Method and device for measuring a battery cell current

Abstract

Method for measuring a battery cell current (I B ) a battery cell unit (20), comprising the steps: - discharging an inductively stored energy of the battery cell unit (20) after switching off the battery cell unit (20) by means of a discharge unit (10), wherein the inductively stored energy of the battery cell unit (20) is energy stored by an inductive behavior of the battery cell unit (20), - Determine a time period (t M ), in which the inductively stored energy of the battery cell unit (20) drops to a given threshold value, and - Determining a battery cell current (I B ) based on the determined time period (t M ).

Description

The present invention relates to a method and an apparatus for measuring a battery cell current.Current battery systems are constructed from a plurality of battery cell control units each having a battery cell, as a result of which individual control of the individual battery cells connected to the battery cell control unit is made possible. The battery cells are connected to each other in a series connection via the battery cell control units. Furthermore, a central control unit is provided for controlling the battery system.To generate a regulated total output voltage of the battery system, individual battery cells are introduced into the series circuit with the aid of the battery cell control units either in positive or negative polarity, relative to the pick-off of the total output voltage (state "positively connected" or "negatively connected"), or are switched off, i.e. the battery cells are separated from the series circuit and the connection terminals of the associated battery cell control unit are electrically conductively connected (state "bridged").In addition, the control of the battery system takes place in such a way that battery cells are activated as a function of their state of charge and further state variables (e.g. SOH-State of Health), as a result of which active balancing of the battery cells is made possible.The SOH can be determined on the basis of the instantaneous cell voltage and the energy already drawn, which is possible with the aid of a current sensor. Since in such a battery system each battery cell can be controlled independently of the other battery cells, each battery cell requires a possibility of determining the charging / discharging current with which it is currently being charged / discharged. The current measurement can be carried out, for example, by a shunt resistance measurement or a Hall sensor on each of the battery cell control units.A battery system or a battery 60 according to the prior art is shown in FIG. 6. The battery system 60 includes a plurality of battery cell control units 61 (also called smart cell units) connected in series. Each battery cell control unit 61 comprises a battery cell 64, and the battery cell control units 61 are connected to a central regulator 63 via a unidirectional communication interface 62. The battery cells 64 of the battery cell control units 61 can be individually inserted into a series circuit of all battery cells 54 or bridged therein via the unidirectional communication interface 62 by a control signal of the central controller 63.The document DE 10 2011 113 798 A1 discloses a battery, the components of which do not have elevated and thus potentially dangerous voltages. The required high voltages and currents are provided only when a proper operating state is given. In the idle or maintenance state and in the event of faults or accidents, potentially dangerous voltages or currents are not present either at the battery output or within the battery.The document DE 10 2012 206 622 A1 discloses the efficient and flexible charging of modular energy storage devices. A central feature is the use of already existing components, in particular the parasitic inductances of the energy storage cells and / or an additional coupling inductance, in order to realize a boost converter function.The document DE 10 2012 203 309 A1 discloses an energy storage device having at least two energy storage modules that can be connected in parallel. Each module includes at least one energy storage cell, a supply switching device, and a bypass switching device connected in parallel. The parasitic inductances of the cells and the switching elements together operate as synchronous converters. This allows the modules to be operated in parallel, which reduces the output voltage and simplifies the design of the downstream components.Document US 2012 / 0 105 078 A1 discloses a method and a system for determining at least one operating characteristic curve of an inductance in a power converter system.The document US 2005 / 0 275 382 A1 discloses a circuit for measuring the charge consumption.Disclosure of the InventionThe method according to the invention for measuring a battery cell current through a battery cell unit comprises the steps of discharging an inductively stored energy of the battery cell unit after the battery cell unit has been switched off by means of a discharge unit, wherein the inductively stored energy of the battery cell unit is an energy stored by an inductive behavior of the battery cell unit, determining a time duration in which the inductively