproduction line 1, 2 is indicated by some vehicles/vessels 3. lt is thus indicatedin figure 1 that the two production lines 1, 2 are located at different sites. Cellproduction and assembling of battery packs may, however, be more or lessintegrated with each other at the same site in which a particular transportationbetween the two separated production lines 1, 2 may not be needed. Fig. 1 isintended to illustrate that the invention is applicable in production lines for bothmanufacturing of cells and assembling of battery packs and that these linesmay or may not be located at a considerable distance from each other.

Fig. 1 further shows that each production line 1, 2 comprises a production linestation in the form of first and second production line robots 7, 8 forgripping/holding, lifting and moving of battery cells, in particular for placingbattery cells 10 (in this case one by one) into a shipping container 9a at thecell production line 1 and for lifting up individual battery cells 10 from a shippingcontainer 9b and placing them in the battery pack 100 at the battery packassembly line 2. ln case the two production lines 1, 2 are integrated there isno need to place battery cells 10 into shipping containers 9a, 9b (but perhapsinto some sort of storage containers) and it may be sufficient to use only oneproduction line robot 7, 8. Any particular production line gripping apparatuses,such as the robots 7, 8 shown in figure 1, are not required for the invention butprovides for a suitable embodiment where the measuring device 5, 6 isintegrated into the production line apparatus 7, 8.

Fig. 1 further shows that the measuring device 5, 6 is arranged onto, orintegrated in, each robot 7, 8 in connection with a gripping tool of the robot 7, 8 used to grip the battery cell 10. This means that the measuring device 5, 6will be positioned close to the cell 10 during gripping, Iifting and moving of thecell 10. This allows the measuring device 5, 6 to be connected to the terminals11, 12 when the robot 7, 8 grips the cell 10 and to maintain the connectionduring lifting and moving of the cell 10 until the cell 10 is released in theshipping container 9a or in the battery pack 100. During the time period spenton Iifting and moving of the cell 10, perhaps around 10-20 s in a typicalproduction line of interest here, the measuring device 5, 6 can carry outmeasurements on the cell 10, and these measurements can thus be carriedout without delaying the production process and even without affecting theproduction process at all. By properly positioning and fixing the measuringdevice 5, 6 to the corresponding robot/production line apparatus 7, 8 andmaking sure that the gripping tool holds the cell 10 steady during Iifting andmoving, the measuring device 5, 6 will not move in relation to the cell 10 duringIifting/moving. This simplifies securement of the electric connection betweenthe cell terminals 11, 12 and the measuring device 5, 6. The connection members may be integrated with the gripping tool of the robots 7, 8.

As will be further described in relation to figures 2 and 3, the impedancemeasuring arrangement 20, 21 is configured to, when connected to the firstand second terminals 11, 12, connect the battery cell 10, C to a measurementcircuit 30, 31. The impedance measuring arrangement 20, 21 is furtherconfigured to apply a time-varying test current through the measurement circuit30,31. ln the examples shown, this is carried out by drawing a DC current fromthe cell 10, C subject to the measurement (i.e. no external power source isneeded in these examples) and applying a time-varying stimulus signal lin tothe drawn current in the measurement circuit 30, 31. The time-varying testcurrent is thus composed of two components; the current drawn from the cell10, C forms a first component and the time-varying stimulus signal lin forms a second component.

The impedance measuring arrangement 20, 21 is further configured tomeasure, while the time-varying test current is applied (and thus also thestimulus signal lin), a voltage Uceii over the battery cell 10, C and the test currentliesi in the measurement circuit 30, 31. The test current liesi is likely to differ atleast slightly from the (in this case known) oscillating pattern of the stimulussignal lin because of non-linearity or temperature effects etc. of variouselectronic circuit components. Therefore, the test current liesi in the measurement circuit 30, 31 is measured in this embodiment.

The impedance measuring arrangement 20, 21 is further configured todetermine a phase shift and a magnitude ratio between the measured voltageUceii and test current liesi. These data can be used to calculate the internalimpedance of the battery cell 10, C. How to calculate the impedance is wellknown to a person skilled in the art.

