Method and apparatus for current excitation for impedance measurement

Abstract

The present invention relates to a method and an apparatus for impedance measurement (EIS measurement). Known apparatuses and methods suffer from the disadvantage that only asymmetric voltages referenced to the battery voltage are provided, such that the voltage across the inductor has unequal magnitudes in different switching states, resulting in limited and asymmetric voltage headroom. The problem is solved by a method for exciting at least one current through an electrochemical element. The at least one current is excited by means of time-varying control of at least two switching elements of at least one half-bridge. In this case, the half-bridge is supplied simultaneously by a first and a second voltage which are different from one another and are both different from the potential at a pole, in particular different from the potentials at both poles, of the electrochemical element.

Description

[0001] RAFFAY & FLECK

[0002] 3017PA029WO

[0003] Method and device for current excitation for impedance measurement

[0004] This document relates to a method and a device for measuring the impedance (EIS measurement) of a battery.

[0005] This is well known from the prior art, for example from “On-line Electrochemical Impedance Spectroscopy for Lithium-Ion Battery Systems Estimation, Compensation and Avoidance of Measurement Deviations” Koch, Reinhold or US 10,386,422 B2 or US 10,566,817 B2.

[0006] A device for transferring charges between half-cells is also known from WO 2021 / 175,623 A1. However, this method is not applicable if only the positive and negative terminals of the accumulator have a sufficient conductor cross-section.

[0007] Furthermore, the internal resistances and impedances of conventional batteries, especially those with high power ratings, are becoming increasingly smaller, which poses major difficulties for conventional devices, particularly during charging and discharging, but also in the case of EMC influences from the environment.

[0008] Known devices and methods have the disadvantage in such cases that only asymmetrical voltages with respect to the battery voltage are provided, so that the voltage across the inductor is unequal in magnitude across switching states, resulting in small and asymmetrical headroom. This leads to the (minimum or maximum) duty cycle of the

[0009] Raffay & Fleck • Stephansplatz 2-6 • 0-20354 Hamburg

[0010] Tel.: +49 (0) 40 47 80 23 • Fax: +49 (0) 40 480 25 02 • info@raffay-fleck.de The individual semiconductor, which changes over the period of the excited current, must be chosen to be very small or very large, leading to poor utilization of the semiconductor or to excitations that deviate significantly from sinusoidal excitations, especially because the necessary current transconductance cannot be achieved. This is particularly problematic for high battery voltages. Furthermore, to establish symmetry, a supply voltage twice as high as the battery voltage must be used, resulting in high component costs and significant equipment complexity, especially if this causes devices to fall under the medium-voltage standard, as is the case, for example, with 800 V batteries.

[0011] Classical systems operate with linear output stages, resulting in low efficiency because the entire voltage, which we refer to as headroom (positive supply voltage of the output stage minus battery voltage, or battery voltage minus negative supply voltage of the output stage), must drop completely across the output transistors, generating corresponding losses. Linear solutions are also difficult to handle with high currents. This necessitates large cooling systems, significantly larger power supplies, filters, and other components, and is also very energy-intensive.

[0012] With switched-mode output stages (see Class-D amplifiers), the efficiency is significantly higher, and it is possible to briefly store energy in the intermediate circuit during the negative half-cycle and reuse it during the positive half-cycle. In another design, it is also advantageous to use a bidirectional voltage source to feed the energy back into the circuit.

[0013] With a bridge circuit and filter choke (buck converter), a certain headroom is required to magnetize a specific current within a given time (di / dt = v_L / L). The necessary di / dt can be derived from the required excitation signal. A distinction is made between positive or upper headroom and lower or negative headroom. In classic configurations, their sum is determined by the supply voltage, and their distribution is determined by the voltage of the electrochemical element. One headroom is equal to the voltage of the electromagnetic element, and the other is equal to the supply voltage minus the voltage of the electrochemical element.

[0014] If the headroom is chosen to be too small or the inductance of the choke too large, it is physically impossible to achieve a sufficient current transconductance. In reality, line and contact resistances also come into play, causing a voltage drop that is not negligible at high currents, thus reducing the voltage across the choke. Therefore, the headroom must always be large enough to achieve a high current transconductance even at high currents.

