Method for operating a battery system

Abstract

The invention relates to a method for operating a battery system (6) comprising at least one electrochemical battery cell (10) in which at least one electrochemical process contributing to a cell internal electrical resistance of the battery cell (10) occurs during operation, wherein the electrochemical process is assigned a time constant, wherein a current pulse (38) is fed into the battery cell (10), and wherein a pulse frequency of the current pulse (38) is set on the basis of the time constant such that the electrochemical process of the battery cell (10) is resonantly excited by the current pulse (38).

Description

[0001] Description

[0002] Method for operating a battery system

[0003] The invention relates to a method for operating a battery system comprising at least one electrochemical battery cell in which, during operation, at least one electrochemical process occurs, which results in an internal electrical cell resistance of the battery cell. The invention further relates to a battery system and software for implementing the method.

[0004] Electrically or electric-powered or drivable motor vehicles, such as electric or hybrid vehicles, typically include an electric motor that can drive one or both vehicle axles. To supply electrical energy, the electric motor is typically connected to an on-board (high-voltage) battery system that serves as an electrical energy storage device.

[0005] An electrochemical battery, in particular, is understood here and below to mean a so-called secondary battery of a motor vehicle. In such a (secondary) vehicle battery, consumed chemical energy can be recovered by means of an electrical charging process.

[0006] Such battery systems or vehicle batteries are designed, for example, as electrochemical accumulators, in particular as lithium-ion accumulators. To generate or provide a sufficiently high operating voltage, such vehicle batteries typically have at least one battery module (battery cell module) in which several individual electrochemical battery cells are interconnected in a modular fashion. Alternatively, a so-called cell-to-pack design is possible, in which the battery cells are directly interconnected to form the vehicle battery, in particular connected in parallel, rather than being pre-combined into modules.

[0007] Lithium-ion battery cells typically have an efficiency of around 95%, with any losses being converted into heat energy. The performance of such lithium-ion battery cells typically decreases below -5°C (degrees Celsius) (depending on the cell chemistry). The absorbable power in the charging direction is generally even more temperature-dependent, with the absorbable power already limited below +20°C. This has a particular impact on rapid charging processes, where the vehicle battery needs to be charged in the shortest possible time.

[0008] The available power of the vehicle battery is therefore essentially dependent on its state of charge (SOC) and its battery temperature. In the case of a fully charged vehicle battery, it is necessary for the vehicle battery to maintain a certain operating or battery temperature to improve the range and available power of the electrically powered or drivable vehicle.

[0009] High-voltage batteries therefore require thermal conditioning (heating / cooling) to ensure consistently optimal operation and the best possible performance. Especially when cold, at temperatures well below room temperature, the electrochemical processes inside the battery cells occur very slowly. This reduces driving performance and fast-charging capability in cold environments. Particularly when the vehicle is stationary, for example during a charging process, the battery temperature may cool down or be reduced to such an extent that the vehicle battery no longer allows optimal power output or power consumption at the start of a driving process, i.e., when the vehicle continues driving or departs. Furthermore, there is a risk, particularly during charging, that the battery cells will be irreversibly damaged by the deposition of metallic lithium on the anode (so-called lithium plating).As a result, the charging currents released by the battery management system are usually very limited at low temperatures, resulting in long charging times.

[0010] In order to ensure sufficient driving and charging performance even in cold environments, electrically powered or drivable vehicles are regularly equipped with heating systems for the battery system.

[0011] For example, heating elements or devices are provided on the battery housing of the vehicle battery or on the cell housings, but these initially only heat the battery or cell housing. The temperature of the internal active material of the battery cells, on the other hand, only begins to rise with a time delay after the battery and cell housings have heated up. Furthermore, heating is energetically ineffective due to the heat losses when heating the cell housing, resulting in an overall insufficient heating effect. Alternatively, an electric heater is used, for example, to heat the cooling water of the battery system. This water is circulated in a battery temperature control circuit and thus fed to the battery system. The heating function via the water heater is comparatively slow, since water must first be heated and then transported to the battery via the temperature control circuit.There, too, the heat must first travel from the cooling plates through gap filler layers, if necessary, to the battery cells. Losses occur everywhere along this heat path, further limiting the efficiency of this heating method.

