Electrochemical system and method for operating an electrochemical system

Abstract

An electrochemical system (1), in particular a battery system, having a plurality of electrochemical cells (3) comprises a fluid-operated cooling system (2) provided for controlling the temperature of the electrochemical cells (3), as well as a measuring and control system (9) which is designed to alternately carry out impedance measurements on the electrochemical cells (3) and to heat same by applying alternating current thereto.

Description

[0001] Electrochemical system and method for operating an electrochemical system

[0002] The invention relates to an electrochemical system, in particular a battery system, comprising several electrochemical cells, which is temperature-controlled and includes means for carrying out measurements. Furthermore, the invention relates to a method for operating an electrochemical system.

[0003] EP 3 875 975 A1 describes a method and apparatus for charge transfer in electrochemical impedance spectroscopy. It proposes that, during the determination of at least one voltage value, charge is transferred back and forth, particularly periodically, between a first number of accumulator cells (i.e., electrochemical cells) and a second number of accumulator cells. The accumulator cells of the first number are connected in series with the accumulator cells of the second number. According to EP 3 875 975 A1, the accumulator cells can be, for example, lithium-ion or lithium-nickel-manganese-cobalt accumulator cells. Charging and discharging are to be carried out using at least one DC / DC converter. The frequency of a periodic charge transfer is specified as being between 0.1 and 10 kHz. The total charge transfer current should reach at least 0.1 A.

[0004] German patent application DE 10 2013214 821 A1 discloses an electrochemical storage module and a method for investigating an electrochemical storage cell within a module. The method comprises the step of balancing the charge state between a first storage cell and a second storage cell using a direct current. Furthermore, the method according to DE 10 2013214 821 A1 comprises superimposing the direct current with an alternating signal and investigating the response of a storage cell to the alternating signal. The method according to DE 10 2013214 821 A1 is intended to be particularly applicable to electrically powered vehicles. The result of the investigation carried out on the electrochemical storage module is intended to be used, in particular, to adjust an operating mode and / or a balancing parameter.

[0005] DE 10 2009 000 336 A1 deals with an electrical circuit for measuring the impedance of cells in an electrochemical energy storage device in a vehicle. The impedance is determined at a given frequency.

[0006] According to the teaching of DE 10 2009 000 336 A1, there are selection switches which can be switched in such a way that the impedance of a selected cell of the electrochemical energy storage device can be measured.

[0007] The invention is based on the objective of improving the usability of electrochemical systems, in particular battery systems, compared to the prior art by means of measurement technology, taking into account in particular the charging process, even at low temperatures.

[0008] This problem is solved according to the invention by an electrochemical system with the features of claim 1. Likewise, the problem is solved by a method for operating an electrochemical system designed according to claim 4. The embodiments and advantages of the invention explained below in connection with the operating method also apply mutatis mutandis to the device, i.e., the electrochemical system, and vice versa.

[0009] The invention is based on the premise that electrochemical systems, particularly battery systems, are operated within a wide temperature range, for example, at ambient temperatures between -40 °C and +50 °C. It also aims to enable charging of a battery system within a wide temperature range. As is known, the maximum currents permissible during charging and discharging are temperature-dependent. To enable rapid battery charging, a predetermined temperature range must be maintained. Due to the heat generated during charging, the battery, for example, a vehicle traction battery, is often cooled during the charging process.

[0010] The application's approach focuses particularly on charging phases in which the electrochemical system has not yet reached its optimal temperature for the charging process. According to unclaimed concepts, the temperature could, for example, be measured at the battery surface or in the cooling system. The application's solution further develops such concepts by obtaining information about the state of the electrochemical cells, especially the temperature within the cells, using impedance measurements. The temperature within the cells is then repeatedly increased by applying alternating current, alternating with these impedance measurements. This results in a close integration of measurement and heating phases, enabling highly accurate temperature control.

[0011] The cooling system of the electrochemical system is primarily an immersion cooling system. In this system, the outer surfaces of the individual electrochemical cells are directly exposed to a non-conductive, dielectric fluid. For calculating the internal temperature of an electrochemical cell, models that combine impedance measurement with temperature measured within the cooling system are particularly suitable.

[0012] The application process relates generally to the operation of an electrochemical system comprising a plurality of electrochemical cells that can be temperature-controlled with a fluid, in particular an immersion fluid. The operating process involves alternately determining the temperature in the electrochemical cells by impedance measurement and heating the cells by applying alternating current.