stored energy has dropped to a given threshold value, and determining a battery cell current on the basis of the determined time duration.The device according to the invention for measuring a battery cell current through a battery cell unit comprises a discharge unit which discharges an inductively stored energy of the battery cell unit after the battery cell unit has been switched off, wherein the inductively stored energy of the battery cell unit is an energy stored by an inductive behavior of the battery cell unit, a measurement unit which determines a time duration in which the inductively stored energy of the battery cell unit has dropped to a given threshold value, and an evaluation unit which determines a battery cell current on the basis of the determined time duration.In this way, a cost-effective possibility for determining a current which has flowed through a battery cell unit immediately before the switching-off is created. In addition, the discharging prevents voltage peaks during the switching on of the battery cell, which are caused by the inductive behavior of the battery cell unit. Since the battery current takes place when the battery cell is switched off, external disturbing influences on the measurement are additionally minimized. In particular, already existing power electronics components of a conventional battery cell control unit can be used. Furthermore, no additional resistance is required in the power path, thereby minimizing power losses. In particular, a cost-effective alternative is provided compared to a measurement by means of a Hall sensor.The dependent claims show preferred developments of the invention.It is advantageous if the battery cell unit is switched off by the discharge unit. By such multiple use of the discharge unit, the number of necessary components of a battery cell control unit can be reduced, whereby a cost advantage is produced.Furthermore, it is advantageous if the inductively stored energy of the battery cell unit is detected by means of a measurement voltage caused by the battery cell unit after the switching off. This enables measurement of the inductively stored energy of the battery cell unit with little effort. In particular, means for measuring a measurement voltage are already provided in many batteries, as a result of which no additional means for detecting the stored energy of the battery cell unit are required and a cost advantage is thus produced.It is likewise advantageous if the discharge of the stored energy of the battery cell unit and / or the deactivation of the battery cell unit takes place via a transistor, in particular a MOSFET, of the discharge unit. As a result of the fast switching times of a transistor, a particularly precise measurement can thus be carried out. In addition, other inductive voltage peaks, as can occur in other switches, are largely avoided. A MOSFET is particularly advantageous since such a MOSFET is optimized for the conduction and blocking of large electrical currents and voltages which occur in a series connection of a plurality of battery cells.Advantageously, the inductively stored energy of the battery cell unit is discharged via a parasitic diode of a MOSFET, which goes into breakdown on account of a voltage caused by the inductively stored energy of the battery cell unit. Thus, the number of necessary devices can be further reduced, resulting in higher reliability and lower cost. In addition, it is ensured in a simple manner that the discharge unit is active only during a discharge phase.In particular, the inductively stored energy of the battery cell unit is used for switching the transistor in order to discharge the inductively stored energy of the battery cell unit via the switched transistor. Thus, an additional switching voltage and means for providing this additional switching voltage can be dispensed with.In addition, it is advantageous if the discharging of the inductively stored energy of the battery cell unit is triggered by means of a diode, in particular by means of a Zener diode, which goes into breakdown on account of a voltage caused by the inductively stored energy of the battery cell unit and thus provides a voltage for switching the transistor. It is thus ensured in a simple manner that the discharge unit is active only during a discharge phase.Furthermore, a battery cell control unit is advantageous, which comprises the device for measuring the battery cell current and the battery cell unit. This is advantageous since a cost-effective battery cell control unit having the aforementioned advantages is thus provided, which can be used in current battery systems.A battery is likewise advantageous which comprises at least one battery cell control unit according to the invention. A cost-effective battery having the aforementioned advantages is thus provided.Brief Description of the DrawingsHereinafter, embodiments of the invention will be described in detail with reference to the accompanying drawings. In the drawing, the following is: FIG. 1 shows a flow diagram of a preferred method according to the invention for measuring a battery cell current, FIG. 2 shows a schematic illustration of a battery cell control unit having a device for measuring a battery cell current in a first preferred embodiment according to the invention, FIG. 3 is a diagram showing voltages and currents at a discharge unit according to the first preferred embodiment of the present invention, FIG. 4 shows a schematic illustration of a battery cell control unit having a device for measuring a battery cell current in a second preferred embodiment according to the invention, FIG. 5 is a diagram showing voltages and currents at a discharge unit according to the second preferred embodiment of the present invention; and FIG. 6 shows a schematic illustration of a battery according to the prior art having a plurality of battery cell control units.Embodiments of the InventionFIG. 