[0012] For example, it is known from CN 106532187 A to alternately charge and discharge battery cells with an alternating current to heat them, i.e., to alternately supply a charging current and withdraw a discharging current. The resulting current flow leads to self-heating of the cells due to losses in their internal resistance (internal cell resistance).

[0013] In what is known as "pulse heating," alternating current is fed in and / or drawn from the vehicle battery, particularly in the form of high-frequency current pulses. Typically, the vehicle battery is coupled to the electric motor via an intermediate circuit and a (pulse) inverter. For pulse heating, it is possible to send high-frequency current pulses to the vehicle battery via the pulse-controlled inverter (PWR). A certain amount of energy is drawn from the vehicle battery and temporarily stored in the magnetic field of the electric motor's stator coils. The polarity is then reversed, the magnetic field dissipates, and the energy is fed back into the vehicle battery or battery cells as a current pulse. This results in energy oscillation between the battery cells and the electric motor, which, due to the internal cell resistance, causes an increase in cell temperatures.

[0014] Advantageously, the heat is generated at the internal resistance of the battery cells directly where it is needed. However, in the drives used in electrified vehicles, the inductance of the stator coils is generally relatively small. This means that the pulse heating process must operate at comparatively high pulse frequencies in order to achieve technically feasible heating outputs. A disadvantage here is that the effective internal resistance of the battery cells (real part of the cell impedance) is very small at high frequencies, which limits the achievable heating output. This can be avoided, for example, with a separate controller and a capacitive energy storage device. Due to the larger storage device, this also enables low-frequency pulses in which the internal resistance of the cells effective for heating is greater, so that comparatively greater heating outputs are possible with the same pulse current.

[0015] The invention is based on the object of providing a particularly suitable method for operating a battery system. In particular, the invention aims to provide reliable and safe temperature control or heating of battery cells. Furthermore, the invention is based on the object of providing a particularly suitable battery system and particularly suitable software.

[0016] With regard to the method, the object is achieved according to the invention by the features of claim 1, with regard to the battery system by the features of claim 8, and with regard to the software by the features of claim 10. Advantageous embodiments and further developments are the subject of the dependent claims (subclaims).

[0017] The advantages and configurations cited with regard to the method are also transferable to the battery system and / or the software, and vice versa. The conjunction "and / or" is to be understood here and below in such a way that the features linked by this conjunction can be implemented both together and as alternatives to one another.

[0018] If method steps are described below, advantageous embodiments for the battery system result in particular from the fact that it is designed to carry out one or more of these method steps.

[0019] The method according to the invention is intended for operating a battery system, and is suitable and configured for this purpose. The method is generally suitable for battery systems that are (cold-)started at low ambient temperatures and are intended to have increased power or performance. The battery system is preferably designed as a vehicle battery of a battery-electric vehicle, for example, a hybrid or fully electric (motor) vehicle. Alternatively, the battery system can be designed, for example, for other battery-electric products or as a stationary energy storage device.

[0020] The battery system comprises at least one electrochemical battery cell. The battery system is preferably designed on a lithium-ion basis, with the electrochemical battery cell correspondingly being a lithium-ion cell, for example, a thin-film cell with a liquid electrolyte. During battery or cell operation, at least one electrochemical process takes place in the battery cell, which contributes to the internal electrical resistance, in particular the cell impedance, of the battery cell.

[0021] According to the method, a time constant is assigned to the at least one electrochemical process, and a current pulse is fed into and / or withdrawn from the battery cell. According to the invention, a pulse frequency of the current pulse is adjusted based on the time constant such that the electrochemical process of the battery cell is resonantly excited by the current pulse. In other words, the pulse frequency is adjusted based on the time constant such that the electrochemical process is stimulated more intensively. This realizes a particularly suitable method for operating a battery system.

[0022] According to the invention, the described behavior is utilized for the pulse heating function known from the prior art by specifically selecting the frequency of the current or heating pulses with regard to the time constants of the electrochemical processes within the battery cell. In contrast to known solutions, in the proposed method, the pulse frequency, as well as preferably the pulse shape and / or pulse amplitude and / or pulse phase during pulse heating, are matched to specific electrochemical processes within the battery cell. This allows resonance with specific electrochemical processes to be utilized for the targeted heating of individual cell components.