The determined impedance (or the determined phase shift and a magnituderatio) can be compared to reference data and be used to identify potentialdefects inside the cell 10, C, such as defects in an electrode or in theelectrolyte or defects in the interface between electrode and electrolyte.Different frequencies of the time-varying test current liesi provide different kindof information on a particular type of cell 10, C and certain frequencies may bemore useful than other for different types of cells. The time-varying test currentliesi (or rather, in this example, the stimulus signal lin) may be adapted to thetype of cell 10, C to be analysed, i.e. the test current may comprise one orseveral selected frequency ranges that have significantly higher amplitudes. ltis also possible to include a very wide frequency range in the test current (or alarge number of more narrow frequency ranges) and make a selection offrequencies in the next step(s) (i.e. in the voltage and current measurementsand/or in the determination/calculation of the impedance).

Fig. 2 shows an impedance measuring arrangement 20 according to a firstembodiment. The poles Po+ 11 and Po- 12 of the cell C (that corresponds to cell 10 in figure 1) are connected in a first measurement circuit 30, whichcomprises also a current controlling transistor T and a combined load andcurrent measurement resistor RLn. The second connection point Po- isconnected to a common ground GND. The transistor, T, is here a MOSFET-transistor, but could also be another type of transistor, such as an NPN-transistor.

The measurement circuit 30 has the function of being loading only, i.e. it iscapable of pulling current from the cell C subject to the impedancemeasurement. This has the advantage that there is no need for a power supplythat charges the cell C, which makes the measurement device 5, 6 simple andsuitable for implementation in a cell and/or pack production line. lt is thearrangement with a transistor T and loading resistor RLu, that limits theoperation so the current can only flow in the direction indicated. This passivedesign makes the circuit able to only pull energy/current from the cell C.

The function ofthe loading resistor, RLn, is twofold: 1) it forms a loading resistorthat dissipates the power extracted from the cell C, and 2) it forms a currentmeasurement resistor where the voltage over the resistor gives informationabout the test current liesi in the measurement circuit 30 through Ohms law; asignal loui is generated that represents the actual test current liesi in themeasurement circuit 30. The dimensioning of the resistor RL+i can for examplebe 1.09, giving a maximum current from the cell C of lmaX=4A for a cell voltageof 4 V, and a maximum power dissipation of 16W, in the resistor. The accuracyof the resistor will determine the precision of the measurement of the testcurrent liesi. (The signal representing the test current liesi is actually a voltagesignal but is denoted loui, and not e.g. Uoui, to indicate that the signal is a measure of a current.) The transistor T together with an operational amplifier OA1, and the resistorRL+i constitute a voltage controlled current stimulus source. This source controls the time-variation of the current in the measurement circuit 30 based on a time-varying stimulus signal lin provided as an output voltage by a digital-to-analog converter in a data acquisition system D. (The time-varying teststimulus signal lin is actually a voltage signal but is denoted lin, and not e.g. Uin,to indicate that this signal controls the test current.) Without the voltage controlled current source and its control stimulus signal lin,the measurement circuit 30 would form a simple DC circuit. The test stimulussignal lin is a time-varying signal, i.e. it varies with certain frequencies etc., andwhen this stimulus signal is applied, via transistor T, to the current drawn fromthe cell C it generates a corresponding time-variation or oscillation in thecurrent in the measurement circuit 30. (The corresponding time variation in themeasurement circuit 30 does not necessarily exactly reflect that of the stimulussignal lin due to influences from electronic components, see above, andtherefore the actual test current liesi is measured in the embodiment described.)The time-varying test current has thus been applied through the measurementcircuit 30. The test current may, however, be applied in other ways.

The time-varying test current liesi may be selected to vary in accordance withthe applied frequencies around an average of linax/Z A, that depends on theselected resistance. Because of the impedance of the cell C there will be, atleast at certain frequencies or frequency ranges, some difference in phasebetween the test current liesi in the measurement circuit 30 and the voltage Uceiiover the cell C. By measuring the test current linsi, via the signal loiii, and thevoltage Uceii simultaneously the impedance and its magnitude and phase canbe calculated.

The transistor T with the operational amplifier OA1 coupled as a voltagefollower is a feedback loop controlling the current in the resistor Ri+i byadjusting the voltage at the gate of the transistor T. The feedback loop isprovided to reduce the influence of nonlinearities in the transistor T and othercomponents. lt produces a transistor gate signal that will make the test currentliesi follow the desired test stimulus lin and so that the desired spectrum of the test current llesl is achieved and the cell behaviour at certain frequencies canbe tested as desired. With the feedback the test signal will be controlled sothat it gets the desired spectrum. Without the feedback there is a risk that thenonlinearity of the transistor would influence the signal and thereby produce atest signal that differs from the desired test stimulus and thus testing the cellat other frequencies than desired.

