DE102021203390B3 - Method for voltage-controlled operation of a battery system during charging - Google Patents

Abstract

The invention relates to a method for voltage-controlled operation of a battery system (4) during a charging process, having at least one electrochemical battery cell (6), a process damaging the battery cell (6) being characterized using at least one stored reference curve (14), the reference curve ( 14) is a state of charge/cell voltage characteristic, in which a cell voltage (UZ) of the battery cell (6) is detected, in which a state of charge (SOC) of the battery cell (6) is determined, in which an aging state of the battery cell (6) is determined at which the cell voltage (UZ) and the state of charge (SOC) are compared with the stored reference curve (14), at which a maximum charging current (IL) for the aging state of the battery cell (6) is determined on the basis of the comparison, and at which the charging current (IL) is set by controlling and/or regulating the cell voltage (UZ).

Description

The invention relates to a method for voltage-controlled operation of a battery system during a charging process, having at least one electrochemical battery cell.

A charging process is to be understood here and in the following in particular as charging or discharging the battery system or the battery cell by means of an electrical charging current (charging current, discharging current). During a charging process, electrical energy is supplied to the battery cell by means of the (charging) charging current, with electrical energy correspondingly being discharged from the battery cell by means of the (discharging) charging current during a discharging process.

Motor vehicles that are driven or can be driven electrically or by an electric motor, such as electric or hybrid vehicles, generally include an electric motor with which one or both vehicle axles can be driven. To supply electrical energy, the electric motor is usually connected to a vehicle-internal (high-voltage) battery as an electrical energy store.

A battery, in particular an electrochemical battery, is to be understood here and in the following in particular as a so-called secondary battery (secondary battery) of the motor vehicle. In such a (secondary) vehicle battery, chemical energy that has been consumed can be restored by means of an electrical (charging) charging process. Such vehicle batteries are designed, for example, as electrochemical accumulators, in particular as lithium-ion accumulators.

In order to generate or provide a sufficiently high operating voltage, such vehicle batteries typically have at least one battery module (battery cell module) in which a plurality of individual electrochemical battery cells are connected in a modular manner. Alternatively, a so-called Cell2Pack design is possible, in which the battery cells are connected directly to the vehicle battery, in particular connected in parallel, and are not combined into modules in advance.

Depending on the (battery cell) temperature, duration of current supply and state of charge, battery cells, in particular lithium-ion cells, can only absorb a certain charging current without being damaged. This limit current changes with the aging of the battery cell. Furthermore, there is the problem that in a battery system, several battery cells can be connected in series or in parallel, which have different temperatures and/or aging states.

It is therefore necessary to know the state of charge and aging of all battery cells in the battery system at all times, as well as the coldest and warmest spots. It is therefore also necessary that temperature gradients within a battery cell must be known in order to actually know the coldest and warmest point in the entire battery system. Together with the charging and aging status of each battery cell, these two points determine the maximum possible charging current at any time. Since a relaxed battery cell can briefly absorb a higher current than the maximum possible continuous current, the time reaction of the battery cell to the charging current must also be known. “Relaxed” or “relaxed to completion” is to be understood here in particular as a constant state of the battery cell, which is reached after a charging or discharging current has ended.

An example of an aging or damage process in lithium-ion battery cells with liquid electrolytes is so-called lithium plating (Li-plating), in which metallic lithium is deposited on the surface of the anode and reacts irreversibly with the components of the electrolyte. As a result, for example, there is a loss of capacity in the battery cell due to the reduction in free lithium ions. Furthermore, this can cause an electrical short circuit within the battery cell, as a result of which the battery cell can catch fire during a charging process. Li plating occurs when the electrical potential of the anode material and the electrolyte falls below the potential of lithium. Overvoltages occur during the charging process, which reduce the potential of the anode. The overvoltage is made up of a separator/electrolyte component and a cathode component as well as an anode component, with the anode component being the critical component with regard to Li plating. At high charging currents, the anode potential falls below the lithium potential (0 volt vs. Li/Li ⁺ ), which leads to the deposition of lithium ions on the anode surface.

The starting point of Li-plating can be determined by measuring a thickness of the battery cell or a force on a bracing device of several battery cells. Alternatively, for example, a measurement by high-precision coulometry (high precision coulombmetry) is possible to determine the starting point of Li-plating.

