Method and device for charging or discharging a battery system

Abstract

Method for charging or discharging a battery system (20), wherein the battery system (20) has at least two battery cells (21-x) connected in parallel and / or in series, which can only be charged or discharged together, wherein a voltage (U) is controlled as a controlled variable by means of a current (I) or a power (P) as a manipulated variable, wherein the current (I) or the power (P) is specified taking into account a first charge state-dependent characteristic curve (3-1) and a second charge state-dependent characteristic curve (3-2), and wherein the first charge state-dependent characteristic curve (3-1) and the second charge state-dependent characteristic curve (3-2) have opposite gradients at least in sections.

Description

The invention relates to a method and an apparatus for charging or discharging a battery system.Li-ion cells can only absorb a limited charging current, depending on the temperature, energization duration and charge state, without suffering damage. This maximum current also changes with an aging state (SOH) of the Li-ion cells. Furthermore, the difficulty arises that a plurality of cells can be connected in series or in parallel in a battery system and the cells can have different temperatures and / or ageing states and / or states of charge. Consequently, it is currently necessary to know at any time the state of charge and aging of all cells in the system, as well as the coldest and warmest location. This also includes knowing temperature gradients within a cell to actually know the coldest and warmest point throughout the battery system. These two points, together with the state of charge and aging of each cell, determine the maximum possible charging current at each point in time. Since an expanded cell can briefly absorb a higher current than the maximum possible continuous current, the time response of the cell to the charging current must also be known. In addition, the maximum permissible charging currents are also dependent on a mechanical pressure or a force which acts / r on the battery cells. Furthermore, a state of charge distribution, i.e. different states of charge, can occur within the battery system between the individual battery cells. In addition, a state of charge distribution can also occur within a battery cell. This charge state distribution must be taken into account when selecting (regulating) the charge current, since the charge current generally depends on the charge state. However, determining the state of charge of each individual cell in the case of parallel-connected battery cells and the state of charge distribution within a battery cell cannot be implemented in a technically meaningful manner in practical operation.DE 10 2021 108 085 A1 discloses a method for determining at least one control parameter for a charging device for supplying electrical charge to a lithium-based electrical energy store, for which purpose the electrical energy store is charged with an electrical charging current as a function of at least one state parameter of the electrical energy store. For determining the at least one control parameter, a limit function is determined, by means of which an electrical limit voltage is assigned to a supplied electrical charge quantity, in such a way that an anode potential is greater than a predefined anode potential value if a terminal voltage is less than the limit voltage, wherein the predefined anode potential value is greater than zero volts with respect to a Li / Li+ reference electrode.CN 1 11 766 523 B discloses a method and a device for determining the optimum charging strategy for lithium-ion batteries, which contributes to improving their performance and service life. By evaluating and comparing the structural integrity of various lithium ion batteries after multiple charge and discharge cycles, the most suitable charge current is selected for each battery, resulting in better overall performance. A segmented charging current approach is also introduced that reduces the expansion voltage and mechanical impact on the microstructure of the battery during the charging process, resulting in a more balanced and secure charging experience.DE 10 2022 200 867 A1 discusses a method for charging a multicellular battery, which detects the battery voltage and adjusts the charging voltage accordingly, thereby ensuring efficient and safe charging. In this case, a charge-quantity-dependent charging voltage characteristic curve is determined, which enables precise control and regulation of the charging voltage as a function of the state of charge of the battery.The object of the invention is to improve a method and a device for charging or discharging a battery system.The object is achieved according to the invention by a method with the features of claim 1 and a device with the features of claim 10. Furthermore, the object is achieved by a method with the features of claim 11 and a device with the features of claim 12. Advantageous embodiments of the invention are evident from the dependent claims.In particular, a method for charging or discharging a battery system is provided, wherein the battery system has at least two battery cells connected in parallel and / or in series, which can only be charged or discharged together, wherein a voltage as controlled variable is controlled by means of a current or a power as controlled variable, wherein the current or the power is predefined in this case taking account of a first characteristic curve dependent on the state of charge and a second characteristic curve dependent on