[0015] Furthermore, it is generally very difficult to achieve high bandwidths at high currents with switched-mode power amplifiers, as this requires very high switching frequencies or multiple parallel phases. This necessitates appropriate components, processing power, and control systems.

[0016] The object of the invention is to provide a more efficient generation of the excitation current, which can achieve high bandwidths in particular with comparatively simple hardware and comparatively low power losses.

[0017] This is solved by a device according to claim 1, a method according to claim 12 and a use according to claim 15.

[0018] The solution according to the invention offers the advantages that, in particular, symmetrical voltages around the battery voltage can be provided, that galvanic coupling or isolation can be achieved using DC-DC converters, and that the voltage across the storage inductance of the half-bridge is averaged over operation and kept at a similar magnitude, thus achieving symmetrical headrooms. This leads to a more uniform and therefore better utilization of the semiconductors. As a result, higher dynamic ranges (higher bandwidth) can be enabled, the power dissipation can be distributed more evenly or even symmetrically across the semiconductors, and thus more efficient cooling can be achieved. Furthermore, the voltage of the input voltage supply, i.e., the supply voltage of the voltage source, needs to be less than twice the battery voltage to obtain symmetrical voltages for the output bridge.

[0019] Considering a 12.8V cell made up of four 3.2V battery cells connected in series, and using a conventional 1.5V supply voltage, one achieves only 1.5V - 12.8V = 2.2V positive headroom and -12.8V negative headroom. However, if one desires 10V headroom for both positive and negative applications, +2.8V and +22.8V would result (unipolar), while 20V headroom for each would result in -7.2V and +32.8V (bipolar). With a conventional configuration and a 40V supply voltage, only -1.8V and 27.2V headroom are achieved.

[0020] The problem is solved by a method for exciting at least one current through an electrochemical element, in particular comprising a plurality of cells connected in series between a first pole and a second pole, in particular for carrying out at least one impedance measurement, in particular electrochemical impedance spectroscopy, in particular comprising the determination of at least one impedance value, in particular a plurality of impedance values, of the element.

[0021] The excitation of at least one current is achieved by means of time-varying control of at least two switching elements of at least one half-bridge. The voltage supply to the half-bridge is simultaneously provided by a first and a second voltage, which are different from and both unequal to the potential of one pole, and in particular unequal to the potentials of both poles, of the electrochemical element.

[0022] The problem is also solved by using two voltages to simultaneously supply a half-bridge for exciting at least one current through an electrochemical element with at least two poles, in particular comprising a plurality of cells connected in series between a first pole and a second pole, in particular for carrying out at least one impedance measurement, in particular electrochemical impedance spectroscopy, in particular comprising the determination of at least one impedance value, in particular a plurality of impedance values, of the element, wherein the excitation of the at least one current is carried out by means of time-varying control of at least two switching elements of the half-bridge, characterized in that the voltages are different and unequal to the potential of one pole, in particular unequal to the potentials of both poles, of the electrochemical element.

[0023] The problem is also solved by a device for exciting at least one current through an electrochemical element, in particular comprising a plurality of cells connected in series between a first pole and a second pole, in particular for carrying out at least one impedance measurement, in particular electrochemical impedance spectroscopy, in particular configured for determining at least one impedance value, in particular a plurality of impedance values, of the element, wherein the device has a first connection for connecting a first pole of the element to be measured and a second connection for connecting a second pole of the element to be measured, wherein the device has at least one half-bridge with two switching elements.the first switching element of which is arranged between a first supply conductor and the first terminal and the second switching element of which is arranged between the first terminal and a second supply conductor (whereby the arrangement can be made directly at the first terminal and / or at the second supply conductor, but does not have to be; further elements can be interposed, such as at least one choke and / or at least one filter), wherein the device is configured to control the switching elements of the at least one half-bridge in a variable manner over time in order to excite the at least one current, and wherein the device has a voltage source, in particular a bidirectional one, which provides a first non-zero voltage with respect to the second terminal on the first supply conductor and a second voltage, in particular a voltage with reverse polarity, with respect to the second terminal and in particular a voltage with respect to the second terminal, and in particular a voltage with respect to the second terminal,on the second supply line.