1 shows a flow chart of a preferred method according to the invention for measuring a battery cell current I B. The method comprises a first step S 1, a second step S 2 and a third step S 3. The method is advantageously triggered by a signal for switching off a battery cell unit 20, through which the battery cell current I B to be measured flows. Such a signal can be output, for example, by a central controller of a battery. If the method has been initiated, first the first step S 1, then the second step S 2 and, last, the third step S 3 are carried out. After execution of the third step S 3, the method is ended.In the first step S 1, an inductively stored energy of the battery cell unit 20 is discharged after the battery cell unit 20 has been switched off by means of a discharge unit 10. A battery cell unit 20 is a unit including at least one battery cell 23. The battery cells 23 of a battery cell unit 20 can be connected in series and / or in parallel with one another. Each of the battery cells 23 has inductive behavior when the battery cell 23 is turned off. Switched off means here that the battery cell 23 is disconnected from a load. The inductive behavior of the battery cell 23 may be caused, for example, by the physical structure of the battery cell 23. Thus, a coil-like behavior of the battery cell 23 can be caused, for example, by a winding of the electrodes of the battery cell 23. The inductive behavior of the individual battery cells 23 is the reason for the inductive behavior of the battery cell unit 20. Similar to a coil, an energy is thus stored by the battery cell unit 20. The energy of the battery cell unit 20 inductively stored by the inductive behavior of the battery cell unit 20 is therefore an energy which is additionally temporarily stored in the battery cell unit 20 in addition to the electrochemically stored energy of the battery cell unit 20. According to the invention, the inductively stored energy of the battery cell unit 20 is completely or partially discharged by a discharge unit 10 after the battery cell unit 20 has been switched off. For this purpose, for example, an electrically conductive connection of a terminal pole 21, 22 of the battery cell unit to a ground point is produced or a connection of a terminal pole 21, 22 of the battery cell unit to a point is produced whose electrical potential is lower than that of the battery cell unit 20 after the disconnection. This electrically conductive connection preferably has a resistor by which a speed of the discharge can be influenced. This electrically conductive connection is preferably disconnected as soon as the inductively stored energy of the battery cell unit 20 is discharged, in order to avoid the electrochemically stored energy of the battery cell unit 20 being discharged via the discharge unit 10.In the second step S 2, a time duration t M, in which the inductively stored energy of the battery cell unit 20 drops to a given threshold value, is ascertained. In this case, the inductively stored energy of the battery cell unit 20 can be measured directly and the measured value can be compared with a threshold value. Likewise, a parameter can be measured which is influenced by the inductively stored energy of the battery cell unit 20. For example, after the battery cell unit 20 has been switched off, a voltage could be measured via the discharge unit 10 and / or the battery cell unit 20 and compared with a threshold value, which in this case is a voltage value S. Likewise, a time interval between the time of the shutdown and an event that occurs when the inductively stored energy of the battery cell unit 20 has reached the threshold value can also be measured as the time duration t M. Such an event could be, for example, termination of the discharge by the discharge unit 10 or switching of a transistor.In the third step S 3, a battery cell current I B is determined on the basis of the ascertained time duration t M. Since the initial inductively stored energy of the battery cell unit 20 is dependent on the battery cell current I B that has flowed through the battery cell unit 20 before being turned off, the determined time period t M increases with the battery cell current I B, since a larger stored energy requires a longer time period to become discharged. Thus, from the determined time duration t M it is possible to infer the battery cell current I B which has flowed through the battery cell unit 20 before the switching off. This can be effected, for example, by a preceding calibration in which a respective battery cell current I B is assigned in the different time periods