the state of charge, and wherein the first characteristic curve dependent on the state of charge and the second characteristic curve dependent on the state of charge have opposite slopes at least in sections.Furthermore, in particular, a device for charging or discharging a battery system is provided, wherein the battery system has at least two battery cells connected in parallel and / or in series, which can only be charged or discharged together, comprising a regulator, wherein the regulator is configured to regulate a voltage as a controlled variable by means of a current or a power as a controlled variable, wherein the current or the power is predefined in this case taking account of a first characteristic curve dependent on the state of charge and a second characteristic curve dependent on the state of charge, and wherein the first characteristic curve dependent on the state of charge and the second characteristic curve dependent on the state of charge have, at least in sections, opposite slopes.Furthermore, in particular a method for charging or discharging a battery system is provided, wherein the battery system has a battery cell, wherein a voltage as controlled variable is controlled by means of a current or a power as controlled variable, wherein in this case the current or the power is predefined taking account of a first state-of-charge-dependent characteristic curve and a second state-of-charge-dependent characteristic curve, and wherein the first state-of-charge-dependent characteristic curve and the second state-of-charge-dependent characteristic curve have opposite slopes at least in sections.A device for charging or discharging a battery system is also provided, wherein the battery system has a battery cell, comprising a regulator, wherein the regulator is configured to regulate a voltage as a controlled variable by means of a current or a power as a manipulated variable, wherein the current or the power is predefined in this case taking account of a first characteristic curve dependent on the state of charge and a second characteristic curve dependent on the state of charge, and wherein the first characteristic curve dependent on the state of charge and the second characteristic curve dependent on the state of charge have, at least in sections, opposite slopes.The method and the device make it possible to regulate charging or discharging on the basis of a state of charge of the battery system as the overall system, in particular without the states of the individual battery cells having to be known for this purpose. One of the basic ideas here is to use two state-of-charge-dependent characteristic curves for regulation, wherein these two characteristic curves have an opposite gradient at least in sections. This makes it possible to maintain limit values for electrical variables during charging and discharging, in particular if an estimate of the state of charge is subject to an error.A voltage, a current, an internal resistance, and a power in charging and discharging are, in particular, a total voltage, a total current, a total internal resistance, and a total power of the battery system. The sizes for the individual battery cells are in particular not detected and / or used.It is provided in particular that a voltage is detected at the battery system, this voltage being used as a controlled variable during the control.It is provided in particular that the characteristic curves specify maximum values (limit values) for suitable electrical variables, which maximum values must not be exceeded. By using two characteristic curves, two such variables can be taken into account and / or used for regulating according to this method. It can be provided here that during the regulation, the compliance with the first characteristic curve is first checked and subsequently the compliance with the second characteristic curve is checked. However, it can alternatively also be provided that during the regulation first the compliance with the second characteristic curve is checked and subsequently the compliance with the first characteristic curve is checked. Furthermore, the checking of the compliance with both characteristic curves can also take place simultaneously.The first characteristic curve dependent on the state of charge and the second characteristic curve dependent on the state of charge are determined in particular under laboratory conditions, in particular in empirical experiments, and / or with the aid of simulations. As a result, for example, a characteristic curve for a state-of-charge-dependent maximum voltage can be determined. For example, measurements can be made on 3-electrode laboratory cells for this purpose. Furthermore, compressive force, thickness or high precision coulombmetry measurements can also be carried out. Furthermore, a predefined current characteristic map, in particular also taking into account a safety margin, can also be applied to the cell and in this way a charge state-dependent characteristic curve for the maximum voltage can be determined. A simulation can be carried out, for example, on the basis of half-cell models.Parts of the device, in particular the controller, can be configured individually or collectively as a combination of hardware and software, for example as program code which is executed on a microcontroller or microprocessor. However, it can also be provided that parts are configured individually or combined as an application-specific integrated circuit (ASIC) and / or field-programmable gate array (FPGA).In one specific embodiment, it is provided that the characteristic curves have opposite slopes