[0024] The problem is further solved by a device for impedance measurement comprising an excitation device according to the invention and a device for impedance measurement comprising a first measuring conductor and a second measuring conductor for connection to an electrochemical element, in particular the electrochemical element, for measuring the impedance when a current flows through the electrochemical element and changes over time, in particular when excited by the excitation device. If the electrochemical element has at least two electrochemical cells connected in series, in particular at least one more measuring conductor than the number of electrochemical cells connected in series, in particular twice the number of electrochemical cells connected in series, is used. In particular, when electrochemical cells are additionally connected in parallel, only the stages of the series connection are counted.

[0025] The problem is further solved by an arrangement consisting of a device for excitation or a device for impedance measurement, wherein the electrochemical element is connected with its first pole to the first terminal and in particular also to the first measuring conductor and with its second pole to the second terminal and in particular also to the second measuring conductor.

[0026] The invention can also be applied to electrochemical elements with electrochemical cells connected in parallel and / or in series.

[0027] The device and / or method or use of a plurality of half-bridges according to the invention is particularly advantageous. The half-bridges can be supplied with a common voltage source or with a plurality and / or separate voltage sources.

[0028] Advantageously, the method is carried out using a device according to the invention, the use includes the use of a device according to the invention, and / or the device is configured to carry out a method according to the invention. For this purpose, it particularly includes a correspondingly configured control device.

[0029] It is known from the prior art to operate the switching elements of a half-bridge for exciting a current by switching the switching elements with duty cycles / modulation levels that vary over time (even within a period of the current to be excited) and thereby, for example, to generate approximately sinusoidal currents. Pulse width modulation and / or pulse density modulation can be used for this purpose. This can also be achieved with the invention.

[0030] Numerous electrochemical cells are possible, including conventional batteries as well as fuel cells. The cell can consist of several electrochemical cells, particularly those connected at least partially in series, and especially identical cells. A cell can be, for example, a battery cell or a fuel cell.

[0031] The poles of the electrochemical element can be formed by an anode and a cathode.

[0032] The switching elements are usually controllable two-poles of the half-bridge, which are controlled / switched in particular by means of a digital switching arrangement.

[0033] Advantageously, the voltage source is / is implemented using a bidirectional voltage source and / or two DC / DC converters, which can be connected and / or linked to a unipolar input voltage supply. This allows for a very efficient provision of the necessary power supply.

[0034] Advantageously, at least one capacitor is / is arranged between the first supply conductor and the second terminal, and between the second supply conductor and the second terminal, and / or a filter and / or a choke is arranged between the half-bridge and the first terminal, and / or between the second and first terminals. This allows for smoothed excitation.

[0035] Ideally, the first and / or second voltage differences (first and / or second headroom) are selected such that the same average power dissipation occurs at both switching elements and / or the possible dynamic range is maximized. Theoretically, this is achieved with equal voltage differences. However, limitations such as the maximum or minimum duty cycle of one or both switching elements lead to limiting boundary conditions in some designs. Therefore, to optimize the power dissipation distribution across both switching elements and / or maximize the possible dynamic range, voltage differences deviating from a symmetrical design must be selected. Similarly, when using a device for electrochemical elements with different nominal voltages or voltages, optimization can only be achieved across the application range, which in some cases must lead to a deviation from the optimum.Nevertheless, the solution according to the invention offers significant advantages over the prior art even in these cases.

[0036] Consider, for example, the case where the switching elements have a minimum duty cycle of at least 5% of the period and can only be used with a maximum duty cycle of 85%. This results in a usable range of 5% to 85%. It would therefore be advantageous not to set the "idle duty cycle" (dO) to 50%, but to 45%, so that it lies in the middle of the usable range and a symmetrical range for magnetization is achieved. However, this would also mean that the headroom would no longer be symmetrical, and ideally the following equation would have to be satisfied: and

[0037] With vhr, P as positive headroom and vkr,n as negative headroom and

[0038] Vout — Vbat — Vout,0 as the voltage of the electrochemical element then results in Vfjr ,n

[0039] Preferably, a first voltage difference (first headroom) between the first voltage and the first terminal, in particular the voltage of the electrochemical element, and / or a second voltage difference (second headroom) between the first terminal and the second voltage is chosen such that the first voltage difference and the second voltage difference are approximately equal in magnitude or approximately adapted to the duty cycle and / or their ratio lies approximately in the range between one and a choice adapted to the duty cycle.