t M.FIG. 2 shows a schematic illustration of a battery cell control unit 50 having a device for measuring a battery cell current I B in a first preferred embodiment according to the invention. The battery cell control unit 50 shown in FIG. 2 executes the above-described method. The battery cell control unit 50 includes a battery cell unit 20, a discharge unit 10, a measurement unit 30, and an evaluation unit 40.The battery cell control unit 50 includes a first terminal 51, a second terminal 52, and a control contact 53. In this first embodiment, the discharge unit 10 comprises a MOSFET 14. The battery cell unit 20 comprises a first terminal pole 21 and a second terminal pole 22. The measuring unit 30 comprises a first measurement input 31 and a second measurement input 32.The MOSFET 14 has the property that, at a breakdown voltage S which is greater than an open circuit voltage of the battery cell unit 20, an avalanche breakdown takes place between a drain contact 11 and a source contact 12 of the MOSFET 14. This avalanche breakdown is a breakdown of a parasitic diode between the drain contact 11 and the source contact 12 of the MOSFET 14.The first connection contact 51 is electrically conductively connected to the first connection pole 21. The second connection pole 22 is electrically conductively connected to the first measurement input 31 and the drain contact 11 of the MOSFET 14. The second connection contact 52 is electrically conductively connected to the second measurement input 32 and the source contact 12 of the MOSFET 14. A gate contact 13 of the MOSFET 14 is connected via a resistor R to the control input 53. The evaluation unit 40 is coupled to the measuring unit 30 in such a way that at least one signal which describes the time duration t M can be transmitted from the measuring unit 30 to the evaluation unit 40.The battery cell control unit 50 further comprises a switching unit 54, which is embodied here as a mechanical switch, but may likewise be a transistor or another electrical switch. This switching unit 54 is connected between the first terminal 51 and the second terminal 52. The switching unit 54 makes it possible in an open state to connect the battery cell unit 20 in series with other battery cell units of other battery cell control units and in a closed state to bridge the battery cell unit 20 in a series connection with other battery cell units.If a corresponding control voltage U GS is applied to the control input 53, the MOSFET 14 switches on and a current flow is thus made possible between the drain contact 11 and the source contact 12. This state is illustrated in a first time period 100 in the diagram from FIG. 3. Here, the battery cell current I B is represented by a dot-and-dash line, a measurement voltage U DS is represented by a solid line, and the control voltage U GS is represented by a dashed line. When a load is connected to the battery cell control unit 50, a battery cell current I B may flow from the first terminal 51 to the second terminal 52 via the battery cell unit 20 and the discharge unit 10. A measurement voltage U DS between the drain contact 11 and the source contact 12 is measured by the measurement unit 30. In this first embodiment, the measurement voltage U DS between the drain contact 11 and the source contact 12 is "0" volts when the control voltage U GS is applied to the control input 53, since the resistance between the drain contact 11 and the source contact 12 of the MOSFET 14 goes to 0 ohm in this case.In this first preferred embodiment according to the invention, the battery cell unit 20 is switched off by the MOSFET 14 of the discharge unit 10 when the control voltage U GS is no longer present at the control input 53. Simultaneously with the disconnection of the battery cell unit 20, the latter is bridged by closing the switching unit 54 in order not to interrupt a current flow through any further battery cell units 20 connected in series with the battery cell unit 20. This state is illustrated in the diagram from FIG. 3 in a second time period 200. The control voltage U GS is interrupted and falls to "0" volts. In this state, the MOSFET 14 turns off and the current flow between the drain contact 11 and the source contact 12 is interrupted. The lack of or at least high-resistance electrical connection between the drain contact 11 and the source contact 12 results in a measurement voltage U DS greater than "0" volts. Due to the inductive behavior of the battery cell unit 20, the measurement voltage U DS exceeds the open circuit voltage of the battery cell unit 20, and thus the avalanche breakdown of the MOSFET 14 occurs and an electrically conductive connection is produced between the drain contact 11 and the source contact 12. The energy stored inductively in the battery cell unit 20 is discharged via this electrically conductive connection and thus via the MOSFET 14 or via the discharge unit 10, since the battery cell unit 20 is short-circuited via the switching unit 54 and the MOSFET 14. Since the inductively stored energy of the battery cell unit 20 decreases, the measurement voltage U DS between the drain contact 11 and the source contact 12 likewise decreases. If the breakdown voltage S of the MOSFET 14 is reached by the dropping measurement