Energy storage for a motor vehicle

Abstract

An energy storage device (10) for a motor vehicle comprises a plurality of electrically interconnected electrochemical cells (12), wherein the energy storage device (10) has several logic cells (16) consisting of at least two electrochemical cells (12) connected in parallel via parallel connectors (14). The logic cells (16) are connected in series via series connectors (22) such that at least a first string (24) and a second string (26) are formed from electrochemical cells (12) connected in series. The parallel connector (14) is a passive resistive element with an electrical resistance in the range of 1 to 1000 Ω.

Description

[0001] The invention relates to an energy storage device for a motor vehicle comprising a plurality of electrically interconnected electrochemical cells and to the use of such an energy storage device in a motor vehicle.

[0002] In order to provide the desired performance, energy storage systems for motor vehicles comprise a large number of electrochemical cells that are electrically connected in series and / or parallel.

[0003] Serial and parallel connectors are used to provide the electrical connections. Parallel connectors connect electrochemical cells that have essentially the same voltage level. These parallel connectors connect them to form so-called "logical cells." The voltage of the logical cells can be monitored by the energy storage system's battery management system.

[0004] In addition, equalizing currents can flow between the electrochemical cells of the respective logic cell via the parallel connector to equalize the voltage levels of the individual electrochemical cells. This process is also referred to as "cell balancing" and is to be distinguished from the balancing of serially connected logic cells, which are connected in parallel on the voltage side via switches such as MOSFETs (active balancing) or "short-circuited" via a balancing resistor and a switch (passive balancing).

[0005] The serial connectors, in turn, connect the logic cells in series, i.e. they electrically connect the logic cells in series, resulting in strings of serially connected electrochemical cells.

[0006] Thus, serial and parallel current paths can be distinguished within the energy storage device.

[0007] It is known that parallel connectors should be designed to be as robust as possible so that, in the event of a short circuit in an electrochemical cell, they have a current-carrying capacity sufficient to trigger a fuse arranged in the serial path. Such fuses trigger at a current in the range of several hundred amperes, for example, approximately 200 to 400 A, requiring correspondingly thick parallel connectors that can carry such currents. This leads to the parallel connectors being oversized with regard to the currents to be handled for cell balancing during fault-free operation of the energy storage system, which only have to compensate for voltage differences of a few millivolts. This has a detrimental effect on the installation space available for other components of the energy storage system and limits the achievable specific energy density of the energy storage system.In addition, high-intensity currents generate thermal energy in the parallel connector, which can be sufficient to heat other electrochemical cells beyond their permissible maximum temperature, causing them to also suffer a short circuit. This effect must be counteracted, for example, with complex cooling devices.

[0008] US 2014 / 0354232 A1 discloses a rechargeable battery in which several modules, each consisting of several rechargeable batteries connected in parallel, are used. The parallel connection is established via current limiters, which limit the current flowing through the parallel connection to a saturation current when a threshold voltage is exceeded. To achieve this behavior, the current limiters contain switching components, such as transistors. However, due to the large number of current limiters required, such a solution is expensive. Furthermore, due to the unavoidable manufacturing tolerances in the production of such current limiters, it is to be expected that an increasing number of current limiters will fail over the service life of the energy storage device, even though a short circuit cannot be handled. This limits the performance and service life of the energy storage device.

[0009] The object of the invention is to provide an energy storage device that is cost-effective and maintenance-free and at the same time enables reliable handling of short circuits in electrochemical cells.

[0010] The object is achieved by an energy storage device for a motor vehicle, comprising a plurality of electrically interconnected electrochemical cells, wherein the energy storage device has a plurality of logic cells composed of at least two electrochemical cells connected in parallel via parallel connectors. The logic cells are connected in series via serial connectors, so that at least a first strand and a second strand are formed from serially connected electrochemical cells. The parallel connector is a passive resistive element with an electrical resistance in the range of 1 to 1000 Ω, in particular in the range of 1 to 100 Ω.

[0011] The term “passive” in this context means that the parallel connector is not a switching component and / or a component that breaks the electrical connection between the electrochemical cells connected in parallel.

[0012] The invention is based on the fundamental idea of ​​using parallel connectors whose resistance is designed such that, in the event of a short circuit in one of the electrochemical cells, a current can flow through the parallel connector(s) in the logic cell containing the short-circuited electrochemical cell. The current can flow below a specified threshold value, thus enabling a controlled discharge of the electrochemical cells of the affected logic cell within a specified period of time. In this way, the temperature increase within the energy storage device that occurs in the event of a short circuit is limited, allowing the heat generated by equalization processes within the energy storage device to be dissipated without having to fear harmful temperature effects on other electrochemical cells of the energy storage device.Specifically, the temperature rise in the electrochemical cells connected in parallel to the short-circuited electrochemical cell via the parallel connector is limited by limiting the current and preventing excessively rapid deep discharge, which would be associated with a significant loss of power. This effectively prevents thermal propagation within the energy storage system. This effect is facilitated by the fact that the current of compensating currents contributes quadratically to the thermal load, meaning that reducing the maximum current significantly limits the resulting thermal load.