[0040] The term "approximately" refers in particular to a deviation from the aforementioned design of less than 25%, in particular less than 15%, in particular half of the sum of the first and second voltage differences and / or the ratio.

[0041] A selection adapted to the duty cycles exists in particular if the adaptation is based on the minimum and maximum duty cycle of the first, second and / or switching elements of the at least one half-bridge, in particular the largest minimum duty cycle and the smallest maximum duty cycle among the switching elements.

[0042] The adjustment to the duty cycles is carried out in such a way that the voltage difference between the higher of the first and second voltages and the voltage difference between the lower of the first and second voltages has a ratio that corresponds to the difference of the reciprocal of the arithmetic mean of the smallest and largest duty cycles minus 1.

[0043] In the example above, the ratio was 1.222 and the range from the symmetrical design to the design adapted to the duty cycle would run from 1 to 1.222, and the range from the approximately symmetrical design to the design adapted to the duty cycle would run from 0.75 to 1.53.

[0044] It is particularly advantageous to design the half-bridge as a buck converter, step-down converter, and / or Class-D amplifier. This allows for a further reduction in power loss.

[0045] It is advantageous to place at least one capacitor in parallel with the input of the voltage source, especially a bidirectional one. This stabilizes the voltage supply, which is particularly important during dynamic operation of the half-bridge.

[0046] A particular advantage is that the switching elements are each formed by at least one transistor, especially a BJT and / or field-effect transistor, such as a MOSFET, IGBT, GIT, HEMT and / or JFET. This allows for high dynamic range and low power dissipation.

[0047] Preferably, the voltage source is configured to generate the first and second voltages from a unipolar DC voltage source (DC-Link).

[0048] It is advantageous to perform at least one impedance measurement as a four-terminal measurement and / or to have a device / arrangement designed for four-terminal impedance measurement.

[0049] The device formed by the generation of the first and / or second voltage, in particular the bidirectional voltage source, the DC link capacitors and / or the half-bridge are used to particular advantage for short-term energy storage. This allows for a further increase in efficiency.

[0050] The following purely schematic and non-limiting figures illustrate some aspects of the invention by way of example, showing:

[0051] Fig. 1 shows an arrangement with power supply, half-bridge and electrochemical element from the prior art,

[0052] Fig. 2 illustrates the essential difference between an arrangement according to Fig. 1 and the invention and

[0053] Fig. 3 as well as

[0054] Fig. 4 advantageous designs and arrangements of measuring leads when several cells are connected in series,

[0055] Fig. 5 shows a very schematic representation of a possible embodiment of a device according to the invention with a plurality of half-bridges according to the invention, and Fig. 6 shows a possible embodiment of a device according to the invention with a plurality of half-bridges according to the invention.

[0056] Fig. 1 shows a half-bridge with two MOSFETs, which is powered via Vin between Vn / GND and Vp. The electrochemical cell (on the right in the diagram) also has one of its poles connected to GND. By switching the two MOSFETs, a variable excitation current Lut is generated.

[0057] Fig. 2 shows the transformation to a device according to the invention by eliminating the direct coupling of Vn and the pole of the electrochemical element (indicated by the cross on the dashed connection) and creating a non-zero voltage Vn between GND and the source of the lower MOSFET.

[0058] Fig. 3 shows a possible embodiment of a device according to the invention with two DC / DC converters as a power supply, which generate two supply voltages (Vp, Vn) from a DC link DC voltage (Vin), with which the half-bridge (output inverter) is powered. Three capacitors are arranged for voltage stabilization. An output filter is provided for smoothing. The electrochemical cell is shown on the far right.

[0059] Fig. 4 shows possible arrangements for measuring conductors when several cells are connected in series. This arrangement can, for example, be used in place of the electrochemical cell in Figure 3.

[0060] Ideally, two measuring leads are used per cell. However, it is also possible, with slight limitations, to use only one measuring lead between two cells.

[0061] Fig. 5 shows a very schematic representation of a possible embodiment of a device according to the invention with a plurality of half-bridges according to the invention, whereby the second connection is not shown. Fig. 6 shows the arrangement from Fig. 5 in a somewhat less schematic representation. The parallel connection of the half-bridges according to the invention can be seen.