With lithium-ion cells, the maximum possible charging current can be determined with the help of 3-electrode cells, for example, by clicking on the Potential of the anode is regulated under load. This is only possible in a 3-electrode cell with a reference electrode. A reduction in the charging current characteristics in proportion to the decrease in capacity or the increase in the internal resistance of the battery cell over the course of aging then takes place in the battery system. This is for example in the article SIEG, Johannes [ua]: Fast charging of an electric vehicle lithium-ion battery at the limit of the lithium deposition process. In: Journal of Power Sources, Vol. 427, 2019, pp. 260-270 and the DE 10 2016 007 479 A1 described. However, this method has the limitation that the minimum and maximum temperature in the battery cell must be known for each battery cell in the system so that the maximum permitted charging current can be selected from the map. Furthermore, the state of aging, especially the current internal resistance, of each battery cell in the battery system must be known, since the procedure described there determines the current from the most aged battery cell. Furthermore, it is necessary for battery systems as vehicle batteries to build experimental laboratory cells with reference electrodes from the industrially manufactured battery cells.

Furthermore, it is, for example, from the DE 10 2019 003 465 A1 It is known that the maximum pulsed current for each state of charge can be determined for a relaxed cell using the 3-electrode cells. However, this method presents similar challenges as the method described above. In addition, a large number of maps must be determined and stored in the system.

In general, if explicit charging current maps are stored in the system, it is necessary to determine the maximum and minimum temperature in the system precisely and to use the large number of stored maps without errors. Stable and safe operation must be secured through complex programming and comprehensive tests.

Furthermore, it is possible, for example, for charging current characteristic diagrams to be checked for their harmfulness as a function of temperature, state of charge and pulse duration by repeated application to the battery and later specified accordingly for the system.

From the U.S. 2011/0 012 563 A1 a battery cell charger is known for rapidly charging a lithium-ion battery cell (or a series of cells connected in series-parallel) with a maximum battery cell voltage, the battery cell charging system comprising: a circuit for charging the battery cell using an adjustable voltage charging profile for applying a charging voltage and a charging current to the battery cell, wherein the adjustable voltage charging profile comprises: a first charging stage with a constant first stage charging current and an increasing battery cell voltage, wherein the first stage charging current is provided up to the first stage charging and the voltage is approximately equal to a first stage completion voltage which is less than the maximum battery cell voltage; one or more intermediate charge stages, each intermediate stage selected from the group consisting of a constant voltage intermediate stage providing a decreasing charge current, a constant current intermediate stage providing an increasing battery cell voltage, and combinations thereof; and a final stage charge having a constant final stage charge voltage approximately equal to an interstage complete voltage and a decreasing final stage charge current, the final stage charge voltage being provided until the final stage charge current reaches a desired full charge level.

From the U.S. 2015/0 022 160 A1 a system is known that manages the use of a battery in a portable electronic device. During operation, the system obtains a battery voltage and a battery state of charge. Next, the system calculates an effective C-Rate of the battery using the voltage and state of charge. Finally, the system uses the effective C rate to manage a charge for the battery.

From the DE 10 2016 007 479 A1 a device and a method for charging a battery cell with an anode and a cathode are known, with a charging current being fed into the battery cell during the charging process, with the anode potential of the anode being regulated indirectly via the charging current to a predeterminable value at least in a predetermined charging phase .

From the DE 10 2015 205 171 A1 a method for charging or discharging an electrochemical energy store is known, the aging behavior of which is modeled by a description of its depth of discharge, in which, in a time-dependent course of the state of charge, a discharge depth value is assigned to each period between two adjacent local maxima and minima, which corresponds to the charging or discharging stroke between the corresponds to state of charge values of the two adjacent local maxima and minima. The energy store is transferred from a first state of charge at a first point in time to a second state of charge at a second point in time by within the period between the first and the second point in time formed period of time, the charging or discharging process is interrupted at least once to form respective charging or discharging sub-blocks with a respective charging or discharging stroke, with a respective charging or discharging stroke being smaller than the total stroke between the first and the second charging state.

The invention is based on the object of specifying a particularly suitable method for operating a battery system. The invention is also based on the object of specifying a particularly suitable battery system for carrying out the method.

With regard to the method, the object is achieved with the features of claim 1 and with regard to the battery system with the features of claim 10. Advantageous refinements and developments are the subject of the dependent claims.