[0013] In particular, each of the logic cells is free of electrical fuses. In other words, no components are required to interrupt the electrical connection between the electrochemical cells of the logic cell affected by the short circuit. This reduces the complexity of the energy storage device and minimizes its manufacturing costs. Furthermore, the service life of the energy storage device is further increased, as unexpected functional failures of such components cannot occur.

[0014] Alternatively or additionally, the serial connectors can be free of electrical fuses. In other words, all strings of electrochemical cells in the energy storage system can be free of electrical fuses. This reduces power losses and further simplifies the design of the energy storage system. Because the maximum current flowing through the parallel connector(s) in the event of a short circuit is already limited by the design of the parallel connector, the maximum current cannot be so large that the current flow would have to be interrupted by a fuse in the serial path to prevent excessive heating of the energy storage system components.

[0015] The design of the parallel connector can be achieved using various measures as long as the desired electrical resistance of the parallel connector is achieved and the parallel connector can act as a passive resistive element.

[0016] For example, the parallel connector can be adjusted to the desired behavior by selecting the materials used in the parallel connector, their melting point, the thermal conductivity, the cross-section of the parallel connector or a sub-combination of these parameters.

[0017] In this context, it should be noted that adjusting a single parameter of the parallel connector may not be sufficient to produce the desired behavior compared to conventional parallel connectors. For example, simply reducing the cross-section of known metal parallel connectors, which are designed for a current-carrying capacity of several hundred amperes, does not necessarily result in suitable parallel connectors, since in this case the parallel connector would merely act as a fuse and could completely break the electrical connection between the connected electrochemical cells. In this case, the behavior according to the invention can be produced by adjusting another parameter of the parallel connector, for example, by using a material with a higher melting point, higher thermal conductivity, and / or higher thermal capacity.

[0018] To further prevent the electrical connection within the logic cell from being severed by a melting of the parallel connector at a time when the basic functionality of the energy storage device is still beyond question, the melting point of the parallel connector can be equal to or higher than the melting point of the serial connector. This ensures that the parallel connector does not melt before the serial connector also melts, thus preventing a scenario in which the energy storage device must be electrically interrupted in any case.

[0019] In particular, the parallel connector is a passive resistive element with an electrical resistance in the range of greater than 1 to 10 Ω. Such an electrical resistance corresponds to a medium resistance range that neither corresponds to an essentially unlimited current flow nor to an essentially completely or almost completely interrupted electrical connection. The current flow still possible at this electrical resistance is associated with an increase in the temperature of the energy storage device, which can be managed by heat transport through the energy storage device components without fear of propagating a thermal event.

[0020] The electrical resistance of the parallel connector is in particular selected such that a controlled discharge of a logic cell affected by a short circuit takes place within a predetermined period of time.

[0021] The specified time period depends in particular on the expected thermal load caused by the current flowing through the parallel connector and the speed at which the heat generated can be dissipated.

[0022] For example, the specified time period is greater than 60 seconds. Preferably, the specified time period is between 10 and 60 minutes.

[0023] The energy storage device may have a voltage measuring device configured to measure the voltage of each of the logic cells.

[0024] The voltage measuring device may have a plurality of voltage measuring units, wherein each voltage measuring unit is assigned to one of the logic cells of the energy storage device and is configured to determine the voltage of the assigned logic cell.

[0025] In particular, the voltage measuring device is configured to measure the voltage of a logic cell affected by a short circuit in an electrochemical cell. Because the parallel connector has an electrical resistance in the range of 1 to 1000 Ω, in particular from 1 to 100 Ω or greater than 1 to 10 Ω, and thus an electrical current can flow even in the event of a short circuit, the voltage measuring device can measure the voltage of the logic cell affected by the short circuit even in the event of a short circuit. This has the advantage that the voltage curve of the logic cell can be reliably monitored even after a short circuit has occurred, so that further measures can be taken if necessary to limit the effects of the short circuit on the energy storage device.

[0026] If the voltage measuring device is configured to measure the electrochemical cell affected by the short circuit, a voltage drop can be detected essentially immediately by the voltage measuring device in the event of a short circuit. However, if the voltage measuring device is configured only to measure the voltage of an electrochemical cell of the logic cell that is not affected by the short circuit, the voltage drop associated with the short circuit may, under certain circumstances, only be measurable after a time delay. This delay in the measurement signal can be accounted for using an algorithm, for example, by means of a filter and / or software function executed by a control unit of a battery management system, wherein the control unit is configured to access the measurement data collected by the voltage measuring device.