Claims

Claims 1. Device for exciting at least one current through an electrochemical element, wherein the device has a first terminal for connecting a first pole of the element to be measured and a second terminal for connecting a second pole of the element to be measured, wherein the device has at least one half-bridge with two switching elements, the first switching element of which is arranged between a first supply conductor and the first terminal and the second switching element of which is arranged between the first terminal and a second supply conductor, wherein the device is configured to control the switching elements of the at least one half-bridge in a variable manner over time in order to excite the at least one current, characterized in that the device has a voltage source, in particular a bidirectional one, which provides a first non-zero voltage with respect to the second terminal on the first supply conductor and a second,in particular, provides a voltage of reverse polarity, a voltage not equal to the first voltage with respect to the second terminal, and in particular a voltage not equal to zero with respect to the second terminal, on the second supply conductor.

2. Device according to claim 1, wherein the voltage source is symmetrical and / or has two DC / DC converters, which are particularly connectable and / or connected for supply with a unipolar input voltage supply.

3. Device according to one of the preceding claims, wherein at least one capacitor is connected between the first supply conductor and the second connection and between the second supply conductor and the second connection, and / or wherein at least one capacitor is connected between the half-bridge and the first supply conductor and the second connection. A filter and / or a choke is arranged between the second and first connection.

4. Device according to one of the preceding claims, wherein a first voltage difference between first voltage and first terminal and / or a second voltage difference between first terminal and second voltage is selected such that the first voltage difference and the second voltage difference are approximately equal in magnitude or approximately adapted to the duty cycle and / or their ratio lies approximately in the range between one and a choice adapted to the duty cycle.

5. Device according to one of the preceding claims, wherein the first and second voltages are arranged symmetrically around a potential of a pole of the electrochemical element and / or the voltage of the electrochemical element, with a deviation of no more than 25% of the higher of the aforementioned voltages.

6. Device according to one of the preceding claims, wherein the device is configured to switch at least one half-bridge as a Class-D amplifier.

7. Device according to one of the preceding claims, wherein at least one capacitor is arranged in parallel to the input of the voltage source and / or wherein the switching elements are formed by at least one MOSFET, BJT, IGBT, GIT, HEMT and / or JFET.

8. Device according to one of the preceding claims, wherein the voltage source is configured to generate the first and second voltages starting from a unipolar DC voltage source (DC-Link).

9. Device for impedance measurement comprising a device according to one of the preceding claims and a device for impedance measurement comprising a first measuring conductor and a second measuring conductor for connection to an electrochemical element for measuring the Impedance of a current flowing through the electrochemical element that changes over time.

10. Arrangement comprising a device according to one of the preceding claims and an electrochemical element, in particular comprising a plurality of cells connected in series, wherein the electrochemical element is connected with its first pole to the first terminal and in particular also to the first measuring conductor and with its second pole to the second terminal and in particular also to the second measuring conductor. 1 1 . Method for exciting at least one current through an electrochemical element, wherein the exciting of the at least one current is carried out by means of time-varying control of at least two switching elements of at least one half-bridge, characterized in that the voltage supply of the half-bridge is carried out simultaneously by a first and a second voltage which are different and both unequal to the potential of one pole, in particular unequal to the potentials of both poles, of the electrochemical element.

12. Method according to claim 1 1 , wherein at least one impedance measurement is performed as a four-terminal measurement.

13. Method according to any one of the preceding claims 1 1 to 12, wherein the device formed by the device for generating the first and / or second voltage and the half-bridge are used for short-term energy storage.

14. Use of two voltages for the simultaneous supply of a half-bridge to excite at least one current through an electrochemical element, wherein excitation of the at least one current is achieved by means of time-varying control of at least two switching elements of the half-bridge. This is characterized in that the voltages are different and unequal to the potential of one pole, in particular unequal to the potentials of both poles, of the electrochemical element.

15. Use according to claim 14, wherein several parallel-connected half-bridges are used to excite the at least one current through the electrochemical element, wherein excitation of the at least one current is effected by means of time-varying control of the at least two switching elements per half-bridge of the plurality, and each of the half-bridges is supplied by means of two voltages for the simultaneous supply of the respective half-bridge, wherein this is realized in particular by a total of two voltage sources, wherein the two voltages are different and unequal to the potential of one pole, in particular unequal to the potentials of both poles, of the electrochemical element.

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