Insofar as method steps are described below, advantageous configurations for the battery system result in particular from the fact that it is designed to carry out one or more of these method steps.

The method according to the invention is intended for operating a battery system, in particular a vehicle battery or high-voltage battery (traction battery) of an electrically driven or drivable motor vehicle, for example an electric or hybrid vehicle, and is suitable and designed for this. The operation of the battery includes both idle phases, in which no current flows from the outside, as well as complex operating scenarios, such as fast charging and recuperation in electrically powered or drivable motor vehicles. The loads on the battery can be combined with one another in any way and can have any time sequence and any form, such as pulses, steps, or a continuous course. In this case, the method is designed for voltage-controlled operation of the battery system during a charging process, ie while the battery system is under load. This means that the battery system is operated under voltage control and/or voltage regulation during the charging process.

The conjunction “and/or” is to be understood here and in the following in such a way that the features linked by means of this conjunction can be designed both together and as alternatives to one another.

In this case, the battery system has at least one electrochemical battery cell. The battery system can also have several battery cells, the battery cells being electrically connected to one another (e.g. series and parallel connection). The following statements relate in particular to lithium-ion cells with liquid electrolytes as battery cells. However, the explanations can also be transferred to solid-state cells or battery cells with different cell chemistry, for example.

A process that damages the battery cell (damage process) is characterized using at least one stored reference curve (characteristic curve, trajectory). The reference curve is a state of charge/cell voltage characteristic.

A damaging process is to be understood here in particular as a process within the battery cell which affects the cell or electrolyte chemistry and leads to reduced cell properties, such as a reduced cell capacity. In particular, the damaging process represents a factor that limits the charging process. The damaging process is, for example, an aging process in the battery cell. In particular, a damaging process is to be understood that leads to a safety-critical state of the battery cell, such as lithium plating, overheating of the battery cell, destruction of the active materials due to excessive load gradients, or decomposition of the electrolyte. In the case of a lithium-ion cell with liquid electrolytes, the damaging process characterized for the process is in particular Li plating.

According to the method, a cell voltage of the battery cell is detected during the charging operation and a state of charge of the battery cell and an aging state of the battery cell are determined. The state of health (SOH) of the battery cell is a measure of the aging processes caused in the battery cell, i.e. of the damaging processes that lead to a change in the cell properties, which, for example, affects the maximum energy storage capacity, the cell internal resistance , the cell voltage, or the cell geometry changes.

The values for the current cell voltage and the current state of charge are compared with the stored reference curve. From the comparison, a maximum possible charging current for which the damaging process does not occur is determined for the respective aging state of the battery cell. The current charging current of the battery cell is then controlled and/or regulated to the maximum current value by setting the cell voltage independently of a battery cell temperature. In other words, according to the method, by controlling and/or regulating the drawer current to the respective aging condition of the battery cell by comparing the cell voltage and the state of charge with the stored reference curve sets an advantageous and/or non-damaging maximum charging current. This maximizes the charging current while avoiding the damage process.

The invention is based on the finding that the determination of the maximum charging current leads to a voltage trajectory that is quasi-independent of the temperature if the damaging process is the limiting factor. In particular, the cell voltage has little or no temperature dependency on the state of charge of the battery cell. This means that the cell voltage has an almost constant curve for different battery cell temperatures compared to the state of charge of the battery cell. This makes it possible to control and/or regulate the charging process using only a single reference curve. In particular, no temperature monitoring of each individual battery cell in the battery system is necessary. Complex temperature monitoring through direct measurement or modeling of each individual battery cell is therefore no longer necessary. A particularly suitable method for voltage-controlled operation of a battery system during a charging process is thus implemented.

If the state of charge of each battery cell in the battery system is known during the charging process, it is sufficient to measure the cell voltage in order to determine the maximum charging current that is currently possible. It is advantageous for the method that, in battery systems, the voltage of each battery cell or each battery cell connected in parallel is individually monitored. Exact knowledge of the coldest and warmest points in the battery system is no longer necessary when using the method.