[0013] Impedance measurement can be performed, for example, at a frequency in the range of 0.1 Hz to 10 Hz, particularly at 1 Hz. For heating the cells, an alternating current with a frequency in the range of 100 Hz to 1 kHz, particularly a frequency of 300 Hz, is used.

[0014] In various possible variations of the procedure, the heating frequency is at least one hundred and at most one thousand times the frequency used for impedance measurement. The comparatively high-frequency excitation during cell heating leads to a rapid current flow that heats the cells due to the relatively small real part of the impedance in this case, compared to the very low-frequency excitation used in impedance measurements performed in other time windows. In contrast, any heating of the cells by the impedance measurement itself occurs only to a very minimal extent, if at all.

[0015] The method for operating the electrochemical system, in particular the battery system, is such that different temperature levels are established on the outer surface of the cells and inside the cells by means of the cooling system, for example in the form of an immersion cooling system, on the one hand, and by supplying the electrochemical cells with alternating current on the other. This allows the temperature inside the cells to be maintained, for example, at least 1 K and at most 3 K, in particular 2 K, above the temperature in the cooling system.

[0016] The immersion cooling system can very quickly raise the temperature of the cell's outer surface to set the optimal charging temperature. The temperature inside the cell can thus be temporarily significantly lower than the outer surface temperature. By applying alternating current to the cell, the interior can be selectively heated.

[0017] As the temperature inside the cells approaches the optimal charging temperature, the heat input, which is supplied to the cells via alternating current heating, can be gradually or abruptly stopped. In later operating phases, including charging, the cells can be cooled as needed using a cooling system, such as an immersion cooling system.

[0018] Even during operating phases in which the cells are not supplied with alternating current for heating, impedance measurements can be carried out at individual time intervals, which are not necessarily limited to determining the temperature prevailing in the cells.

[0019] If the electrochemical system is designed as a battery system, it could, for example, be a lithium-ion battery. Alternatively, it could be, for example, a nickel-cadmium battery, a sodium-ion battery, or another type of rechargeable battery.

[0020] An embodiment of the invention is explained in more detail below with reference to a drawing. The drawing shows:

[0021] Fig. 1 in a block diagram features of a method for operating an electrochemical system, namely a battery system,

[0022] Fig. 2 Properties of the electrochemical system in a Nyquist diagram,

[0023] Fig. 3 shows the electrochemical system in a schematic representation.

[0024] An electrochemical system, designated 1, is configured in the exemplary embodiment as a battery system, which is used, for example, in a motor vehicle. The method described below can be carried out, in particular, when the electrochemical system 1 is installed in the motor vehicle. It is also possible to carry out the method immediately after the manufacture of the electrochemical system 1, in this case a lithium-ion battery, even before its installation in the vehicle.

[0025] The electrochemical system 1, i.e., the rechargeable battery system, is equipped with a cooling system 2, which can be used as needed in various operating phases, for example, during charging or ferry operation. The electrochemical cells, generally referred to as 3, i.e., battery cells, can also be heated using the cooling system 2. Thus, the cooling system 2 represents a versatile temperature control system.

[0026] The individual electrochemical cells 3 are arranged in a housing 4 containing an immersion fluid, which is circulated by a pump 12 belonging to the cooling system 2. The cooling system 2 is thus designed as an immersion cooling system. Coolant lines are designated 5.

[0027] Each electrochemical cell 3 is constructed in a manner known per se from two half-cells 6, 7, which are separated from each other by a proton-permeable membrane 8. The electrical connections of the individual electrochemical cells 3 are not shown in Figure 3. All electrochemical cells 3 are connected to a measuring and control system 9, where electrical lines are generally designated by 10. The previously mentioned pump 12 and a temperature sensor 11, which detects the temperature of the immersion fluid, are also connected to the measuring and control system 9 via lines 10.

[0028] In the following, it is assumed that the electrochemical cells 3 are to be charged at low ambient temperatures, and thus initially at a low temperature of the immersion fluid. To be able to adjust the charging parameters, the most accurate possible information about the internal state of the electrochemical cells 3 is required. The temperature of the immersion fluid alone does not provide sufficient information. Therefore, a state monitoring procedure (ZE) is performed, which is illustrated in Figure 1. When the state monitoring procedure (ZE) is to be carried out, the electrochemical cell 3 is subjected to an alternating current signal in a so-called measurement phase (MP), the frequency of which is 1 Hz in this case. Various possible response signals, which depend on the state of the electrochemical cell 3, are shown in Figure 2.The response signal is influenced by factors including temperature differences (AT) and changes in the state of charge (ASOC) and maintenance state (ASOH).