[0023] For example, if the pulse frequency (f) exactly corresponds to the inverse of the time constant (T) of one of the electrochemical processes occurring within the battery cell (f = 1 / T), the process is excited and brought into resonance (resonance frequency). This allows for the targeted introduction of heating energy to specific regions of the battery cell. If, for example, the kinetics of the intercalation reaction at the anode limit a rapid charging process, the active centers at this electrode can be specifically heated in this way.

[0024] "Resonant excitation" or "resonant excitation" is understood here and below to mean, in particular, an intensified (periodic) excitation of the electrochemical process when it is subject to the time-varying influence of the current pulse. This can result in a stronger effect of the electrochemical process on the cell's internal resistance / impedance than with a constant influence (direct current charging / discharging). With periodic excitation of the electrochemical process, the excitation or pulse frequency (or an integer multiple thereof) is close to the resonant frequency of the electrochemical process, i.e., in a certain frequency range around the resonant frequency or the inverse of the time constant of the electrochemical process.

[0025] The method according to the invention thus generates heat directly within the battery cell without detours or losses, for example, in a water circuit. The resulting heating power (heating performance) is achieved by specifically addressing individual electrochemical processes within the battery cell. In particular, this enables the targeted introduction of heating power to performance-critical cell components (e.g., the anode during rapid charging).

[0026] An "electrochemical process" is understood here and below to mean, in particular, an electrochemical reaction or an electrochemical process inside the battery cell that causes a (local) change in a state within the battery cell due to an electric current, which influences the electrical conductivity of the battery cell and thus changes the internal electrical resistance or impedance of the battery cell. An "electrochemical process" is also understood to mean movements or changes in the charge carriers (electrons, ions) in the battery cell or its components (e.g., conductors).

[0027] Within a battery cell, a whole series of different electrochemical processes take place, each contributing to the cell's internal resistance or cell impedance. Examples include the charge transfer of lithium ions at the anode and cathode, the formation of electrochemical double layers, and diffusion processes. Diffusion processes can also be divided into diffusion within the electrolyte (i.e., within the electrolyte-filled pore structure of the separator and electrodes) and diffusion within the solid (i.e., within the active material). Each of these processes contributes to the effective cell impedance and has its own time constant.

[0028] If the impedance spectrum of the battery cell is plotted in the complex plane (Nyquist plot), the individual processes appear in the form of semicircles. Equivalent circuit models are often used in the electrical modeling of battery cells. The individual processes can be represented by a parallel connection of a capacitance and a resistance (RC element) (see, for example, Gaberscek, M: Impedance spectroscopy of battery cells: Theory vs. Experiment, Current Opinion in Electrochemistry, 2022, 32:100917). The time constant of the respective process is then the product of the (total) capacitance and the (total) resistance (T = R*C). A “current pulse” is understood in particular to be an alternating current signal of a specific (pulse width) and with a predetermined alternating current frequency (signal frequency).The alternating current signal can comprise only one (pulse) signal component with a single pulse frequency, so that the current pulse is essentially a sinusoidal signal. However, the alternating current signal can also comprise multiple signal components with different pulse frequencies, pulse amplitudes, and / or pulse shapes, with the current pulse essentially being the resulting superposition of the individual signal components.

[0029] For the technical implementation of the current pulse, the methods known from the state of the art can be used, e.g. generation of the current pulse via a pulse inverter with inductive storage in stator coils of an electrical machine / electric motor, via a separate actuating unit with separate, e.g., capacitive energy storage, or via combinations thereof.

[0030] Preferably, the current pulse is applied periodically to the battery cell to ensure reliable resonant heating of the battery cell. For example, a number of consecutive current pulses are applied to heat the battery cell. The periodicity or repetition frequency of the current pulse is also advantageously selected with regard to the resonant excitation of the electrochemical process.

[0031] In addition to or alternatively to supplying a charging current pulse, a corresponding withdrawal of a discharging current pulse can also be used. This generally improves cell performance, also in the discharging direction, e.g. for faster preparation of driving readiness in cold environments or faster provision of required driving performance. To warm or heat the battery cell, energy oscillation between the battery cell and an energy storage device (e.g. inductances of stator coils) is preferably used, with current pulses having components in both the charging and discharging directions. In a preferred embodiment, asymmetrical current pulses are used in particular, which have a lower C-rate in the charging direction than in the discharging direction.