The voltage trajectory of the reference curve indicates the maximum voltage under continuous current for each state of charge. If a current is applied to a relaxed cell, it takes time until all overvoltages have built up in the cell and the cell voltage reaches the voltage limit value for continuous current application. Since the anode overvoltage only accounts for a certain proportion of the total overvoltage and is decisive in this respect, the estimation of the maximum charging current is always conservative. Consequently, if charging is to be carried out with the maximum current, the current only has to be regulated in such a way that the cell voltage is exactly on the voltage trajectory for continuous currents.

The method according to the invention ensures a particularly advantageous operation of the battery system, in which damage mechanisms and damage processes are avoided by means of voltage regulation or voltage control of the individual battery cells, part battery or entire battery. The method makes it possible to ensure an optimal distribution of the charging currents of the connected battery cells for all operating states without achieving unfavorable operating states (Li-plating or the like).

The method is explained in more detail below based on an application during a rapid charging process for a vehicle battery. When fast charging the vehicle battery or battery system, high currents flow into the individual battery cells. Here, unfavorable conditions can occur in the vehicle battery in a variety of ways. In order to prevent Li plating in particular, the anode potential must assume a specific value greater than 0 volts, which can be determined in advance. However, heat is generated during the charging process, which can increase the battery temperature to an unfavorable value. The power loss that occurs is, among other things, a function of the overvoltage of the battery cell. The overvoltage of a battery cell is, for example, a function of the temperature, the state of charge, the history and the state of aging of the battery cell. In order to lower the battery temperature, corresponding additional control circuits are required, for example for the charging current or battery cooling. In addition, any external influences can change the battery temperature and the heating behavior. As a result, the charging current can no longer be regulated to a previously determinable anode potential. The stored reference curve specifies the maximum voltage under continuous current for which Li-plating does not occur for each state of charge. Since the reference curve corresponds to an essentially temperature-independent course of the cell voltage, all dynamics of the battery and the cooling as well as any external influences are taken into account by the stored reference curve, whereby the fast charging operation can be controlled reliably using this reference curve.

The method according to the invention can also be used, for example, as an evaluation method or for characterizing the battery system or its battery cells.

In a suitable development of the method, the aging state of the battery cells is determined regularly, for example periodically. The at least one reference curve is expediently adjusted with regard to the state of aging. This enables advantageous operation of the battery system over its entire service life. A control on the stored reference curve thus results in a at any time conservative estimate for the maximum charging current.

To determine the aging status, for example, the current capacity of all battery cells or battery cells connected in parallel is determined. This can be done, for example, based on curve fitting of an open circuit voltage (OCV). To determine the aging state, it is sufficient to determine only a decrease in capacity of the battery cells and not the internal resistance.

The change in internal resistance is automatically included in the measurement of the load voltage (cell voltage). This makes it possible to dispense with a time-consuming determination of the maximum possible pulse currents, e.g. by calculating a PT1 behavior, and instead to implement a conservative and inherently safe estimate. This means that it is not necessary to determine the internal resistance by measuring the pulsed current to determine the aging condition in order to adjust the reference curve. An additional implementation of pulsed currents in addition to the continuous charging currents can therefore be omitted.

In a preferred embodiment, different reference curves are stored for a charging process and for a discharging process of the battery cell. In particular, only one reference curve for the charging process and only one reference curve for the discharging process is stored. Only one voltage characteristic is stored in the system for each charging process, so that errors do not have to be interpolated between any number of current characteristics. In order to prevent these unfavorable states, an upper limit voltage is defined for charging the battery as a function of the state of charge for the individual battery cells. For discharging the battery, a corresponding lower limit voltage is defined depending on the state of charge for the individual battery cells. It is possible to define and implement separate limit voltages for each battery cell using the reference curves.

In an advantageous embodiment, the at least one reference curve is pre-characterized with a laboratory cell. The laboratory cell is a 3-electrode cell with an anode and a cathode as well as a reference electrode that is operated without current. The 3-electrode cell enables direct measurement and acquisition of continuous voltage trajectories or current maps. The voltage trajectory determined in the 3-electrode cells is stored in the battery system as a reference curve. Instead of directly using the voltage trajectory from the 3-electrode laboratory cell, it is possible, for example, to store the specific current map with a safety margin for the original battery cell in the battery system.

Instead of a measurement on a 3-electrode laboratory cell, the charging voltage trajectory can also be determined on the original cell using other methods, such as compressive force, thickness or high-precision coulombmetry measurements. In particular, the reference curve can be determined by measuring the Coulomb's efficiency or the cell expansion and/or cell geometry change.