Thanks to the feedback loop an lln=3V will give a voltage over RL+i of 3V andthus l0ul=3V which, with a resistance of RL+|=1Q, will correspond to a currentof lleSi=3A. With the resistance RL+|=1Q, used for illustration in this example,we have one-to-one correspondence between voltage and current and othervalues of the resistance RL+| will give other current and voltage relations according to Ohm's law ltest= low/RW.

The measurement circuit 30 is connected to the data acquisition system D thathas four connections: Uceii, lin, loui 0Ch GND.

GND is a common ground and forms a reference level. GND is connected tothe negative pole 12 of the cell C and to the loading resistor RLu.

The test signal lin that is used as a stimulus for the current in the measurementcircuit 30 is generated by a digital-to-analog converter (not shown) and sent tothe current control amplifier OA1.

The test current llesl is measured with the aid of the loading resistor RL+i and afirst analog-to-digital (AD) converter (not shown) in the data acquisition systemD arranged to be connected with the input for the signal loul that represents thetest current liesl. The measured voltage of the signal loul is used to calculate the test current ltest with the aid of Ohm's law ltest= lout/RLH.

The fourth connection, Uceii, is connected to a second AD converter (notshown) that measures the cell voltage and registers how it responds to the testcurrent liesl, i.e. how it responds to the current formed by applying the time-varying stimu|us signal lin to the current drawn from the cell C. The first andsecond AD converters are synchronized so that the amplitude ratios and phaseshifts between the representation loul of the test current liesi and the voltageUceii can be detected/measured at different frequencies. lnstead of using first and second analog-to-digital converters for providingvoltage and current signals, it is possible to use a single analog-to-digitalconverter provided with additional sample and hold circuits that are triggeredsimultaneously and use a multiplexer to shift the inputs and thereby providevoltage and current signals that are measured synchronously.

Fig. 3 shows a second embodiment of the impedance measurementarrangement 21. lt works according to the same basic principle as the firstembodiment (figure 2) where a current is drawn out of the cell C and iscontrolled in a time-varying manner by the transistor T. The main difference isthat the resistor RL+| of the first embodiment (figure 2) is split into two resistors:one loading resistor RL and one current measuring shunt resistor Ri. Theloading resistor RL takes up/dissipates the power extracted from the cell C,and it is particularly dimensioned to be capable of being heated up and todissipate the power as heat. ln this second embodiment, the shunt resistor Rlis selected small so that it will dissipate only small amounts of power and heatup only to a low degree. This reduces the potential influence of the temperatureon the current control and measurement, since the resistance of the shuntresistor Rl can be dependent on the temperature. With this arrangement, atemperature variation that would cause changes in RL will not influence theaccuracy of the signal loui representing the test current liesi thanks to the feedback loop.

Since the shunt resistor Ri is small the voltage over it will also be low and thevoltage signals loui representing the actual test current liesi will be small. Toincrease the accuracy of the current measurement/calculation at the dataacquisition system D and the accuracy of the current control at OA1, a secondamplifier OA2 is implemented to amplify the original iom-signal generated at theshunt resistor Ri. The gain of the second amplifier OA2 is selected with theresistors Ri and R2. The amplified signal loui is measured in the first ADconverter and also fed to the current feedback controller that controls thetransistor, T. The data acquisition system D measures loui and Uceii in the same way as described in relation to the first embodiment in Fig. 2.

The resistance of the load resistor RL is preferably selected so as to beadapted to the capacity and voltage of the battery cell C to be measured andto how much current is desired to be drawn from the battery cell C. As anexample, if it is of interest to perform tests with high currents reaching 60 A theload resistor RL may have a resistance of 0.06 Ohm. However, almost anyresistance might be used.

The resistance of the shunt resistor Ri should in principle be as small aspossible (without generating a too weak signal) and it should also be calibratedand thermally stable in order to ensure proper measurement. As an example,the shunt resistor Ri may have a resistance of 0.1 mOhm. However, almostany other resistance may be used.

The third resistor R1 may have a resistance of 10 Ohm while the fourth resistorRz may have a resistance of 10 kOhm. However, other values can be chosenfor these components. ln order to achieve a desired amplification with regardsto the second operational amplifier OA2, to give a high gain, the fourth resistorR2 may have a significantly larger resistance than the third resistor Ri. Thethird resistor R1 may for instance have a resistance of 1-100 Ohm, while thefourth resistor Rg may have a resistance of 1-100 kOhm.