[0032] In addition to the resonant heating of the battery cell, the triggering of age-sensitive processes can be avoided in a corresponding manner. At least one ageing process takes place in the battery cell during operation, to which a time constant can also be assigned. Therefore, in a conceivable further development, the pulse frequency of the current pulse can be set such that a specific ageing process in the battery cell is stimulated little or not at all by the current pulse. This can be useful if the respective ageing process has been identified as critical with regard to cell ageing. Preferably, the pulse frequency of the current pulse is selected such that the electrochemical process is resonant and the ageing process is not stimulated as much as possible. The service life of the battery cell is improved by the fact that age-sensitive processes are not triggered during heating.Furthermore, the resonant heating prevents the cell power from being operated in an otherwise unheated temperature range that would strongly promote aging of the battery cell.

[0033] An "aging process" is understood here and below to mean, in particular, an electrochemical process inside the battery cell that affects the aging of the battery cell. The aging process is particularly irreversible and permanently influences the cell properties. Such aging processes irreversibly affect, among other things, the (remaining) capacity of the battery cell as well as the internal cell resistance or cell impedance. Aged battery cells typically have reduced capacity and a (permanently) higher internal cell resistance than at the beginning of their life. Relevant aging processes include Li plating, layer thickness growth, and gas formation within the battery cell.

[0034] In a particularly simple embodiment of the method, a sinusoidal current pulse with only one excitation or pulse frequency is used, which, for example, specifically targets the diffusion in the anode.

[0035] Within the scope of the described method, a superposition of several excitation or pulse frequencies in the current pulse, also referred to below as a heating pulse, can also be used. These frequencies are matched to the time constants of various electrochemical processes. In a suitable embodiment, a time constant is assigned to each of the different electrochemical processes of the battery cell, and a current pulse is generated with a superposition of a corresponding number of pulse frequencies. The pulse sequences are adjusted based on the time constants such that the respective electrochemical processes of the battery cell are resonantly excited by the current pulse. This means that the heating pulse is designed as a non-sinusoidal current pulse containing several excitation frequencies.In this way, not just one but several time constants and consequently several electrochemical processes within the battery cell can be specifically addressed / stimulated, thereby improving the heating performance.

[0036] In a preferred embodiment of the invention, a total of three frequency components are superimposed to form a single heating pulse. These differ significantly in their time constants. This enables optimized heating performance by addressing multiple processes with widely differing time constants.

[0037] The first excitation frequency is specifically tuned to the movement of electrons in the current collectors of the battery cell. The current collectors are made, for example, of a copper material (at the anode) or an aluminum material (at the cathode). The movement of the electrodes in the metallic current collector material is a very fast process with a small (low) time constant. Accordingly, the first excitation frequency is a high-frequency excitation. At very high pulse frequencies in the range of 100 Hz (hertz) to greater than 5 kHz (kilohertz), only pure electrical conduction within the battery cell (busbars, collector foils) and, if necessary, charge reversal of internal double-layer capacitances are excited. For this purpose, high C-rates of the pulse current (pulse current rate) of 5 C to 10 C (depending on the cell design) would be possible.

[0038] The second excitation frequency is specifically tuned to the movement of solvated lithium ions dissolved in the electrolyte. This movement exhibits a medium time constant, so the second excitation frequency is a medium excitation frequency. In a medium frequency range between 50 mHz (millihertz) and 10 Hz, the intercalation and de-intercalation reactions of lithium ions at the anode and cathode are particularly stimulated. For this purpose, the tolerable pulse current rate is reduced to, for example, 0.25 °C to 1 °C (depending on the cell temperature).

[0039] The third excitation frequency is particularly suited to the comparatively slow movement of the lithium ions stored in the electrodes (anode, cathode) through diffusion or changes in lattice positions. This movement has a large (high) time constant, so that the third excitation frequency in particular causes low-frequency excitation. For example, in a frequency range between 5 mHz and 50 mHz, diffusion processes in the electrolyte are stimulated. At frequencies slower than 5 mHz, diffusion processes within the active material (i.e., solid-state diffusion) also come into play. Depending on the cell temperature, it is therefore necessary to reduce the pulse excitation to, for example, 0.1 °C to 0.2 °C.

[0040] In an advantageous embodiment, different pulse amplitudes and / or pulse phases are assigned to the different pulse frequencies. This means that the pulse components of the heating pulse differ from one another in terms of frequency, amplitude, and phase. In particular, the amplitudes or current strengths of the low-frequency pulse components are selected to prevent accelerated cell aging.