In an alternative, equally advantageous embodiment, the at least one reference curve is determined by means of a simulation. In particular, the voltage trajectory is determined by simulating the cell voltage, for example on the basis of half-cell models. This enables the reference curve to be determined in a particularly cost-effective and effort-reduced manner.

The voltage regulation or voltage control of the battery or cell voltage, taking into account the reference curve, can be carried out using the terminal voltage of the battery cells. For technical reasons, however, the cell voltage cannot usually be tapped directly at the battery cell. Additional voltages can arise from the other voltage measuring points, for example from busbars. In a conceivable development, the reference curve is provided with a safety factor before the reference curve is stored. The safety factor makes it possible to take such additional stresses into account in the reference curve. This simplifies the voltage measurement or detection of the cell voltage.

An additional or further aspect of the invention provides that the method is interrupted or paused when a predefined or stored state of charge of the battery cell is reached. In other words, the state of charge of the battery cell is monitored during the method and compared to the stored state of charge or state of charge value, with the method being paused or interrupted if the stored state of charge is reached or exceeded. The pause or interruption can take place in the form of a predetermined waiting time or period of time, after which the method is automatically continued. During the pause or interruption, advantageous balancing processes can take place or be brought about in the battery system and/or the battery cell. Preferably will a predefined charging current and/or a reference curve is set during the pause.

The battery system according to the invention is provided, for example, as a vehicle battery or high-voltage battery of a motor vehicle, in particular an electrically driven or drivable motor vehicle, such as an electric or hybrid vehicle, and is suitable and set up for this. The battery system is in particular a lithium-ion battery system. In this case, the battery system has at least one electrochemical battery cell. In particular, several battery cells are connected to one another in series or in parallel. In addition, each battery cell or each battery cell connected in parallel is equipped with a voltmeter to measure the voltage. A voltage regulator and a controller, ie a control unit, are also provided. In this case, the controller is part of a battery management system of the battery system, for example.

In this case, the controller is generally set up—in terms of program and/or circuitry—to carry out the method according to the invention described above. The controller is thus specifically set up to determine the state of charge of the battery cells and to receive the measurement data from the voltmeter. For example, the controller has a memory in which the reference curve is stored. The controller evaluates the current cell voltages and state of charge based on the reference curve and controls the voltage regulator accordingly. The voltage regulator regulates the charging power or the charging current at all times in such a way that all cell voltages in the system remain below or equal to the reference curve. The state of charge of each battery cell or battery cells connected in parallel is suitably determined at any time and regularly adjusted using balancing concepts. Furthermore, the controller regularly uses the voltage data to determine the current capacity (state of health) of all battery cells or battery cells connected in parallel and adjusts the reference curve accordingly. Methods based on OCV curve matching can be used here.

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 programmatically in the form of operating software (firmware), so that the method - optionally in interaction with a device user - is performed automatically upon execution of the operating software in the microcontroller. Alternatively, within the scope of the invention, the controller can also be formed by a non-programmable electronic component, such as an application-specific integrated circuit (ASIC), in which the functionality for carrying out the method according to the invention is implemented with circuitry means.

A particularly suitable battery system is thereby realized.

• 1 in schematischer Darstellung ein Kraftfahrzeug mit einem Batteriesystem,
• 2 ein Ladezustands-C-Rate-Diagramm für unterschiedliche Temperaturen einer Batteriezelle,
• 3 ein Ladezustands-Zellspannungs-Diagramm für unterschiedliche Temperaturen einer Batteriezelle mit einer Referenzkurve,
• 4 schematische Darstellung eines Simulationsmodells,
• 5 ein Ladezustands-C-Rate-Diagramm zum Vergleich des Simulationsmodells mit einer 3-Elektroden-Messung,
• 6 ein schematisches Diagramm zur Zusammensetzung einer Überspannung in der Batteriezelle, und
• 7 ein Ladezustands-C-Rate-Diagramm zum Vergleich des Simulationsmodells mit Grenzwerten.

An exemplary embodiment of the invention is explained in more detail below with reference to a drawing. Show in it:

• 1 a schematic representation of a motor vehicle with a battery system,
• 2 a state of charge C-rate diagram for different temperatures of a battery cell,
• 3 a state of charge cell voltage diagram for different temperatures of a battery cell with a reference curve,
• 4 schematic representation of a simulation model,
• 5 a state of charge C-rate diagram to compare the simulation model with a 3-electrode measurement,
• 6 a schematic diagram of the composition of an overvoltage in the battery cell, and
• 7 a state-of-charge C-rate plot comparing the simulation model to limit values.