To increase accuracy of current control and measurements it is beneficial toapply state of the art in the selection of components (e.g. A/D, D/A and OP).Further, the A/D converters could be coupled for differential measurement sothat the voltage over each component is measured, instead of making themeasurement against a common ground reference. This would make thesignals less sensitive to noise on the common ground. Additionalimprovements can be achieved by employing a system with four-wiremeasurement, i.e. using two wires to measure the current and generate loui,using RL+| or Ri as in circuits 20 and 21 respectively, and two wires directlyconnected to the cell poles 11 &12 for measuring the cell voltage. ln this case,differential measurements are also preferable, where both pairs of wires arecoupled to differential A/D input channels. lt is possible to design themeasurement circuit in other ways. As an example, the current sensing resistorRi could be replaced with another current sensor technology, e.g. an inductor based transducer. ln order for the impedance measurement to be conducted during a short timeinterval, e.g. the brief time-window of attaching/gripping, lifting, transporting,placing and detaching the battery cell 10/C as described in relation to figure 1,there is a need of having a time-efficient measurement. This is achieved by amulti-frequency signal lin sent from the digital-to-analog converter of the dataacquisition system D to the control gate of the transistor T for controlling thecurrent drawn from the battery cell 10/C, which generates a similar multi-frequency test current liesi. The multi-frequency signal can for instance be amulti-sine signal comprising sine signals corresponding to a predeterminedrange of frequencies or predetermined set of frequency ranges to bemeasured. lt can also be another type of signal comprising the frequencies tobe measured, such as for instance a pseudo random binary sequence or a burst signal comprising a selected set of frequency ranges.

Figure 4 shows an example of how a test stimulus signal could be designed. ltshows the signal in the frequency domain (top) and time domain (bottom). The upper diagram shows the signal's energy contents in the frequency domain.The areas that have energy are indicated by black areas, the signal energy isconcentrated to low frequencies; the lowest range 0-5 kHz have highestenergy contents and the range 5-10 kHz has a lower energy, while the range10-50 kHz does not have any energy and is thus zero. The spectrum for realsignals is symmetric around half the sampling rate (100 kHz), this explains whythe low frequency region 0-10 kHz is mirrored to 100-90 kHz. Furthermore thephase of the individual frequencies are randomly altered between 0 and 180degrees, this is performed so that the time domain signal will have a limited and balanced amplitude.

The lower diagram in figure 4 shows the resulting time-varying test signal, i.e.the test stimulus signal lin. The time domain signal is generated from thespectrum using the inverse discrete fourier transform and therefore containsthe selected frequency contents. The symmetry of the spectrum and therandom phase shift gives time domain signal that starts with an amplitude of 0and attains the highest amplitude in the center without growing to too largeamplitudes. There are many signals that have the same spectral properties,but preferably a test signal is selected that has a balanced and limitedamplitude, like the one in the lower diagram of figure 4, so that it is possibleand easy to realize it in the D/A converter and current controller. Signal generation as such is well known to a person skilled in the art.

Analysis of the measured voltage Uceii over the cell 10, C and the test currenthest, via the signal lout, in the measurement circuit 30, 31 may be carried outusing e.g. fast fourier transform (FFT) or any other suitable signal processingmethod for frequency analysis.

The measuring device 5, 6, and in particular the impedance measuringarrangement 20, 21, is preferably designed to be capable of generating thedesired frequency range(s) to the test current applied to the measurementcircuit 30, 31 whatever is deemed interesting for the cell type concerned. (lt should of course also be designed to be capable of measuring the voltage andcurrent at these frequencies and be capable of analyzing the measured data.) The stimulus signal lin can be composed of several sub signals coveringdifferent frequency ranges of interest. These frequency ranges may havedifferent amplitudes in the multi-frequency stimulus signal depending on theimportance of the specific frequencies, like in the upper plot in figure 4. Thisgives a higher signal to noise ratio in the most important frequencies in thepredetermined range of frequencies to be measured. The predeterminedranges of frequencies may be determined so that the signal components thatrepresent the frequencies are orthogonal to each other, this reduces thedisturbances between the sub signals Figure 5 shows an example of a spectrogram that results from such ananalysis. ln the plot the amplitude and phase of the frequency response isvisualized in a complex valued plot, called Nyquist diagram, where the x-axiscorresponds to the real value and the y-axis to the imaginary values withnegative sign (this representation is selected for agreement with thespectroscopy community that use this representation for historical reasons).