[0027] The voltage measuring device can be part of a battery management system of the energy storage device.

[0028] The object of the invention is further achieved by the use of an energy storage device as described above in a motor vehicle.

[0029] The advantages and properties of the energy storage device according to the invention apply accordingly to the use of the energy storage device according to the invention and vice versa.

[0030] The motor vehicle is in particular a motor vehicle that is at least partially electrically powered, for example a hybrid or electric vehicle.

[0031] Further features and characteristics will become apparent from the following description of exemplary embodiments, which are not intended to be limiting, and from the drawings, which show: - Fig. 1 schematically shows an energy storage device according to the invention, and - Fig. 2 the energy storage Fig. 1 in case of a short circuit.

[0032] Fig. 1 schematically shows an energy storage device 10 according to the invention for a motor vehicle, for example a hybrid or electric vehicle.

[0033] The energy storage device 10 has a plurality of electrically interconnected electrochemical cells 12. The electrochemical cells 12 are configured to store and release electrical energy. In other words, the energy storage device 10 according to the invention is a secondary battery, for example, a lithium-ion accumulator.

[0034] Selected electrochemical cells 12 are connected via parallel connectors 14 (in Fig. 1 by dot-dash lines) are electrically connected to each other to form logic cells 16, i.e. they are electrically connected in parallel to each other.

[0035] In the embodiment in Fig. 1, three electrochemical cells 12 are connected to form a logic cell 16. It is understood that only two or more than three electrochemical cells 12 can form a logic cell 16.

[0036] The capacity of the energy storage device 10 can be influenced in particular by the number of electrochemical cells 12 per logical cell 16, whereby a higher number of electrochemical cells 12 per logical cell 16 results in a higher capacity.

[0037] Within each of the logic cells 16, the associated electrochemical cells 12 exhibit essentially the same voltage level. This means that all electrochemical cells 12 of a logic cell 16 exhibit essentially the same voltage, except for unavoidable fluctuations, which are particularly due to manufacturing and aging-related effects. Such fluctuations are typically in the range of a few millivolts.

[0038] Balancing currents can therefore flow through the parallel connectors 14, which connect the respective electrochemical cells 12, allowing the voltages of the electrochemical cells 12 to be equalized. This process is also referred to as cell balancing and contributes to extending the service life of the energy storage device 10.

[0039] In order to be able to determine the voltage level of each of the logic cells 16, the energy storage device 10 has a voltage measuring device 18 which comprises a plurality of voltage measuring units 20, namely one voltage measuring unit 20 for each of the logic cells 16.

[0040] The voltage measuring device 18 is in particular a component of a battery management system (not shown further) of the energy storage device 10.

[0041] The parallel connectors 14 have an electrical resistance in the range of 1 to 1000 Ω, in particular in the range of greater than 1 to 10 Ω. The electrical resistance, in turn, determines the maximum current intensity that can flow between the electrochemical cells 12 of a logic cell 16.

[0042] As in Fig. 1, the logic cells 16 are electrically connected to one another via serial connectors 22. In other words, the electrochemical cells 12 of the logic cells 16 are connected in series via the serial connectors 22 and form strings of electrochemical cells, wherein the number of strings corresponds to the number of electrochemical cells 12 per logic cell 16.

[0043] Accordingly, in the embodiment shown, the energy storage device 10 has a first string 24, a second string 26 and a third string 28 of serially connected electrochemical cells 12.

[0044] By connecting the logic cells 16 in series, the voltage provided by the energy storage device 10 via electrical connections 30 and 32 is set to a desired value, for example to a value of 800 V or more.

[0045] The number of electrochemical cells 12 per string 24 to 26 depends on the desired voltage of the energy storage device 10, as indicated by a dotted connection in Fig. 1. Accordingly, it is understood that the energy storage device 10 may have more or fewer electrochemical cells 12 and logic cells 16 than in Fig. 1 shown.

[0046] The logic cells 16 are free of electrical fuses. In other words, the parallel connectors 14 represent a resistive element that specifically limits the current flow. However, no fuse is provided that would break the electrical connection between the electrochemical cells 12 of a logic cell 16 at a predetermined current level.

[0047] Likewise, the serial connectors 22, and overall the strings 24 to 26 of serially connected electrochemical cells 12, are free of electrical fuses.

[0048] Thus, neither the parallel connectors 14 need be designed to have a current carrying capacity that allows current to flow to blow a fuse in the serial connector 22 or one of the strands 24, 26 or 28, nor do the serial connectors 22 need to be designed to have a current carrying capacity that allows current to flow to blow a fuse in the parallel connector 14.