over all states of charge. This makes it possible to provide a particularly simple implementation of the method and of the device and to ensure compliance with limit values at all times.In one specific embodiment, it is provided that the first characteristic curve dependent on the state of charge specifies a maximum voltage during charging or discharging, which increases with the state of charge. As a result, a voltage during charging and discharging can be limited. This is based on the idea that if the maximum charging or discharging current is determined for each state of charge and the assumption is made that the anode potential is the limiting factor, a state-of-charge-dependent full cell voltage trajectory results which is independent of the temperature. In particular, this full cell voltage trajectory then forms the state-of-charge-dependent first characteristic curve. This full cell voltage trajectory increases as the state of charge increases.In one specific embodiment, it is provided that the first characteristic curve dependent on the state of charge specifies a maximum internal resistance during charging or discharging, which increases with the state of charge. The internal resistance can be calculated here on the basis of a current and an overvoltage (=outward voltage, i.e. a voltage under load minus the open circuit voltage). The overvoltage can be estimated from a model in a manner known per se.In one specific embodiment, it is provided that the second characteristic curve dependent on the state of charge specifies a maximum current that falls with the state of charge during charging or discharging. As a result, a current during charging and discharging can be limited.In one specific embodiment, it is provided that the second characteristic curve dependent on the state of charge specifies a maximum power during charging or discharging, which decreases with the state of charge.As a result, power during charging or discharging can be limited. The power may be a power calculated from the current and the voltage (i.e., P=U*I; where P is the power, U is the voltage, and I is the current). In principle, however, the power can also be a power loss which is calculated, for example, from the internal resistance and the current (i.e. P=R*I 2) or from the overvoltage and the current (i.e. P=U Ü* I).It can also be provided that the first characteristic curve dependent on the state of charge and / or the second characteristic curve dependent on the state of charge depicts a combination of electrical variables and / or specifies limit values for such a combination of the electrical variables.In one specific embodiment, it is provided that a temperature of the battery system is detected and / or estimated, the first characteristic curve dependent on the state of charge and / or the second characteristic curve dependent on the state of charge being temperature-dependent, the detected and / or estimated temperature being taken into account. Temperature effects can thereby be taken into account, so that a regulation during charging or discharging can be further improved.In one specific embodiment, it is provided that a state of charge of the battery system is estimated on the basis of a temporal integration of the current during charging or discharging. For this purpose, a state-of-charge estimator integrates a current flowing during charging or discharging. This balances an amount of charge that has flowed into and / or out of the battery system. Starting from a starting value and a total charge quantity that the battery system can store, the current state of charge can be determined in this way. The state of charge (SOC) can be specified, for example, as a percentage or absolutely in the unit of ampere hours (Ah) with reference to a maximum storable charge quantity.In one specific embodiment, it is provided that a state of charge of the battery system is estimated on the basis of an open-circuit voltage-state of charge (OCV / SOC) characteristic curve and / or by means of an electric battery model. The open-circuit voltage-state of charge (OCV / SOC) characteristic curve makes it possible to estimate the state of charge on the basis of a measurement of the open-circuit voltage in the non-loaded state of the battery system. The electric battery model makes it possible to estimate a state of charge in a manner known per se on the basis of measurable electrical variables (open-circuit voltage, voltage, current, internal resistance, etc.). The open-circuit voltage-state-of-charge (OCV / SOC) characteristic curve and / or the electric battery model are determined, for example, with the aid of measurements under laboratory conditions, that is to say with the aid of empirical experiments, and / or with the aid of simulations.Further features for the configuration of the devices emerge from the description of configurations of the method. The advantages of the devices are in each case the same as in the embodiments of the method. The embodiments of the method and of the device for charging and discharging a battery system having only one battery cell are in particular the same as for a battery system having a plurality of battery cells.The invention is explained in more detail below with reference to preferred exemplary embodiments with reference to the figures. The following are shown here: FIG. 1 is a schematic illustration of an embodiment of the apparatus for charging or discharging a battery system; FIGS. 2 a, 2 b, 2 c are schematic representations for illustrating the mode of operation of an embodiment of the method and of the device; FIG. 3 shows results of a