voltage U DS the MOSFET 11 blocks the electrically conductive connection between the drain contact 11 and the source contact 12. The battery cell control unit 50 thus enters the state represented by the third period 300 in FIG. 3. The inductively stored energy of the battery cell unit 20 is therefore discharged by means of the discharge unit 10 after the battery cell unit 20 has been switched off, and the first step S 1 described above is therefore carried out.The measuring unit 30 measures the measuring voltage U DS and thus also the increase of the measuring voltage U DS above the open circuit voltage after switching off and the decrease of the measuring voltage U DS, when the breakdown voltage S is undershot again. The measuring unit 30 is configured to determine the time duration t M between this increase and decrease of the measuring voltage U DS. This determined time duration t M is transmitted to the evaluation unit 40 as an analog or digital value. The time duration t M during which the inductively stored energy of the battery cell unit has dropped to a given threshold value is thus determined by the measuring unit 30. The threshold value is defined by the breakdown voltage S of the MOSFET 14 in this first embodiment. Thus, the above-described second step S 2 is performed by the measurement unit 30.In this first embodiment, the determined time duration t M is converted into a digital value and transmitted from the measuring unit 30 to the evaluation unit 40. In the evaluation unit 40, the determined time duration t M is compared with a predefined table and the determined time duration t M is therefore converted into a value which describes the battery cell current I B. Such a table can be specified at the factory, for example, and can be created in advance by a series of tests with simultaneous direct measurement of the battery current I B. An inductance of the battery cell unit 20 could likewise be determined and the battery cell current I B could be calculated by the evaluation unit 40. The evaluation unit 40 determines the battery cell current I B thus on the basis of the determined time duration t M. Thus, the above-described third step S 3 is performed by the evaluation unit 40.FIG. 4 shows a schematic illustration of a battery cell control unit having a device for measuring a battery cell current in a second preferred embodiment according to the invention. The battery cell control unit 50 shown in FIG. 4 executes the above-described method. The battery cell control unit 50 includes a battery cell unit 20, a discharge unit 10, a measurement unit 30, and an evaluation unit 40.The battery cell control unit 50 includes a first terminal 51, a second terminal 52, and a control contact 53. The discharge unit 10 includes a MOSFET 14, a diode 15, and a Zener diode 16 in this second embodiment. the battery cell unit 20 includes a first terminal 21 and a second terminal 22. a plurality of battery cells 23 are connected in series between the first and second terminal 21, 22. The measuring unit comprises a first measurement input 31 and a second measurement input 32.The Zener diode 16 has the property that breakdown occurs at a Zener voltage S that is greater than an open circuit voltage of the battery cell unit 20.The first connection contact 51 is electrically conductively connected to the first connection pole 21. The second connection pole 22 is electrically conductively connected to the first measurement input 31 and the drain contact 11 of the MOSFET 14. The second connection contact 52 is electrically conductively connected to the second measurement input 32 and the source contact 12 of the MOSFET 14. A gate contact 13 of the MOSFET 14 is electrically conductively connected to the control input 53 via a resistor R. An anode of the diode 15 is electrically conductively connected to the drain contact 11 and a cathode of the diode 15 is electrically conductively connected to a cathode of the zener diode 16. An anode of the zener diode 16 is electrically conductively connected to the gate contact 13. The evaluation unit 40 is coupled to the measuring unit 30 in such a way that at least one signal which describes the time duration t M can be transmitted from the measuring unit 30 to the evaluation unit 40.The battery cell control unit 50 further comprises a switching unit 54, which is embodied here as a mechanical switch, but may likewise be a transistor or another electrical switch. This switching unit 54 is connected between the first terminal 51 and the second terminal 52. The switching unit 54 makes it possible in an open state to connect the battery cell unit 20 in series with other battery cell units of other battery cell control units and in a closed state to bridge the battery cell unit 20 in a series connection with other battery cell units.If a corresponding control voltage U GS is applied to the control input 53, the MOSFET 14 switches on and a current flow is thus made possible between the drain contact 11 and the source contact 12. The diode 15 ensures that no current defined by the resistor R and the control voltage U GS flows away via the first or second connection contact 51, 52. This state is illustrated in a first time period 101 in the diagram from FIG. 5. Here, in FIG. 5, the battery