Switchable storage system and method for reducing a charge or voltage difference between at least two battery modules of such a switchable storage system

Abstract

Switchable storage system (1) for providing electrical power for propulsion of a motor vehicle comprising at least two battery modules (2a, 2b) of the same nominal voltage, namely at least a first battery module (2a) and a second battery module (2b), a switching unit (3), wherein the switching unit (3) is configured to connect the first battery module (2a) and the second battery module (2b) in parallel for a charging process and to connect them in series for propulsion of the motor vehicle, a control unit (7) and a power electronics unit (4), wherein the power electronics unit (4) is configured toto reduce a difference in the state of charge of the first battery module (2a) and in the state of charge of the second battery module (2b) in preparation for a parallel connection of the first battery module (2a) and the second battery module (2b), and a method for reducing a charge difference between at least two battery modules of such a switchable storage system.

Description

Field of invention

[0001] The present invention relates to a switchable storage system for providing electrical power for the propulsion of a motor vehicle, comprising at least two battery modules of the same nominal voltage, namely at least a first battery module and a second battery module, a switching unit, wherein the switching unit is configured to connect the first battery module and the second battery module in parallel for a charging process and to connect them in series for the propulsion of the motor vehicle, and a power electronics unit, wherein the power electronics unit is configured toto reduce a difference in a charge state or a voltage state of the first battery module and in a charge state or a voltage state of the second battery module in preparation for a parallel connection of the first battery module and the second battery module, and a method for reducing a charge or voltage difference between at least two battery modules of a switchable storage system. State of the art

[0002] Hybrid and electric vehicles currently use an energy storage system as an energy source, which typically consists of several individual battery or storage cells. These battery or storage cells are usually lithium-ion cells connected in series or in a combination of series and parallel circuits.

[0003] Charging such an energy storage system is typically done by connecting it to an external charging station that is connected to a power grid. The available connection power (charging power) for charging the energy storage system depends on the charging station.

[0004] Electric vehicles with an energy storage system, i.e., a battery system, operating at a voltage higher than 500 V (DC) require a system to boost the voltage supplied by the charging station to a higher level or to adjust the battery system voltage accordingly when charging at lower voltage charging stations. One possible solution is to divide the battery into two or more strings, namely two or more battery modules, with the same nominal voltage. Depending on the requirements, the individual battery modules can be connected in parallel during charging and in series for driving, or vice versa, via a dedicated control unit.

[0005] At least two battery modules can exhibit a charge difference at the time of switching from series to parallel operation due to a variety of factors. This charge difference leads to a voltage difference between the battery modules in parallel operation. As a result, high currents and potentially the formation of high-energy arcs can occur during the switchover. To control these arcs, additional design measures, such as gas-filled switching chambers or magnets for cooling and arc quenching, must be implemented. Summary of the invention

[0006] It is an object of the invention to provide an improved switchable storage system for supplying electrical power for the propulsion of a motor vehicle, which is characterized in particular by a safe switching between driving operation and charging operation of the motor vehicle.

[0007] Furthermore, it is an object of the invention to provide a method that reduces in a simple manner a difference in a charge state or a voltage state of at least two battery modules connected in series in preparation for a parallel connection of the at least two battery modules.

[0008] This need can be met by the subject matter of the present invention according to independent claims 1 and 4. Advantageous embodiments of the present invention are described in the dependent claims.

[0009] A switchable storage system according to the invention serves to provide electrical power for the propulsion of a motor vehicle.

[0010] The switchable storage system according to the invention comprises at least two battery modules, namely at least a first battery module and a second battery module, a switching unit, a control unit and a power electronics unit.

[0011] According to the invention, the battery modules have the same nominal voltage.

[0012] According to the invention, the switching unit is designed to connect the first battery module and the second battery module in parallel for a charging process and to connect them in series for the propulsion of the motor vehicle.