[0041] When a number of electrochemical processes are excited, which upon excitation have a purely ohmic effect on the cell's internal resistance / impedance, the processes are generally in phase, so that the pulse phases of the pulse components are preferably the same. When the ions are solvated, phase differences between 0° and 45° can occur, with the phase difference for diffusion in a solid state, for example, being up to 90°.

[0042] The time constants of the electrochemical processes depend, for example, on (cell) temperature, state of charge (SOC) and aging of the battery cell (state of health (SOH)), and can therefore change during operation. In a practical embodiment, the selected pulse frequencies and amplitudes and / or pulse shapes of the current pulse are tracked based on a cell state. A "cell state" here is understood to mean, in particular, an operating parameter of the battery cell, such as the cell temperature, the state of charge or the cell aging. The frequency and pulse shape are therefore advantageously tracked, for example, as heating increases. In particular, the processes that serve to heat the cell but prevent damage to the cell should also be stimulated as the heating process progresses. The tracking can be carried out, for example, using stored characteristic curves or tables.

[0043] In one conceivable implementation, the time constant(s) is pre-characterized and stored. The determination of the time constant(s) and the identification of the individual electrochemical processes can be performed, for example, ex-situ via laboratory measurements on the battery cells for different cell states, followed by modeling. The model and / or derived or corresponding characteristic curves and / or tables are stored in a memory of the battery system and are thus available on demand during operation.

[0044] In an alternative embodiment, the or each time constant is determined during operation of the battery cell. In particular, an in-situ determination of the time constant is possible by evaluating a voltage response for the impressed or injected current pulses in the battery system. For this purpose, the current pulses are impressed, and an impedance spectrum is derived. From this, the specific processes—stimulated / influenced by the current pulse—are identified, and a corresponding heating or current pulse is subsequently set. The battery system according to the invention has at least one electrochemical battery cell in which, during operation, at least one electrochemical process takes place that contributes to the internal electrical resistance of the battery cell.The battery system further comprises a feed device for feeding a current or heating pulse into the battery cell, and a controller (i.e. a control unit) for carrying out a method described above.

[0045] The controller is generally configured—in terms of programming and / or circuitry—to carry out the method according to the invention described above. The controller is thus specifically configured to adjust a pulse parameter (e.g., pulse frequency, pulse amplitude, pulse shape), in particular a pulse frequency, of the supplied current pulse based on a time constant for an electrochemical process inside the battery cell in such a way that the electrochemical process is resonantly excited.

[0046] In a preferred embodiment, the controller is formed, at least in its core, by a microcontroller with a processor and a data memory, in which the functionality for carrying out the method according to the invention is implemented in the form of operating software (firmware), so that the method - possibly in interaction with a device user - is carried out automatically when the operating software is executed in the microcontroller. Within the scope of the invention, however, the controller can alternatively also be formed by a non-programmable electronic component, such as an application-specific integrated circuit (ASIC) or an FPGA (Field Programmable Gate Array), in which the functionality for carrying out the method according to the invention is implemented using circuitry.

[0047] In an advantageous embodiment, the feed device is coupled to a device for generating the current pulse, which is controlled and / or regulated by the controller. The device is designed as a pulse-controlled inverter with inductive energy storage in the stator coils of an electric machine / electric motor or as a separate actuating unit with a separate, e.g., capacitive energy storage device. Alternatively, the device for generating the current pulse can also be designed as a combination of pulse direction change and actuating unit. This ensures reliable generation of the current or heating pulse.

[0048] An additional or further aspect of the invention provides software on a medium or data carrier for carrying out or executing the method described above when the software runs on a computer or controller. This means that the software is stored on a data carrier and is intended to carry out the method described above, as well as being suitable and designed for this purpose. This results in particularly suitable software for operating an electrically powered or drivable motor vehicle, with which software the functionality for carrying out the method according to the invention is implemented in programming terms. The software is thus in particular operating software (firmware), the data carrier being, for example, a data memory of the controller.