simulated charging process on a battery system having two battery cells connected in parallel and a regulation according to the method.FIG. 1 shows a schematic illustration of an embodiment of the device 1 for charging or discharging a battery system 20. The battery cells 21- xare in particular Li ion cells. A device for charging and discharging a battery system having only one battery cell is basically designed in the same manner as the embodiment shown in FIG. 1, so that it will not be discussed separately here.The device 1 comprises a regulator 2. the regulator 2 is configured to regulate a (total) voltage U, which is composed of the voltages U 1, U 2 at the battery cells 3- x, as a controlled variable by means of a current I or a power P as a controlled variable. The current I or the power P is divided between the currents I 1, I 2 or the powers P 1, P 2 at the individual battery cells 21- x. The voltage U and the current I or the power P relate here to the battery system 20 as the overall system, that is to say the corresponding values for the individual battery cells 21- xare not known and are also not detected. The voltage U is detected at the battery system 20 by means of a sensor system (not shown) configured for this purpose.It is provided that the current I or the power P is predefined during the regulation taking into account a first characteristic curve 3- 1 dependent on the state of charge and a second characteristic curve 3- 2 dependent on the state of charge. The first characteristic curve 3- 1 dependent on the charge state and the second characteristic curve 3- 2 dependent on the charge state have an opposite gradient at least in sections. The regulation is effected in particular in such a way that predefined maximum values (limit values) are not exceeded by the characteristic curves 3- x.A state of charge SOC is estimated by means of a state of charge estimator 4 of the device 1. It can be provided that the state of charge SOC of the battery system 20 is estimated on the basis of a temporal integration of the current I during charging or discharging. Alternatively or additionally, it can be provided that the state of charge SOC of the battery system 20 is estimated on the basis of an open-circuit voltage-state of charge (OCV / SOC) characteristic curve and / or by means of an electric battery model.In the example shown, it is provided in particular that the first characteristic curve 3- 1 dependent on the state of charge specifies a maximum voltage Ulim rising with the state of charge SOC during charging or discharging.Furthermore, in the example shown, it is also provided in particular that the second characteristic curve 3- 2 dependent on the state of charge specifies a maximum current Ilim, which falls with the state of charge SOC, during charging or discharging.In the example shown, it is provided in particular that the characteristic curves 3- 1, 3- 2 have opposite slopes over all states of charge SOC.Alternatively, it can be provided that the first characteristic curve 3- 1 dependent on the state of charge specifies a maximum internal resistance during charging or discharging, which increases with the state of charge SOC.Alternatively, it can be provided that the second characteristic curve 3- 2 dependent on the state of charge specifies a maximum power during charging or discharging, which decreases with the state of charge SOC.It can be provided that a temperature T of the battery system 20 is detected and / or estimated, wherein the first characteristic curve 3- 1 dependent on the state of charge and / or the second characteristic curve 3- 2 dependent on the state of charge are temperature-dependent, and wherein the detected and / or estimated temperature T is taken into account. It can be provided, for example, that the characteristic curves 3- xare selected or parameterized on the basis of the detected and / or estimated temperature T.FIGS. 2 a, 2 band 2 c show schematic representations for illustrating the mode of operation of an embodiment of the method and of the apparatus. Each of the three figures shows a specific case of states of charge at a single time.In FIG. 2 a, the state-of-charge estimator 4 estimates a state-of-charge SOC corresponding to an average of the states of charge SOC 1, SOC 2 of the two battery cells 21- x(FIG. 1 ). For example, due to a higher temperature in the battery cell 21- 2 compared to the battery cell 21- 1, the battery cell 21- 2 has a lower internal resistance. As a result, the (total) charging current I is divided into two unequally large partial currents I 1 and I 2. The charging current I2is greater than the charging current I2. As a result, a state of charge SOC 2 of the battery cell 21- 2 rises more rapidly than a state of charge SOC 1 of the battery cell 21- 1. Since the state-of-charge estimator 4 (FIG. 1 ) can only detect the (total) current I or the (total) voltage U of the parallel-connected battery cells 21- x, only a resulting average value of the state-of-charge SOC is detected from the states of charge SOC 1, SOC 2 of the battery cells 21- x. The charging control by the charging controller 2, which releases a maximum permissible voltage Ulim on the basis of the state of charge SOC on the basis of the first state-of-charge-dependent characteristic curve 3- 1, would release an excessively high voltage (here Ulim) in the case of battery cell 21- 1. However, according to the method described in this disclosure, the current I would be limited to the current Ilim that is smaller than Ilim 1 on the basis of the second state-of-charge dependent characteristic curve 3- 2. Since both limits