[0013] According to the invention, the control unit is configured to determine a charge or voltage state of the first battery module and a charge or voltage state of the second battery module, to detect a difference in the charge or voltage states of the two battery modules, and to calculate a necessary current pulse based on this difference.

[0014] The charge or voltage state of the battery modules can be determined, for example, by directly measuring the voltage of the two battery modules or by summing the individual cell charge or cell voltage measurements by the control unit.

[0015] According to the invention, the power electronics unit is configured to reduce the difference in the charge or voltage state of the first battery module and in a charge or voltage state of the second battery module, as determined by the control unit, in preparation for a parallel connection of the first battery module and the second battery module, by applying an opposite current pulse, calculated by the control unit, to the terminals of the first battery module and to the terminals of the second battery module, thus achieving a local voltage equalization between the battery modules.

[0016] An “opposite current pulse at the terminals of the first battery module and at the terminals of the second battery module” is understood to mean a charging current pulse at the terminals of the first battery module and a discharging current pulse at the terminals of the second battery module, or vice versa.

[0017] The power electronics unit is preferably a DC / DC converter unit comprising at least two DC / DC converters. The term "DC" stands for "Direct Current." A DC / DC converter is a DC voltage converter. However, the power electronics unit can also be implemented as an inverter, on-board charger, etc.

[0018] The DC / DC converter unit can continue to provide the low-voltage power supply to the vehicle.

[0019] The opposing current pulse, namely a charging current pulse at one battery module and a discharging current pulse at the other battery module, which is impressed by the power electronics unit at the terminal connections of the battery modules, leads to a locally pronounced voltage equalization and thus reduces the voltages and currents occurring during the switching process from a series to a parallel connection of the battery modules, thereby counteracting arc formation within the switching unit.

[0020] The method according to the invention serves to reduce a charge or voltage difference between at least two battery modules connected in series of a switchable storage system according to the invention in preparation for a parallel connection of the two battery modules and comprises at least the following steps: - Determination of a charge or voltage state of at least two battery modules, namely at least a first battery module and a second battery module, via a control unit, - Calculation of a necessary current pulse based on a difference in the charge or voltage state of the two battery modules detected by the control unit, - Applying the calculated current pulse in opposite directions to the terminals of the first battery module and the second battery module via the power electronics unit, - Further determination of the charge or voltage state of the first battery module and the charge or voltage state of the second battery module via a control unit up to a defined optimal switching time, namely a time at which the two battery modules have approximately the same charge or voltage state, - Parallel connection of the previously series-connected battery modules via a switching unit, - Interruption of the applied current pulse via the power electronics unit. Brief description of the drawings

[0021] The invention is described below by way of example with reference to the drawings. Fig. Figure 1 shows a schematic circuit diagram of an exemplary switchable storage system. Fig. Figure 2a shows a current and voltage curve of the first battery module over time when switching from a series connection to a parallel connection of the two battery modules. Fig. Figure 2b shows a current and voltage curve of the second battery module over time when switching from a series connection to a parallel connection of the two battery modules. Detailed description of the invention

[0022] In Fig. Figure 1 schematically shows an exemplary switchable storage system 1 for providing electrical power for the propulsion of a motor vehicle.

[0023] The exemplary switchable storage system 1 comprises two battery modules 2a, 2b of the same nominal voltage, namely at least a first battery module 2a and a second battery module 2b, a switching unit 3, a control unit 7 and a power electronics unit 4.

[0024] The battery modules 2a, 2b have a nominal voltage of 400 V.

[0025] The control unit 7 is configured to monitor the two battery modules 2a and 2b, specifically to determine their charge or voltage state. Furthermore, the control unit 7 is configured to detect any difference in charge or voltage state between the first battery module 2a and the second battery module 2b and, based on this detected difference, to calculate a necessary current pulse.

[0026] The power electronics unit 4 is designed as a DC / DC converter unit with two DC / DC converters, namely a first DC / DC converter 5a and a second DC / DC converter 5b, with split coil bodies.