[0049] By omitting electrical fuses and switching components for establishing the parallel and serial current paths, the power loss of the energy storage device 10 is reduced and the reliability of the energy storage device 10 is increased because the risk of component-related failures is minimized.

[0050] The following describes the operation of the energy storage device 10 with reference to Fig. 2 described in more detail.

[0051] Fig. 2 shows a state of the energy storage device 10 in which one of the electrochemical cells 12 has suffered a short circuit, as indicated by a schematic lightning bolt, more precisely one of the electrochemical cells 12 of the second logic cell 16, counted from the electrical terminal 30 towards the electrical terminal 32.

[0052] A compensating current flows via the parallel connector 14 assigned to the short-circuited logic cell 10 to the other electrochemical cells 12 of the affected logic cell 10, as indicated by arrows in Fig. 2 is indicated.

[0053] Because there are no electrical fuses or switching components in the parallel path, i.e., within the affected logic cell 16, the compensating current is not prevented or limited by the blown fuse or a switching component. This makes it possible to perform a voltage measurement via the voltage measuring device 18, more precisely the voltage measuring unit 20, which is assigned to the logic cell 16 affected by the short circuit, and thus obtain precise information about the behavior of the energy storage device 10 and the flowing compensating currents.

[0054] The compensating current is limited by the selected electrical resistance of the parallel connectors 14 used. This limits the flowing compensating current to a current intensity below a predetermined threshold, which is determined by the fact that the discharge of the electrochemical cells 12 of the logic cell 16 affected by the short circuit takes place within a predetermined period of time. In this context, the predetermined period of time is sufficiently long to prevent other electrochemical cells 12 of the energy storage device 10 from heating to a temperature above a permissible operating temperature.

[0055] For example, the specified time period is greater than 60 seconds; in particular, the specified time period is 10 to 30 minutes. Such periods during discharging of the electrochemical cells 12 ensure that the heat flow occurring during discharging of the electrochemical cells 12 remains sufficiently low that the heat generated by the compensating current can be dissipated via the components of the energy storage device 10 into the environment of the energy storage device 10 quickly enough to prevent local overheating. Thermal propagation is thus particularly effectively prevented.

[0056] The other components of the energy storage device 10 thus represent a thermal mass that is sufficiently large to be able to safely handle the heating that still occurs.

[0057] The parallel connectors 14 are also designed such that they have a melting point which is in particular equal to or higher than the melting point of the serial connectors 22.

[0058] According to the invention, compensating currents with a current intensity of several hundred amperes, which would be necessary to trigger fuses in the strands 24 to 28, can be dispensed with, so that the associated rapid temperature increases can be avoided.

[0059] The energy storage device 10 according to the invention is thus characterized by a particularly simple and low-maintenance design as well as high reliability. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] US 2014 / 0354232 A1

[0008]

Claims

[1] Energy storage device (10) for a motor vehicle, comprising a plurality of electrically interconnected electrochemical cells (12), wherein the energy storage device (10) comprises a plurality of logic cells (16) comprising at least two electrochemical cells (12) connected in parallel via parallel connectors (14), wherein the logic cells (16) are connected in series via serial connectors (22) so that at least a first strand (24) and a second strand (26) are formed from serially connected electrochemical cells (12), and wherein the parallel connector (14) is a passive resistive element having an electrical resistance in the range of 1 to 1000 Ω. [2] Energy storage device (10) according to claim 1, wherein the serial connectors (22) are free of electrical fuses. [3] Energy storage device (10) according to claim 1 or 2, wherein each of the logic cells (16) is free of electrical fuses. [4] Energy storage device (10) according to one of the preceding claims, wherein the melting point of the parallel connector (14) is equal to or higher than the melting point of the serial connector (22). [5] Energy storage device (10) according to one of the preceding claims, wherein the parallel connector (14) is a passive resistive element with an electrical resistance in the range of greater than 1 to 10 Ω. [6] Energy storage device (10) according to one of the preceding claims, wherein the electrical resistance of the parallel connector (14) is selected such that a controlled discharge of a logic cell (10) affected by a short circuit takes place within a predetermined period of time. [7] Energy storage device (10) according to claim 6, wherein the predetermined time period is greater than 60 seconds, preferably 10 to 60 minutes. [8] Energy storage device (10) according to one of the preceding claims, wherein the energy storage device (10) has a voltage measuring device (18) which is arranged to measure the voltage of each of the logic cells (16). [9] Energy storage device (10) according to claim 8, wherein the voltage measuring device (18) is configured to measure the voltage of a logic cell (16) affected by a short circuit of an electrochemical cell (12). [10] Use of an energy storage device (10) according to one of the preceding claims in a motor vehicle.

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