are taken into account in the regulation, neither a current limit violation nor a voltage limit violation would occur as a result. A regulation with a single characteristic curve, for example only the first characteristic curve 3- 1 dependent on the state of charge, on the other hand, would have led to the maximum voltage at the battery cell 21- 1 being exceeded. In the case of the battery cell 21- 2, an excessively large value for the current limitation (Ilim instead of Ilim 2) would first be assumed, but the value for the voltage limitation Ulim is smaller than Ulim 2, as a result of which the resulting current I is likewise limited. Since both limits are taken into account in the regulation, therefore, neither a current limit violation nor a voltage limit violation would occur as a result. A regulation with a single characteristic curve, for example only the second characteristic curve 3- 2 dependent on the state of charge, on the other hand, would have led to the maximum current at the battery cell 21- 2 being exceeded.In FIG. 2 b, the state-of-charge estimator 4 (FIG. 1 ) estimates, by an error, a state-of-charge SOC that is greater than the average value SOC from the states of charge SOC 1, SOC 2 of the two battery cells 3- x. The charging control, which releases a maximum voltage U on the basis of the state of charge SOC, would release an excessively high voltage Ulim in the case of battery cell 3- 1 and battery cell 3- 2. However, according to the method, the current is limited to Ilim by the second characteristic curve 3- 2 dependent on the state of charge, so that the value is smaller than Ilim 1 and Ilim 2. Since both limits are taken into account in the regulation, therefore, neither a current limit violation nor a voltage limit violation would occur as a result. A control with a single characteristic curve, for example only the first characteristic curve 3- 1 dependent on the state of charge, on the other hand, would have led to the maximum voltage in both battery cells 21- 1, 21- 2 being exceeded.In FIG. 2 c, the state-of-charge estimator 4 (FIG. 1 ) estimates a state-of-charge SOC that is less than the average of the states of charge SOC 1, SOC 2 of the two battery cells 3- x. The charging control would initially assume too large a value for the current limiting (Ilim instead of Ilim 1 and Ilim 2). According to the method, however, the value of the voltage limitation Ulim is smaller than Ulim 1 and Ulim 2, as a result of which the resulting current I during charging is also limited. Since both limits are taken into account in the regulation, therefore, neither a current limit violation nor a voltage limit violation would occur as a result. A control with a single characteristic curve, for example only the second characteristic curve 3- 2 dependent on the state of charge, on the other hand, would have led to the maximum current in both battery cells 21- 1, 21- 2 being exceeded.The scenarios shown are merely exemplary in nature and are for purposes of illustrating the method and apparatus described in this disclosure. In principle, the characteristic curves 3- xmay also represent other variables and / or have a different profile.FIG. 3 shows results of a simulated charging process on a battery system having two battery cells connected in parallel, as is schematically shown in FIG. 1, and of a regulation according to the method. One battery cell has a temperature of 10° C. and the other a temperature of 50° C. The simulation parameters used here are as follows:cell chemistry: NMC- Cell capacity: 100 Ahheat capacity of a cell: 2000 J / K- Isothermal boundary conditionsheat exchange coefficient between the parallel-connected cells: 0 W / KStarting state of charge at the beginning of the simulation: 5% (of the maximum charge quantity)resistance of the connection elements of the parallel-connected cells: 0 ohm- contact resistances: 0 ohmsInternal cell resistance at 10° C.: 1 mOhm to 3 mOhmInternal cell resistance at 50° C.: 0.5 mOhm to 1.5 mOhmfirst characteristic curve dependent on the state of charge specifies the maximum voltage Ulim (limit voltage)second characteristic curve dependent on the state of charge presets the maximum current Ilim (limit current)In FIG. 3, at the upper left, a (total) current I, the respective maximum currents Ilim 1, Ilim 2 and the currents I 1, I 2 of the battery cells are shown in A, respectively; at the upper right, the (total) battery current SOC and the states of charge SOC 1 and SOC 2 of the battery cells are shown in %; at the lower left, a (total) voltage U, a respective voltage U 1, U 2 at the battery cells and a maximum voltage Ulim (limit voltage) and the maximum voltages Ulim 1 and Ulim 2 resulting from SOC 1 and SOC 2, respectively, are shown in V; and at the lower right, a temperature T 1, T 2 of the two battery cells is shown in ° C. The magnitudes are shown over the time axis, respectively, with the unit of seconds.It is noted that only Ulim, I and SOC and the temperatures are used in particular for the regulation. In particular, I 1, I 2, ULIM 1, ULIM 2, SOC 1 and SOC 2 are unknown in reality and serve within the scope of the simulation only to illustrate the method described in this disclosure.It can be clearly seen that the currents I 1, I 2 always remain below the relevant maximum current ILIM 1, ILIM 2 by the regulation according to the method. The voltages U1, U2 also always remain below the maximum voltage Ulim.List of reference characters1 Device 2 Controller 3- 1 First state-of-charge-dependent characteristic curve 3- 2 Second state-of-charge-dependent characteristic curve 4 State-of-charge estimator 20 Battery system 21- x Battery cell I Current I 1 Current (first battery cell) I 2 Current (second battery cell) Ilim Maximum current Ilim 1 Maximum current (starting from SOC 1) Ilim 2 Maximum current (starting from SOC 2) P Power P 1 Power (first battery cell) P 2 Power (second battery cell) SOC (total) Battery SOC 1 State of charge (first battery cell) SOC 2 State of charge (second battery cell) T Temperature U (total) voltage U 1 Voltage (first battery cell) U 2 Voltage (second battery cell) Ulim Maximum voltage Ulim 1 Maximum voltage (starting from from SOC 1) Ulim 2 maximum voltage (starting from SOC 2)