[0027] The first DC / DC converter 5a is connected to the battery terminals of the first battery module 2a; the second DC / DC converter 5b is connected to the battery terminals of the second battery module 2b.

[0028] The switching unit 3 is configured to connect the first battery module 2a and the second battery module 2b in parallel for charging (operating state "Charging") and in series for powering the vehicle (operating state "Driving"). For this purpose, the switching unit 3 has three switches 6a, 6b, and 6c, namely a first switch 6a, a second switch 6b, and a third switch 6c, which can be switched to states that allow parallel and series connections of the two battery modules 2a and 2b. In this way, the operating states "Charging" and "Driving" of the switchable energy storage system 1 of the vehicle can be realized.

[0029] Through the in Fig. In the current switching states of switches 6a, 6b, 6c, the two battery modules 2a, 2b are disconnected from each other and from the loads A, B (all switches 6a, 6b, 6c are open). With the second switch 6b closed, the first switch 6a open, and the third switch 6c open, the two battery modules 2a, 2b are connected in series. With the second switch 6b open, the first switch 6a closed, and the third switch 6c closed, the two battery modules 2a, 2b are connected in parallel.

[0030] By switching from a series connection to a parallel connection of the two battery modules 2a, 2b, the existing charging infrastructure, which often only offers charging voltages up to a maximum of 400V or 500V, can be used. On the other hand, by connecting the two battery modules 2a, 2b in series in the present embodiment, an operating voltage of 800 V can be achieved.

[0031] The power electronics unit 4 is designed to reduce the difference in the state of charge of the first battery module 2a and the state of charge of the second battery module 2b in preparation for a parallel connection of the first battery module 2a and the second battery module 2b by applying an opposite current pulse, previously calculated (necessary) via the control unit 7, to the terminals of the first battery module 2a and the terminals of the second battery module 2b, thus achieving a local voltage equalization between the battery modules 2a and 2b.

[0032] The energy of the opposing current pulses can be transferred directly from one half to the other half through the shared coil bodies of the two DC / DC converters 5a, 5b of the DC / DC converter unit.

[0033] By applying opposing current pulses to the two battery modules 2a and 2b, the corresponding switches 6a, 6b, and 6c of switching unit 3 can be switched from series to parallel with minimal or no arcing. Furthermore, this type of parallel connection reduces or even eliminates the time-consuming charge equalization operations of the two battery modules 2a and 2b.

[0034] In Fig. Figure 2 schematically illustrates the switching of the two battery modules 2a and 2b from a series connection to a parallel connection, depicted using a voltage U and current I over time t. Assuming that the first battery module 2a has a higher charge level (voltage) than the second battery module 2b, at time t0 a discharge current pulse is applied to the first battery module 2a via the first DC / DC converter 5a, and a charging current pulse is applied to the second battery module 2b via the second DC / DC converter 5b. The discharge current pulse reduces the voltage U0 of the first battery module 2a. The charging current pulse increases the voltage U0' of the second battery module 2b. If the current pulse is correctly chosen, the voltages of both battery modules 2a and 2b will be equal at time t1 (U1 in the example shown). Fig.2) At time t1, the two battery modules 2a, 2b are electrically connected in a parallel arrangement. This ensures that the necessary switches 6a, 6b, 6c of switching unit 3 can operate with minimal or no arcing. Furthermore, this type of switching from series to parallel connection of the two battery modules 2a, 2b reduces or even eliminates time-consuming charge balancing operations.

[0035] Applying a discharge current pulse to a battery module 2a, 2b leads to a spontaneous voltage drop proportional to the current pulse, caused on the one hand by the internal resistance of the individual battery cells of battery modules 2a, 2b and on the other hand by polarization effects. With prolonged discharge, the voltage of the respective battery module 2a, 2b decreases further due to the decrease in the state of charge (SOC).