cell current I B is represented by a dot-and-dash line, a measurement voltage U DS is represented by a solid line, and the control voltage U GS is represented by a broken line. When a load is connected to the battery cell control unit 50, a battery cell current I B may flow from the first terminal 51 to the second terminal 52 via the battery cell unit 20 and the discharge unit 10. A measurement voltage U DS between the drain contact 11 and the source contact 12 is measured by the measurement unit 30. In this first embodiment, the measurement voltage U DS is "0" volts when the control voltage U GS is present at the control input 53, since the electrical resistance between the drain contact 11 and the source contact 12 of the MOSFET 14 goes to "0" ohms.In this second embodiment, the battery cell unit 20 is switched off by the MOSFET 14 of the discharge unit 10 when the control voltage U GS is no longer present at the control input 53. Simultaneously with the disconnection of the battery cell unit 20, the latter is bridged by closing the switching unit 54 in order not to interrupt a current flow through any further battery cell units 20 connected in series with the battery cell unit 20. This state is shown in the diagram from FIG. 5 in a second time period 201. The control voltage U GS is interrupted and falls. In this state, the MOSFET 14 turns off and the current flow between the drain contact 11 and the source contact 12 is interrupted. The lack of or at least high-resistance electrical connection between the drain contact 11 and the source contact 12 results in a measurement voltage U DS greater than "0" volts. Due to the inductive behavior of the battery cell unit 20, the measurement voltage U DS exceeds the open circuit voltage of the battery cell unit 20, as a result of which the increased voltage level at the drain contact 11 of the MOSFET 14 exceeds the zener voltage S of the zener diode 16 arranged between these contacts in this state, and a current can flow from the drain contact 11 to the gate contact 13 of the MOSFET 14. Therefore, a reduced control voltage U GS is present at the MOSFET 14, which is fed from the inductively stored energy of the battery cell unit 20. The inductively stored energy of the battery cell unit 20 is thus used for switching the MOSFET 14. The MOSFET is thereby operated in its linear working range and a resistive electrically conductive connection is established between the drain contact 11 and the source contact 12. The energy stored inductively in the battery cell unit 20 is discharged via this electrically conductive connection and thus via the MOSFET 14 or via the discharge unit 10, since the battery cell unit 20 is short-circuited via the switching unit 54 and the MOSFET 14. Since the inductively stored energy of the battery cell unit 20 decreases, the increased voltage level at the drain contact 13 of the MOSFET 14 also decreases. If the Zener voltage S of the Zener diode 16 falls below, the control voltage U GS falls to 0 volt and the MOSFET 14 blocks the electrically conductive connection between the drain contact 11 and the source contact 13. The battery cell control unit 50 enters the state represented by the third period 301 in FIG. 5. Thus, after the battery cell unit 20 has been switched off, the inductively stored energy of the battery cell unit 20 is discharged by means of the discharge unit 10, and the above-described first step S 1 is thus carried out.The measuring unit measures the voltage U DS and thus also the increase of the measuring voltage U DS above the open circuit voltage after the switching off and the decrease of the measuring voltage U DS, when the Zener voltage S is undershot again. The measuring unit 30 is configured to determine the time duration t M between this increase and decrease of the measuring voltage U DS. This determined time duration t M is transmitted to the evaluation unit 40 as an analog or digital value. The time duration t M during which the inductively stored energy of the battery cell unit 20 has dropped to a given threshold value is thus determined by the measuring unit 30. The threshold value is defined by the zener voltage S of the zener diode 16 in this second embodiment. Thus, the above-described second step S 2 is performed by the measurement unit 30.In this second embodiment, the determined time duration t M is converted into a digital value and transmitted from the measuring unit 30 to the evaluation unit 40. In the evaluation unit 40, the determined time duration t M is compared with a predefined table and the determined time duration t M is therefore converted into a value which describes the battery cell current I B. Such a table can be specified at the factory, for example, and can be created in advance by a series of tests with simultaneous direct measurement of the battery current I B. An inductance of the battery cell unit 20 could likewise be determined and the battery cell current I B could be calculated by the evaluation unit 40. The evaluation unit 40 determines the battery cell current I B thus on the basis of the determined time duration t M. Thus, the above-described third step S 3 is performed by the evaluation unit 40.Generally speaking, battery cell units 20 in a battery or a battery system are