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

In the 1 is a schematic and simplified representation of an electrically driven or drivable motor vehicle 2 with a battery system 4 for the electrical supply of a traction drive, not shown. The battery system 4 is designed as a lithium-ion system with a number of electrochemical battery cells 6 . The battery system 4 also has a voltmeter 8 and a voltage regulator 10 as well as a controller 12 .

In this case, the controller 12 is suitable and set up for voltage-controlled operation of the battery system 4 during a charging process. In a memory of the controller 12 is a reference curve 14 ( 3 ) deposited, which characterizes a process that damages the battery cells 6 . The reference curve 14 is preferred way around a state of charge cell voltage characteristic. In this exemplary embodiment, the damaging process characterized is in particular Li plating.

Different reference curves 14 are preferably stored for a charging process and for a discharging process of the battery cells 6 . Only one charging process, in which electrical energy is fed into the battery cells 6 by means of a charging current IL, is explained in more detail below by way of example. In other words, the term charging process is to be understood below as meaning in particular a charging process, for example a rapid charging process.

During a charging process, the cell voltages UZ of the battery cells 6 are recorded using the voltmeter 8 . The controller 12 determines or monitors a state of charge SOC of the battery cells 6 during the charging process. Furthermore, the controller 12 regularly uses the voltage data from the voltmeter 8 to determine the current capacity (state of health) of all battery cells 6 or parallel circuits of battery cells 6. Methods based on OCV -Curve matching are used.

The respective current states of charge SOC and the current cell voltages UZ are compared with the stored reference curve 14 . The controller 12 evaluates the current cell voltages UZ and states of charge SOC based on the reference curve 14 . The values for the cell voltages UZ and states of charge SOC are compared with the stored reference curve 14, with the controller 12 using the comparison and the aging state (SOH) determining a maximum possible charging current IL for the battery cell 6 for which the damaging process does not occur. The aging state can be taken into account here, for example, by the controller 12 adapting the reference curve 14 accordingly to the determined aging state. The current charging current IL is then controlled and/or regulated to a maximum current value by means of the controller 12 by activating the voltage regulator 10 independently of a battery cell temperature.

The voltage regulator 10 thus regulates the charging power or the charging current IL at all times in such a way that all cell voltages UZ in the system remain below or equal to the reference curve 14 . The state of charge SOC of each battery cell 6 or parallel connection of battery cells 6 is suitably determined by the controller 12 at any point in time and this is regularly adjusted using balancing concepts. In this case, the adjustment can take place, for example, during a pause or interruption of the method carried out by the controller 12 . For this purpose, for example, a predefined state of charge value is stored in the controller 12, with the controller 12 pausing the method described above for balancing when the state of charge value is reached or exceeded.

If the battery system 2 is to consume power, the battery system 2 or the controller 10 for each battery cell 6 compares the current cell voltage UZ at the current state of charge SOC with the stored charging voltage trajectory of the reference curve 14 . The charging current IL can be increased or maintained as long as the cell voltage UZ is less than or equal to the stored reference curve 14 at the current state of charge SOC.

The inventive method is based on the 2 until 7 explained in more detail.

the 2 shows a state-of-charge C-rate diagram of a battery cell 6. The C-rate CR describes the charging (or discharging) current of a battery cell 6 in relation to its capacity. Horizontally, i.e. along the abscissa axis (X-axis), the state of charge SOC is plotted in percent (%), and the unitless C-rate CR is plotted along the vertical ordinate axis (Y-axis). The diagram shows the course of the charging current IL or the C rate CR for different temperatures of the battery cell 6 . In particular, nine curves are shown for temperatures between 10°C (degrees Celsius) and 50°C with temperature steps of 5°C.

As based on the current map of the 2 can be seen comparatively clearly, the charging current IL has a comparatively large temperature dependency. For example, the C-rates CR at 10 °C and 50 °C differ by a factor of about 2.