[0036] The behavior of battery modules 2a and 2b is reversed when a charging current pulse is applied. For a fast and arc-free parallel connection of the two battery modules 2a and 2b, the current pulse is selected such that, at a voltage difference (open-circuit voltage) ΔU between the two battery modules 2a and 2b, the sudden voltage drop (application of a discharge current pulse) of one battery module ΔU1 and the voltage rise (application of a charging current pulse) of the other battery module ΔU2 sum exactly to the original voltage difference ΔU: ΔU = ΔU1 + ΔU2. At this point, the battery modules 2a and 2b can be connected in parallel without a load or with a low load. After the battery modules 2a and 2b are connected in parallel, the respective current pulses (charging current pulse, discharge current pulse) can be removed. Now, until the equilibrium voltage is reached, a balancing current flows between the two battery modules 2a, 2b without damaging the components used for switching.If a power electronics unit 4 is used for each application of the current pulses, which are connected to each other via a shared coil, the energy extracted from one battery module 2a, 2b can be charged into the other battery module 2a, 2b, thus minimizing losses. Reference symbol list 1 Switchable storage system 2a First battery module 2b Second battery module 3 switching unit 4 Power electronics unit 5a First DC / DC converter 5b Second DC / DC converter 6a First switch 6b Second switch 6c Third switch 7 Control unit Consumers A, B t time I Electricity U voltage t0, t1 Time U0, U0', U1 Voltage of the respective battery module at a defined time

Claims

Switchable storage system (1) for providing electrical power for propelling a motor vehicle, comprising: - at least two battery modules (2a, 2b) of the same nominal voltage, namely at least a first battery module (2a) and a second battery module (2b); - a switching unit (3), wherein the switching unit (3) is configured to connect the first battery module (2a) and the second battery module (2b) in parallel for charging and to connect them in series for propelling the motor vehicle; - a control unit (7), wherein the control unit (7) is configured to determine a charge or voltage state of the first battery module (2a) and a charge or voltage state of the second battery module (2b), to detect a difference in the charge or voltage states of the two battery modules (2a, 2b) and to calculate a necessary current pulse based on this difference; - a power electronics unit (4).wherein the power electronics unit (4) is configured to reduce the difference in the charge or voltage state of the first battery module (2a) and in a charge or voltage state of the second battery module (2b) in preparation for a parallel connection of the first battery module (2a) and the second battery module (2b) by applying an opposite current pulse, calculated via the control unit (7), to the terminals of the first battery module (2a) and to the terminals of the second battery module (2b), thus achieving a local charge or voltage equalization between the battery modules (2a, 2b). Switchable storage system (1) according to claim 1 , characterized in that the line electronics unit (4) is a DC / DC converter unit comprising at least two DC / DC converters (5a, 5b). Switchable storage system (1) according to claim 2, characterized in that the DC / DC converter unit provides the low voltage supply of the motor vehicle. A method for reducing a charge or voltage difference between at least two battery modules (2a, 2b) connected in series of a switchable storage system (1) in preparation for connecting the two battery modules (2a, 2b) in parallel according to any one of claims 1 to 3, comprising the following steps: - Determining a charge or voltage state of the at least two battery modules, namely at least a first battery module (2a) and a second battery module (2b), via a control unit (7), - Calculating a necessary current pulse based on a difference in the charge or voltage state of the two battery modules (2a, 2b) detected via the control unit (7), - Applying the calculated current pulse in opposite directions to the terminals of the first battery module (2a) and the second battery module (2b) via the power electronics unit (4),- Further determination of the charge or voltage state of the first battery module (2a) and the charge or voltage state of the second battery module (2b) via a control unit (7) up to a defined optimal switching time, namely a time at which the two battery modules (2a, 2b) have approximately the same charge or voltage state, - Parallel connection of the previously series-connected battery modules (2a, 2b) via a switching unit (3), - Interruption of the applied current pulse via the power electronics unit (4).

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