frequently switched on and off, so that uniform loading of all cells takes place. During each switch-off operation, the energy which is stored in the inductance of the cell by the charging or discharging current has to be dissipated. This can be effected, for example, by utilizing the avalanche operation of a MOSFET 14 or, with the aid of a Zener diode, in the linear operation of the MOSFET 11. In both variants, a source voltage of the MOSFET rises very rapidly after the battery cell current I B has been switched off up to the blocking voltage of the MOSFET or the Zener voltage of the Zener diode until the energy has been dissipated and then falls back to the level of the battery voltage. The time duration t M, which the source contact 12 of the MOSFET 14 remains at the higher voltage level, is proportional to the dissipated energy and thus proportional to the battery current I B. which has previously flowed, given the same blocking voltage and the same inductance. This time duration t M can be easily measured by the microcontroller unit typically present in each battery cell control unit 50.Each battery cell control unit 50 knows the time period with which it was charged or discharged. By means of the method according to the invention, the battery cell control unit 50 likewise identifies the associated battery cell current I B. Thus, it is possible for the battery cell control unit 50 to calculate which energy has already released it. Considering the open circuit voltage of the battery cell unit 20 in direct comparison with the already discharged energy, the battery cell control unit 50 is allowed to determine the state of health (SOH) of the battery cell unit 20. Suitable algorithms can thus contribute to the fact that an already very heavily aged battery cell unit 20 is saved. The maximum service life of the battery or of the battery system is thus improved.In addition to the above written disclosure, explicit reference is made to the disclosure of FIGS. 1 to 6.

Claims

Method for measuring a battery cell current (I B) of a battery cell unit (20), comprising the steps of: - discharging an inductively stored energy of the battery cell unit (20) by means of a discharge unit (10) after the battery cell unit (20) has been switched off, wherein the inductively stored energy of the battery cell unit (20) is an energy stored by an inductive behavior of the battery cell unit (20), - determining a time duration (t M), in which the inductively stored energy of the battery cell unit (20) falls to a given threshold value, and - determining a battery cell current (I B) on the basis of the determined time duration (t M).Method according to Claim 1, characterized in that the battery cell unit (20) is switched off by the discharge unit (10).Method according to one of the preceding claims, characterized in that the inductively stored energy of the battery cell unit (20) is detected on the basis of a measurement voltage (U DS) caused by the battery cell unit (20) after the battery cell unit (20) has been switched off.Method according to one of the preceding claims, characterized in that the discharge of the inductively stored energy of the battery cell unit (20) and / or the disconnection of the battery cell unit (20) takes place via a transistor, in particular a MOSFET (14), of the discharge unit (10).Method according to Claim 4, characterized in that the inductively stored energy of the battery cell unit (20) is discharged via a parasitic diode of a MOSFET (14), which goes into breakdown on account of a voltage caused by the inductively stored energy of the battery cell unit (20).Method according to Claim 4, characterized in that the inductively stored energy of the battery cell unit (20) is used for switching the transistor in order to discharge the inductively stored energy of the battery cell unit (20) via the switched transistor.Method according to Claim 6, characterized in that the discharge of the inductively stored energy of the battery cell unit (20) is triggered by means of a diode, in particular by means of a Zener diode (16), which goes into breakdown on account of a voltage caused by the inductively stored energy of the battery cell unit (20), and thus provides a voltage for switching the transistor.Device for measuring a battery cell current (I B) of a battery cell unit (20), comprising: - a discharge unit (10) which discharges an inductively stored energy of the battery cell unit (20) after the battery cell unit (20) has been switched off, wherein the inductively stored energy of the battery cell unit (20) is an energy stored by an inductive behavior of the battery cell unit (20), - a measurement unit (30) which determines a time duration (t M) in which the inductively stored energy of the battery cell unit (20) has dropped to a given threshold value (S), and - an evaluation unit (40) which determines a battery cell current (I B) on the basis of the determined time duration (t M).Battery cell control unit (50), characterized in that the battery cell control unit (50) comprises a device for measuring a battery cell current (I B) according to claim 8 and the battery cell unit (20).A battery, characterized in that the battery comprises at least one battery cell control unit (40) according to claim 9.

Images