In the 3 is a state of charge-cell voltage diagram for the charging processes of the 2 shown. Horizontally, i.e. along the abscissa axis (X-axis), the state of charge SOC is plotted in percent (%), and along the vertical ordinate axis (Y-axis) the cell voltage UZ in volts (V) is plotted.

How based on 3 can be seen comparatively clearly, the cell voltage UZ has a lower temperature dependency than the charging current IL. An examination using an electrical cell model suggests that the variance in the voltage curve is more due to the moderate quality of the 3-electrode laboratory cell used, and the voltage curve actually for all temperatures above a critical temperature, in which the cell behavior changes significantly is always the same. The fact that the voltage curves show a comparatively large deviation below a state of charge SOC of 10% is due to the fact that the charging process was started in this state of charge range with finite currents selected due to the test bench until the anode potential reached 0V vs. Li/Li+, and the test bench controlled the control of the current could take over.

Due to the low spread of the voltage curves for the different temperatures, it is possible to approximate the behavior using a single reference curve 14, which as a barrier specifies the cell voltages UZ at which no Li plating occurs with continuous current flow.

A safety margin M is used for the transfer of the characteristic diagrams from laboratory cells to the battery cells 6 of the automotive battery system 2 in order to take into account the inhomogeneity of temperature and state of charge in the large battery cell 6 . Therefore the in 3 shown reference curve 14 reduced by about 10% compared to the measured voltage trajectories. The safety margin M can be implemented as a multiplicative factor or as a voltage offset. The reference curve 14 thus determined is stored in the battery system 4 or in the controller 14 .

The voltage regulation or voltage control of the battery or cell voltage UZ, taking into account the reference curve 14, can take place by means of the terminal voltage of the battery cells 6. It is also possible that the voltage measuring points of the voltmeter 8 have additional voltages, for example due to busbars. In a conceivable development, a corresponding safety factor is taken into account in the safety margin M.

To explain an advantageous operation of the battery system 2 with a parallel connection of battery cells 6, where the temperature of the individual cells is not known, a simulation was carried out in which the voltage of the cell parallel connection was regulated to the voltage trajectory of the reference curve.

the 4 shows a schematic sketch of the simulation scenario. The scenario has two battery cells 6a and 6b connected in parallel, battery cell 6a having a temperature of 10° C. and battery cell 6b having a temperature of 50° C. The voltage profile for 10° C. is used here as the reference curve 14 .

In the 5 a state-of-charge C-rate diagram is shown for comparison of the simulated and measured maximum possible charging currents IL. Horizontally, i.e. along the abscissa axis (X-axis), the state of charge SOC is plotted in percent (%), and along the vertical ordinate axis (Y-axis), the C-rate CR or the charging current IL is plotted. In the 5 four curves 16, 18, 20, 22 are shown. Curve 16 shows the curve for the 3-electrode measurement of battery cell 6a at 10° C., curve 18 showing the correspondingly simulated curve. Correspondingly, curve 20 shows the curve for the 3-electrode measurement of battery cell 6b at 50° C., curve 22 showing the simulated curve. How based on 5 can be seen comparatively clearly, the simulated cell currents for both temperatures are within the maximum possible charging currents, which were determined with the 3-electrode measurement (3E measurement).

The determination of the charging current pulses as a function of the amount of charge is explained below. In the article Grimsmann et al., Determining the maximum charging currents of lithium-ion cells for small charge quantities, 2017, Power Sources, it is explained that the maximum pulse current and the amount of charge or the state of charge SOC have approximately a 1/VSOC dependency ( One through root state of charge).

Using a simulation and the voltage trajectory from the 3 electrode measurements, the achievable amounts of charge were determined as a function of the current.

The diagram of 6 comprises two vertically stacked sections 24, 26.

Section 24 shows a schematic state-of-charge-current diagram. The state of charge SOC is horizontal, ie along the abscissa axis (X-axis), and a current I is plotted along the vertical ordinate axis (Y-axis). The level of the current curve, which is shown with a double arrow in section 24, indicates the maximum charging current IL.

Section 26 shows a schematic state-of-charge-voltage diagram. The state of charge SOC is horizontal, ie along the abscissa axis (X-axis), and a voltage U is plotted along the vertical ordinate axis (Y-axis). The section 26 shows two curves 28, 30. The curve 28 corresponds to the voltage trajectory of the reference curve 14, with the curve 30 representing the overvoltage.