[0049] Exemplary embodiments of the invention are explained in more detail below with reference to a drawing. In the drawings:

[0050] Fig. 1 shows a schematic representation of a motor vehicle with a battery system,

[0051] Fig. 2 shows a schematic representation of the battery system,

[0052] Fig. 3 shows a schematic representation of a battery cell and a feed unit of the battery system,

[0053] Fig. 4 is a cell impedance diagram for electrochemical processes of the battery cell, Fig. 5 is a flow chart for a method for operating the battery system, and

[0054] Fig. 6 is a flowchart for an alternative method for operating the battery system.

[0055] Corresponding parts and sizes are always provided with the same reference symbols in all figures.

[0056] Figure 1 shows an electrically powered or drivable motor vehicle 2, for example, an electric or hybrid vehicle. The motor vehicle 2 has a three-phase electric motor 4, shown in Figure 2, as an electric traction drive, which is connected to a battery system 6 for supplying electrical energy.

[0057] The battery system 6 has an electrochemical energy storage device 8 with a number of electrochemical battery cells 10. To generate or provide a sufficiently high operating voltage, the battery cells 10 can be modularly interconnected in at least one battery module. Alternatively, a so-called Cell2Pack design is possible, in which the battery cells 10 are directly interconnected and not previously combined into modules. A pulse-controlled inverter is interconnected as a feed-in device 12 between the energy storage device 8 and the electric motor 4. In this exemplary embodiment, the battery system 6 further comprises a (DC voltage) intermediate circuit 14 connecting the energy storage device 8 and the feed-in device 12, which extends at least partially into the feed-in device 12.The intermediate circuit 14 is at least partially led into the feed device 12, in which an intermediate circuit capacitor 16 and a bridge circuit 18 are connected.

[0058] During operation of the electric motor 4, an input current supplied to the bridge circuit 18 is converted into a three-phase output current (motor current, three-phase current) for the three phases of the electric motor 4. The output currents, also referred to below as phase currents, are fed to the corresponding phases (windings) of a stator (not shown in detail).

[0059] In this embodiment, the bridge circuit 18 is implemented as a B6 circuit. In this configuration, during operation, each of the phase windings is switched between the voltage levels of the intermediate circuit 14 at a high switching frequency. This clocked control is implemented as a PWM control by a controller 20 of the feed-in device 12, which enables control and / or regulation of the speed, power, and direction of rotation of the electric motor 4.

[0060] The battery system 6 further comprises a (battery) controller 22 for controlling and / or regulating the energy storage operation. The controllers 20, 22 can be controlled by a battery management controller or integrated into it.

[0061] Figure 3 shows the structure of a battery cell 10 in more detail. The electrochemical battery cell 10 is designed, in particular, as a lithium-ion cell. In the illustrated embodiment, the battery cell 10 has a cathode 24 with a (cathode) conductor 26 and an anode 28 with an (anode) conductor 30, which are separated by a separator 31 and coupled by a (liquid) electrolyte 32.

[0062] In the battery cell 10, during battery or cell operation, at least one electrochemical process takes place within the arresters 26, 30 and / or the electrode layers 24, 28 and / or the electrolyte 32, which process contributes to an internal electrical cell resistance, in particular a cell impedance Z, of the battery cell 10. Fig. 4 shows an idealized profile of the cell impedance Z in a schematic and simplified cell impedance diagram. In the cell impedance diagram, the real part of the cell impedance Z is plotted along the abscissa axis (X-axis), and the negative imaginary part of the cell impedance Z is plotted along the vertical ordinate axis (Y-).

[0063] The diagram shows a curve 34 for the cell impedance Z, or rather, the cell impedance spectrum. The curve 34 has different sections determined by different electrochemical processes. The curve 34 essentially has six regions: 34a, 34b, 34c, 34d, 34e, and 34f.

[0064] In the low-resistance region 34a, the cell impedance Z is determined by the electronic resistance and the resistance of the electrolyte 32. Region 34b is determined by the contact impedance between the collectors 26, 30 and the associated electrodes 24, 28. In region 34c, the cell impedance Z is determined in particular by charge transfer and double-layer storage. Region 34d is characterized by diffusion processes in the pore structure of the electrodes, with region 34e being determined in particular by diffusion in the separator 31. Region 34f corresponds to the diffusion processes in the active material of the electrodes 24, 28.

[0065] The following describes a method for operating the battery system 6, which, taking into account the electrochemical processes, causes local heating of the battery cells 10 or their (battery) cell components 24, 26, 28, 30, 31, 32. The method is carried out, for example, by the battery management controller or by the controller 20 and / or the controller 22.