Figure 5 illustrates how impedance measurements can be used to identify acell with potential defects or deviations, and figure 5 also forms an example of(sets of) reference values that can be compared to identify cells with potentialdefects or deviations. A first (upper/left) series of dots 51 shows the impedancespectrum of a new Li-ion battery cell and a second (lower/right) series of dots52 shows the impedance spectrum of the same battery cell after ageing. Thatis, the first series 51 represents a set (a series) of reference values and thesecond series 52 represents a similar cell with defects. Various methods maybe applied to automatically (mathematically) compare spectra and decidewhether a certain cell exhibiting a certain impedance spectrum should beclassified as defect or not. lf the internal impedance of an individual cell deviates from a reference value(or from a series of reference values) with more than a certain margin, thisindividual cell may be sorted out from the production line 1, 2. Alternatively orsupplementary, data related to the internal impedance of the battery cell maybe stored in a database. Such data may for instance be the determined internalimpedance at discrete frequencies, or may be a parameterization of thespectrum constituted by the internal impedance at the discrete frequencies.This data could then for instance be used later to follow up discrepancies whenhandling a faulty battery pack in use.

The invention is not limited by the embodiments described above but can bemodified in various ways within the scope of the claims. For instance, themeasuring device 5, 6 need not necessarily be integrated into an assemblyrobot or other production line apparatus. Further, the test current, or at leastthe first component thereof, may be supplied from an external power sourceinstead of being drawn from the battery cell subject to measurement.

Claims (28)

1. Production line station (7, 8) for a production line (1, 2) for producing battery cells (10, C) or for assembling of battery packs (100) comprising a plurality of battery cells (10, C), wherein the production line station (7, 8) comprises a measuring device (5, 6) configured to be connected to first and second terminals (11, 12) of a battery cell (10, C) in the production line (1, 2) and to determine at least one electrical property of the battery cell (10, C), characterized in that the measuring device (5, 6) comprises an impedance measuring arrangement (20, 21) configured to, when connected to the first and second terminals (11, 12), connect the battery cell (10, C) to a measurement circuit (30, 31), wherein the impedance measuring arrangement (20, 21) is further configured to: - provide a time-varying test current (Itest) through the measurement circuit (30, 31); - measure a voltage (Ucell) over the battery cell (10, C) while the time-varying test current (Itest) is provided; and - determine a phase shift and a magnitude ratio between the measured voltage (Ucell) and the time-varying test current (Itest) in the measurement circuit (30, 31).

2. Production line station (7, 8) according to claim 1, wherein the impedance measuring arrangement (20, 21) is configured to calculate an internal impedance of the battery cell (10, C) based on the determined phase shift and magnitude ratio.

3. Production line station (7, 8) according to claim 1 or 2, wherein the impedance measuring arrangement (20, 21) is configured to measure the test current (Itest) in the measurement circuit (30, 31) .

4. Production line station (7, 8) according to any one of the above claims, wherein the time-varying test current (Itest) provided through the measurement circuit (30, 31) comprises at least a first component forming a direct current component that centres the current through the measurement circuit (30, 31) at a certain current magnitude and a second component that is a time-varying test stimulus signal (Iin) and that generates the timevariation of the time-varying test current (Itest) when combined with the first component.

5. Production line station (7, 8) according to claim 4, wherein the first component of the time-varying test current (Itest) is drawn from the battery cell (10, C).

6. Production line station (7, 8) according to claim 4 or 5, wherein the impedance measuring arrangement (20, 21) comprises current stimulus circuitry (OA1, T) configured to generate the time-varying test stimulus signal (lin)·

7. Production line station (7, 8) according to anyone of claims 4-6, wherein the impedance measuring arrangement (20, 21) comprises a transistor (T) configured to receive the time-varying test stimulus signal (lin) and to control the first component of the time-varying test current (Itest).

8. Production line station (7, 8) according to any one of the above claims, wherein the impedance measuring arrangement (20, 21) comprises current sensing circuitry (RL+I, RI, OA2) configured to generate a signal (Iout) representing the test current (Itest) in the measurement circuit (30, 31).

9. Production line station (7, 8) according to any one of the above claims, wherein the impedance measuring arrangement (20, 21) comprises voltage sensing circuitry (D) configured to measure a voltage (Ucell) between the first and second terminals (11, 12).

10. Production line station (7, 8) according to claims 6, 8 and 9, wherein the impedance measuring arrangement (20, 21) comprises a data acquisition system (D) connected to the current stimulus circuitry (OA1, T), the current sensing circuitry (RL+I, RI, OA2) and the voltage sensing circuitry, wherein the data acquisition system (D) comprises a computation circuitry configured to calculate, based on the test current (Itest) and the voltage (Ucell), an internal impedance of the battery cell (10, C).