Claims

Method for charging or discharging a battery system (20), wherein the battery system (20) has at least two battery cells (21-x) connected in parallel and / or in series, which can only be charged or discharged together, wherein a voltage (U) as controlled variable is controlled by means of a current (I) or a power (P) as controlled variable, wherein the current (I) or the power (P) is predefined in this case taking account of a first characteristic curve (3-1) dependent on the state of charge and a second characteristic curve (3-2) dependent on the state of charge, and wherein the first characteristic curve (3-1) dependent on the state of charge and the second characteristic curve (3-2) dependent on the state of charge have opposite slopes at least in sections.Method according to Claim 1, characterized in that the characteristic curves (3-x) have opposite slopes over all states of charge (SOC).Method according to Claim 1 or 2, characterized in that the first characteristic curve (3-1) dependent on the state of charge specifies a maximum voltage (Ulim) which increases with the state of charge (SOC) during charging or discharging.Method according to Claim 1 or 2, characterized in that the first characteristic curve (3-1) dependent on the state of charge specifies a maximum internal resistance which increases with the state of charge (SOC) during charging or discharging.Method according to one of the preceding claims, characterized in that the second characteristic curve (3-2) which is dependent on the state of charge specifies a maximum current (Ilim) which falls with the state of charge (SOC) during charging or discharging.Method according to one of Claims 1 to 4, characterized in that the second characteristic curve (3-2) which is dependent on the state of charge specifies a maximum power which falls with the state of charge (SOC) during charging or discharging.Method according to one of the preceding claims, characterized in that a temperature (T) of the battery system is detected and / or estimated, wherein the first characteristic curve (3-1) dependent on the state of charge and / or the second characteristic curve (3-2) dependent on the state of charge are temperature-dependent, and wherein the detected and / or estimated temperature (T) is taken into account.Method according to one of the preceding claims, characterized in that a state of charge (SOC) of the battery system (20) is estimated on the basis of a temporal integration of the current (I) during charging or discharging.Method according to one of the preceding claims, characterized in that a state of charge (SOC) of the battery system (20) is estimated on the basis of an open-circuit voltage-state of charge (OCV / SOC) characteristic curve and / or by means of an electrical battery model.Device (1) for charging or discharging a battery system (20), wherein the battery system (20) has at least two battery cells (21-x) connected in parallel and / or in series, which can only be charged or discharged together, comprising: a regulator (2), wherein the regulator (2) is configured to regulate a voltage (U) as a controlled variable by means of a current (I) or a power (P) as a controlled variable, wherein the current (I) or the power (P) is predefined here taking account of a first state-of-charge-dependent characteristic curve (3-1) and a second state-of-charge-dependent characteristic curve (3-2), and wherein the first state-of-charge-dependent characteristic curve (3-1) and the second state-of-charge-dependent characteristic curve (3-2) have opposite slopes at least in sections.Method for charging or discharging a battery system (20), wherein the battery system (20) has a battery cell (21-x), wherein a voltage (U) as controlled variable is controlled by means of a current (I) or a power (P) as controlled variable, wherein the current (I) or the power (P) is predefined in this case taking account of a first characteristic curve (3-1) dependent on the state of charge and a second characteristic curve (3-2) dependent on the state of charge, and wherein the first characteristic curve (3-1) dependent on the state of charge and the second characteristic curve (3-2) dependent on the state of charge have opposite slopes at least in sections.Device (1) for charging or discharging a battery system (20), wherein the battery system (20) has a battery cell (21-x), comprising: a regulator (2), wherein the regulator (2) is configured to regulate a voltage (U) as a controlled variable by means of a current (I) or a power (P) as a manipulated variable, wherein the current (I) or the power (P) is predefined here taking into account a first state-of-charge-dependent characteristic curve (3-1) and a second state-of-charge-dependent characteristic curve (3-2), and wherein the first state-of-charge-dependent characteristic curve (3-1) and the second state-of-charge-dependent characteristic curve (3-2) have opposite slopes at least in sections.

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