[0066] According to the method, the battery cells 10 are heated by pulse heating, in which alternating current signals are fed into the battery cells 10 from the feed device 12 as a current or heating pulse 38, causing heating there due to electrical loss processes. The current pulse 38 has components in both the charging and discharging directions.

[0067] To generate the current pulse 38, a certain amount of energy is taken from the energy storage device 8 and temporarily stored in the magnetic field of the stator coils of the electric motor 4. The polarity is then reversed, the magnetic field dissipates, and the energy is fed back into the energy storage device 8 or into the battery cells 10 as a current pulse 38. According to the method, a time constant is assigned to at least one of the electrochemical processes of the battery cell 10. A pulse frequency of the current pulse 38 is adjusted based on the time constant such that the electrochemical process of the battery cell 10 is resonantly excited by the current pulse 38, so that one or more specific electrochemical processes are used by means of the resonance to specifically heat individual or multiple cell components 24, 26, 28, 30, 31, 32.

[0068] A first embodiment of the method is explained in more detail below with reference to Fig. 5.

[0069] The method is started in a method step 36. The method is started, for example, at cold ambient temperatures of the motor vehicle 2 when the battery system 6 is to have increased power or performance.

[0070] In a first method step 40, it is first checked whether the actual temperature of the battery cell 10 is less than or equal to a desired or stored target temperature. The controller 22 is suitable and configured to monitor the temperature of the battery cell 10, for example, using a temperature sensor or based on stored temperature curves. If the actual temperature is greater than or equal to the target temperature, the method is terminated with method step 42.

[0071] If the actual temperature is lower than the desired cell temperature, method step 44 is executed. In method step 44, a current pulse 38 is generated and fed into the battery cells 10 by the feed device 12 in a method step 46. Subsequently, in a method step 48, a voltage response of the battery cells 10 is recorded. For example, the controller 22 monitors the battery cells 10 using a voltmeter.

[0072] Based on the recorded voltage response, the corresponding cell impedance spectrum is then determined in a method step 50. In a method step 52, the electrochemical processes stimulated or influenced by the current pulse 38 are identified. For example, the recorded voltage response is fitted with a stored model for the cell impedance, thus identifying the dominant processes. A time constant is assigned to at least one of the processes, with a corresponding pulse frequency subsequently being determined based on the inverse of the time constant. In other words, the time constant is determined in situ by evaluating the voltage response for the impressed or injected current pulses 38.

[0073] The specific pulse frequency is set by means of the controller 20 or the PWM control of the feed-in device 12. If the current cell temperature is still lower than the target temperature, the next current pulse 38 is generated in method step 44 with the newly set pulse frequency, thus causing a resonant excitation of the associated electrochemical process in the battery cell 10.

[0074] A second exemplary embodiment of the method is explained in more detail below with reference to Fig. 6. In this case, the time constants are determined and the individual electrochemical processes are identified, particularly ex situ, via preliminary characterizations or laboratory measurements on the battery cells 10 for different cell states, followed by modeling. The time constants are stored in a memory of the controller 22, particularly using characteristic curves and tables.

[0075] After starting the process in process step 36, the time constant for a specific electrochemical process is determined in a process step 56 based on the current cell states (operating states) of the battery cell 10 (cell temperature, charge state, aging state, etc.) and the stored information. From this, a corresponding pulse frequency is determined based on the inverse of the time constant, and in a subsequent process step 58, the pulse frequency is set using the controller 20 or the PWM control of the feed-in device 12. Subsequently, in process step 44, the current pulse 38 is generated and fed into the battery cell(s) 10.

[0076] Subsequently, process step 40 checks whether the actual cell temperature is less than or equal to a desired or stored target temperature. If the actual temperature is greater than or equal to the target temperature, the process ends with process step 42. If the actual temperature is lower than the desired cell temperature, process step 60 is executed.

[0077] In process step 60, the pulse frequency is adjusted. This means that, based on the actual cell temperature (and / or other operating conditions) present after the current pulse 38, the time constant or pulse frequency is updated using the stored information. Subsequently, process step 44 is executed again. This ensures that reliable resonant excitation of the desired processes always occurs, even as the heating process progresses.