11. Production line station (7, 8) according to claims 7 and 8, wherein the impedance measuring arrangement (20, 21) comprises a feedback loop configured to: measure the signal (Iout) representing the test current (Itest) in the measurement circuit (30, 31); compare this signal (Iout) with the timevarying test stimulus signal (Iin); and adjust an input voltage to the transistor (T) so as to improve agreement between the two signals (Iout, Iin) in the measurement circuit (30, 31).

12. Production line station (7, 8) according to any one of the above claims, wherein the time-varying test current (Itest) contains at least one frequency or frequency range within the interval 10 mHz to 100 kHz.

13. Production line station (7, 8) according to claim 12, wherein the timevarying test current (Itest) contains a plurality of superimposed frequencies or frequency ranges within the interval 10 mHz to 100 kHz.

14. Production line station (7, 8) according to any one of the above claims, wherein the production line station is a production or assembly line machine/robot (7, 8).

15. Production line station (7, 8) according to claim 14, wherein the measuring device (5, 6) is integrated into the production or assembly line machine/robot (7, 8).

16. Production line station (7, 8) according to claim 14 or 15, wherein the production or assembly line machine/robot (7, 8) is configured to grip and/or lift the battery cell (10, C).

17. Production line station (7, 8) according to any one of the above claims, wherein the first and second terminals (11, 12) form a positive pole and a negative pole, respectively, of the battery cell (10, C).

18. Production line station (7, 8) according to any one of the above claims, wherein the battery cells (10, C) are of the Li-ion type.

19. Production line (1, 2) for producing battery cells (10, C) or for assembling of battery packs (100) comprising a plurality of battery cells (10, C), characterised in that the production line (1, 2) comprises a production line station (7, 8) according to any one of the above claims.

20. A method for producing battery cells (10) or for assembling of battery packs (100) comprising a plurality of battery cells (10), wherein the method comprises the step of: - determining at least one electrical property of the battery cell (10, C) by connecting a measuring device (5, 6) to first and second terminals (11, 12) of a battery cell (10, C) in a production line (1, 2), characterized in that the method comprises the steps of: - connecting the battery cell (10, C) to a measurement circuit (30, 31) of an impedance measuring arrangement (20, 21) arranged in the measuring device (5, 6); - providing a time-varying test current through the measurement circuit (30, 31); - measuring a voltage (Ucell) over the battery cell (10, C) while the timevarying test current is provided; and - determining a phase shift and a magnitude ratio between the measured voltage (Ucell) and the time-varying test current (Itest) in the measurement circuit (30, 31).

21. Method according to claim 20 comprising the step of: - calculating an internal impedance of the battery cell (10, C) based on the determined phase shift and magnitude ratio. 22. Method according to claim 20 or 21 comprising the step of: - measuring the test current (Itest) in the measurement circuit (30, 31) .

22. Method according to any one of claims 20-22 comprising the step of: - providing the time-varying test current (Itest) through the measurement circuit (30, 31) by: providing at least a first component forming a direct current component that centres the current through the measurement circuit (30, 31) at a certain current magnitude; providing a second component that is a time-varying test stimulus signal (Iin) and that generates the time-variation of the time-varying test current (Itest); and combining the first and second components.

23. Method according to claim 22 comprising the step of: - drawing the first component of the time-varying test current (Itest) from the battery cell (10, C).

24. Method according to claim 22 or 23 comprising the step of: - generating the time-varying test stimulus signal (Iin) by means of a current stimulus circuitry (OA1, T)

25. Method according to any one of claims 22-24 comprising the step of: - receiving the time-varying test stimulus signal (Ιin) at a transistor (T) arranged in the impedance measuring arrangement (20, 21), and - controlling the first component of the time-varying test current (Itest) by means of the transistor (T).

26. Method according to any one of claims 20-25 comprising the step of: - generating a signal (Iout) representing the test current (Itest) in the measurement circuit (30, 31) by means of a current sensing circuitry (RL+I, RI, OA2) arranged in the impedance measuring arrangement (20, 21).

27. Method according to any one of claims 20-26 comprising the step of: - gripping and/or lifting the battery cell (10, C) by means of a production or assembly line machine/robot (7, 8) into which the measuring device (5, 6) is integrated while measuring the voltage (Ucell) over the battery cell (1 0, C).