Section 26 of the diagram shows the composition of the overvoltage, wel che is made up of a separator/electrolyte portion 30a and a cathode portion 30b and an anode portion 30c, with only the anode portion 30c being of importance for the (rapid) charging process. From the relationships it can be concluded that an advantageous charging current range is always ensured by the voltage regulation of the cell voltage to the voltage trajectory.

To determine the achievable amounts of charge as a function of the current, the battery cell 6 was charged with a current plus (section 24) until the overvoltage of the battery cell 6 and the voltage trajectory intersect. The resulting change in state of charge due to the current pulse is in the 6 labeled ΔSOC.

A comparison of the determined maximum charging current pulses with manufacturer information from the manufacturer of the battery cell 6 and the theoretical value according to Grimsmann is in FIG 7 shown. the 7 shows a diagram in which the change in the state of charge ΔSOC is plotted horizontally, ie along the abscissa axis (X axis), and the C rate CR or the charging current IL is plotted along the vertical ordinate axis (Y axis). The curves 32, 34 show the courses for two simulated charging current pulses in the diagram. The curve 32 shows the course for a state of charge of 12% and the curve 34 shows the course for a state of charge of 60%. the 7 also shows a line 36, which corresponds to the theoretical value according to Grimsmann, and two curves 38, 40. Curve 38 corresponds to the curve according to the manufacturer's information for a state of charge of 12% and curve 40 corresponds to the curve according to the manufacturer's information for a state of charge of 60% . Based on 7 it becomes clear that the simulated charging current pulses are almost completely below the manufacturer's specifications and the theoretical value according to Grimsmann.

The claimed invention is not limited to the embodiments described above. Rather, other variants of the invention can also be derived from this by the person skilled in the art within the scope of the disclosed claims without departing from the subject matter of the claimed invention. In particular, all of the 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.

Reference List

2

motor vehicle

4

battery system

6

battery cell

8th

tension meter

10

voltage regulator

12

controllers

14

reference curve

16, 18, 20, 22

Curve

24, 26

section

28, 30

Curve

30a

separator/electrolyte content

30b

cathode portion

30c

anode fraction

32, 34

Curve

36

line

38, 40

Curve

u

tension

IP

cell voltage

I

electricity

IL

charging current

SOC

state of charge

CR

C rate

M

margin of safety

Claims (10)

Method for voltage-controlled operation of a battery system (4) during a charging process, having at least one electrochemical battery cell (6), a process damaging the battery cell (6) being characterized using at least one stored reference curve (14), the reference curve (14) indicating a state of charge -cell voltage characteristic is, - at which a cell voltage (UZ) of the battery cell (6) is detected, - In which a state of charge (SOC) of the battery cell (6) is determined, - in which an aging condition of the battery cell (6) is determined, - in which the cell voltage (UZ) and the state of charge (SOC) are compared with the stored reference curve (14), - In which a maximum charging current (IL) for the aging state of the battery cell (6) is determined on the basis of the comparison, and - At which the maximum charging current (IL) is set by controlling and/or regulating the cell voltage (UZ). procedure after claim 1 , characterized in that the aging condition of the battery cell (6) is determined regularly. procedure after claim 1 or 2 , characterized in that different reference curves (14) are stored for a charging process and for a discharging process of the battery cell (6). Procedure according to one of Claims 1 until 3 , characterized in that the at least one reference curve (14) is pre-characterized with a laboratory cell. Procedure according to one of Claims 1 until 4 , characterized in that the at least one reference curve (14) is determined by measuring Coulomb's efficiency. Procedure according to one of Claims 1 until 4 , characterized in that the at least one reference curve (14) is determined by measuring a cell expansion and/or a change in cell geometry. Procedure according to one of Claims 1 until 3 , characterized in that the at least one reference curve (14) is determined by means of a simulation. Procedure according to one of Claims 1 until 7 , characterized in that the at least one reference curve (14) is provided with a safety factor before it is stored. Procedure according to one of Claims 1 until 8th , characterized in that the method is interrupted and/or paused at a predetermined state of charge (SOC), a predefined charging current (IL) and/or a reference curve (14) being set during the pause. Battery system (4), having at least one battery cell (6), a voltmeter (8) and a voltage regulator (10) being coupled to the battery cell (6), and a controller (12) for carrying out a method according to one of Claims 1 until 9 .

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