[0029] Overall, the measurement phase MP allows usable conclusions to be drawn about the internal state of electrochemical cell 3, including the prevailing temperature, without direct temperature measurement. After completion of the measurement phase MP, which lasts, for example, a few seconds or several tens of seconds, a heating phase HP is started. During the heating phase HP, electrochemical cell 3 is subjected to an alternating current with a frequency of 300 Hz.

[0030] As shown in Figure 2, at 300 Hz the real component of the impedance Z is relatively small compared to the previous measurement phase MP. This results in a relatively high current flowing in the electrochemical cell 3, which heats the cell 3. The heating phase HP can also last for a period of, for example, a few seconds up to several tens of seconds. Afterwards, the system switches back to the measurement phase MP.

[0031] The alternating switching between measurement phase MP and heating phase HP is repeated several times, as indicated in Figure 1. Once the electrochemical cells 3 have reached a state favorable for charging, the system switches to charging mode LB. An overlap between charging mode LB and the alternating heating and measurement phases HP and MP is also possible. The state of the electrochemical cell 3, including a determined temperature level, as determined in measurement phase MP using theoretical models, is compared with the temperature output by the temperature sensor 11 via the measurement and control system 9. Temperature measurements at various points in the cooling system 2 are also possible, though not shown.During the heating phase of the electrochemical cell 3, the temperature inside the cell 3 can be maintained at a value several Kelvin, for example 2 K, above the temperature at the outer surface of the cell 3, where the latter temperature corresponds at least approximately to the temperature measured by the temperature sensor 11. Instead of an abrupt termination of the heating phases HP, a gradual decrease in the duration of the heating phases HP can also be provided as the temperature inside the cell 3 approaches the ideal temperature level for charging.

[0032] List of reference signs

[0033] 1 Electrochemical system, battery system

[0034] 2 Cooling system

[0035] 3 electrochemical cell

[0036] 4 cases

[0037] 5 Coolant line

[0038] 6 half-cell

[0039] 7 half-cell

[0040] 8 Membran

[0041] 9 Measurement and control system

[0042] 10 electrical lines

[0043] 11 Temperature sensor

[0044] 12 Pump

[0045] HP heating phase

[0046] LB charging operation

[0047] Measurement phase

[0048] ZE condition assessment

Claims

Patent claims 1. Electrochemical system (1) comprising a plurality of electrochemical cells (3), a fluid-operated cooling system (2) for temperature control of the electrochemical cells (3), and a measuring and control system (9) designed to alternately perform impedance measurements on the electrochemical cells (3) and to heat them by applying alternating current.

2. Electrochemical system (1) according to claim 1, characterized in that the cooling system (2) is designed as an immersion cooling system.

3. Electrochemical system (1 ) according to claim 1 or 2, characterized in that the measuring and control system (9) is configured to determine an internal temperature of the electrochemical cell (3) based on a temperature detected in the cooling system (2) by means of a temperature sensor (11 ) and the impedance measurement.

4. Method for operating an electrochemical system (1) comprising a plurality of electrochemical cells (3) which can be temperature controlled with a fluid, wherein the temperature in the electrochemical cells (3) is determined alternately by impedance measurement and the cells (3) are heated by applying alternating current.

5. Method according to claim 4, characterized in that the impedance measurement is carried out at a frequency in the range of 0.1 Hz to 10 Hz.

6. Method according to claim 5, characterized in that the impedance measurement is performed at a frequency of 1 Hz.

7. A method according to any one of claims 4 to 6, characterized in that the heating of the cells (3) is carried out at a frequency in the range of 100 Hz to 1 kHz.

8. A method according to claim 7, characterized in that the heating of the cells (3) is carried out at a frequency of 300 Hz.

9. A method according to any one of claims 4 to 8, characterized in that different temperature levels are established on the outer surface of the cells (3) and inside the cells (3) by means of the cooling system (2) on the one hand and the application of alternating current to the cells (3) on the other hand, wherein the temperature inside the cells (3) is maintained at least 1 K and at most 3 K above the temperature in the cooling system (2).

10. A method according to claim 9, characterized in that inside the Cells (3) are set to a temperature which is 2 K above the temperature measured in the cooling system (2).

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