[0078] The methods described above are also suitable for preventing aging-sensitive processes in a corresponding manner. In particular, the pulse frequency of the current pulse 38 is adjusted in method steps 52, 60 such that an aging process of the battery cell 10 is stimulated as little as possible, or not at all, by the current pulse 38.

[0079] In a particularly simple embodiment of the method, a sinusoidal current pulse 38 with only one excitation or pulse frequency is used, which, for example, specifically targets the diffusion in the anode (region 34f). Alternatively, a superposition of several excitation or pulse frequencies in the current pulse 38 can be used, which are matched to the time constants of various electrochemical processes. For this purpose, in method steps 52, 56, a number of different electrochemical processes of the battery cell are each assigned a time constant, and a current pulse 38 is generated with a superposition of a corresponding number of pulse frequencies.

[0080] Preferably, in method steps 52, 56, in addition to the pulse frequencies, further pulse properties, in particular a pulse shape and / or a pulse amplitude, are set for the respective pulse frequency.

[0081] The claimed invention is not limited to the exemplary embodiments described above. Rather, other variants of the invention can also be derived therefrom by those skilled in the art within the scope of the disclosed claims without departing from the subject matter of the claimed invention. In particular, all individual features described in connection with the various exemplary embodiments can also be combined in other ways within the scope of the disclosed claims without departing from the subject matter of the claimed invention. List of reference symbols

[0082] Motor vehicle

[0083] electric motor

[0084] Battery system

[0085] Energy storage

[0086] 10 battery cells

[0087] 12 Feed-in device

[0088] 14 intermediate circuit

[0089] 16 DC link capacitor

[0090] 18 bridge circuit

[0091] 20 controllers

[0092] 22 controllers

[0093] 24 Cathode

[0094] 26 arresters

[0095] 28 Anode

[0096] 30 arresters

[0097] 31 Separator

[0098] 32 Electrolyte

[0099] 34 History

[0100] 34a ... 34f area

[0101] 35 process steps

[0102] 38 current pulse

[0103] 40 ... 66 process steps

[0104] Z cell impedance

Claims

Patent claims 1. A method for operating a battery system (6) comprising at least one electrochemical battery cell (10) in which at least one electrochemical process takes place during operation, which contributes to an internal electrical cell resistance of the battery cell (10), - where a time constant is assigned to the electrochemical process, - wherein a current pulse (38) is fed into the battery cell (10), and - wherein a pulse frequency of the current pulse (38) is set based on the time constant such that the electrochemical process of the battery cell (10) is resonantly excited by the current pulse (38).

2. Method according to claim 1, characterized in that in the battery cell (10) during operation at least one aging process takes place, to which a time constant is assigned, wherein the pulse frequency of the current pulse (38) is set such that the aging process of the battery cell (10) is not stimulated by the current pulse (38).

3. Method according to claim 1 or 2, characterized in that - that a number of different electrochemical processes of the battery cell (10) are each assigned a time constant, and - that a current pulse (38) is generated with a superposition of several pulse frequencies, wherein the pulse sequences are adjusted on the basis of the time constants in such a way that the respective electrochemical processes of the battery cell (10) are resonantly excited by the current pulse (38).

4. Method according to claim 3, characterized in that different pulse amplitudes and / or pulse phase positions are assigned to the different pulse frequencies.

5. Method according to one of claims 1 to 4, characterized in that the or each pulse frequency and / or a pulse shape of the current pulse (3) is tracked based on a cell state of the battery cell (10).

6. Method according to one of claims 1 to 5, characterized in that the or each time constant is pre-characterized and stored.

7. Method according to one of claims 1 to 6, characterized in that - that a voltage response of the battery cell (10) to the current pulse (38) is detected, and - that the or each time constant is determined from the voltage response.

8. Battery system (6), comprising - at least one electrochemical battery cell (10) in which at least one electrochemical process takes place during operation, which contributes to an internal electrical cell resistance of the battery cell (10), - a feed device (12) for feeding a current pulse (38) into the battery cell (10), and - a controller (20, 22) for carrying out a method according to one of claims 1 to 7.

9. Battery system (6) according to claim 8, characterized in that the feed device (12) has a pulse inverter coupled to an electric motor (4) or an actuating unit.

10. Software on a data carrier for carrying out a method according to one of claims 1 to 7, when the software runs on a computer.

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