Method for controlling a battery unit of a motor vehicle

Abstract

A method for controlling a motor vehicle battery unit configured as a traction battery providing electrical energy to an electric drivetrain M. The battery unit comprises a plurality of battery module

Description

The present invention relates to a method for controlling a battery unit of a motor vehicle according to claim 1. It is known from the prior art to use a battery unit having a plurality of battery modules. In the context of this specification, a battery module is understood in particular to mean a rechargeable voltage source. The battery modules of a battery unit can be connected to one another differently via semiconductor elements. Such battery units are typically used in an electrically driven motor vehicle in order to provide electrical energy to the drivetrain. It is also known to interconnect a plurality of battery modules of a battery unit parallel to one another. This is done for example in the case of an insulation measurement. It is also possible to interconnect a plurality of battery modules parallel to one another in order to be able to quickly provide more power for the drivetrain, for example if a 0J higher acceleration of the motor vehicle is desired. When operating with battery modules connected in parallel, it is important that voltage -t— differentials of the battery modules do not become too large. If voltage differentials are too large, comparatively large equalizing currents occur in the parallel circuit due to the capacities present in the circuit and the low-ohmic connection between the battery modules. A method for voltage compensation in the on-board network of an electrically operated vehicle is known from DE 10 2018004891 A1, in which a voltage differential is formed from the sensed voltages of two batteries and powering-on of a consumption to the battery whose sensed voltage lies outside a target voltage range. This discharges the battery until the voltage is within the target voltage range. By contrast, the present invention addresses the problem of reducing the risk of large voltage differentials between the battery modules in case of longer downtimes of the motor vehicle. Aspects of the invention provide a method according to claim 1, a control unit according to claim 9, and a system according to claim 10. Optional embodiments of the invention are specified in the dependent claims. The battery unit is configured so as to provide electrical energy to an electric drivetrain of the motor vehicle. For example, the battery unit and the drivetrain can be connected to one another via electrical leads. The electric drivetrain is configured so as to drive the motor vehicle. The battery unit comprises a plurality of battery modules. It is monitored whether the battery unit is in a powered-off state. In the context of this specification, the powered-off state is understood in particular to mean that the battery unit is separated by switching elements from electrical consumers, in particular from the drivetrain of the motor vehicle. A period of time is measured during which the battery unit is in the powered-off state. If the time period exceeds a time threshold, the battery unit is transitioned from the powered-off state into a powered-on state. The battery modules are electrically connected in parallel to one another when powered-C\J on. The powered-on state in the context of this specification is understood in particular to mean that the battery unit is electrically connected to at least one electrical consumer. Preferably, in the powered-on state, the battery unit can be electrically connected, for example via a low-voltage tap, to an on-board network of the motor vehicle, whose operating voltage is less than the voltage provided by the battery unit. Various electrical consumers can be a component part of this on-board network. By transitioning into the powered-on state with battery modules connected in parallel and the electrical connection to the on-board network, the voltages of the battery modules are inherently matched to one another. In this way, the risk of large voltage differentials between the battery modules is reduced when the motor vehicle is not used for an extended period of time, because the individual battery modules are also discharged at different rates when powered-off. According to one embodiment of the invention, the time threshold can be constant. In this way, it can be achieved that the battery unit is switched to the powered-on state at regular intervals during an extended downtime of the motor vehicle and voltage differentials between the battery modules are thus compensated. The time threshold can be changed. For example, the time threshold can be changed after the battery unit has been transitioned back from the powered-on state into the powered-off stateElectrical voltages present on the battery modules are monitored in the powered-on state. The time threshold is decreased if it has been detected that a voltage differential between a highest one of the voltages and a lowest one of the voltages is greater than a first battery module threshold. Thus, the time threshold can be adapted to how quickly the battery modules discharge differently. The time threshold can be increased if it has been detected that the voltage differential between the highest of the voltages and the lowest of the voltages is less than a second battery module threshold. In this way, unnecessary transitions into the powered-on state can be avoided. According to one embodiment of the invention, the battery unit can be configured so as to output an AC voltage. For this purpose, the battery unit can comprise, for C\J example, an inverter. For example, the inverter can consist of several components and does not have to be present as a separate component. i— According to one embodiment of the invention, the battery unit can be configured so as to output a DC voltage. After powering the unit on, an insulation measurement is carried out with the battery modules connected in parallel. In the context of this specification, an insulation measurement is understood in particular to mean a measurement of the insulation resistance of the circuit to which the battery unit is electrically connected. The parallel connection of the battery modules to one another can be part of the insulation measurement. In this way, the parallel circuitry can be accomplished by an operation already implemented in the control unit for the battery unit. The insulation measurement can alternatively also be carried out when the battery modules are interconnected in a bypassing manner. According to one embodiment of the invention, after transitioning to the powered-on state, a respective voltage of the battery modules can be measured. The parallel circuitry of the battery modules and the powered-on state can be maintained until a difference between a highest one of the voltages and a lowest one of the voltages is less than a voltage threshold. This has the advantage that the parallel circuitry is maintained for a sufficient length of time in order to match the voltages to the desired extent. According to one embodiment of the invention, a respective state of charge of the battery modules can be determined upon powering the unit on. This can be done in particular directly after powering the unit on, for example even before the insulation measurement. The state of charge can be determined via, for example, the open circuit voltage and the cell temperature. The respective state of charge significantly affects the voltage of the respective battery module. Preferably, voltage differentials between the battery modules are determined in order to ensure that a parallel circuitry of the battery modules is possible without the risk of damage. The control unit for a battery unit of a motor vehicle according to claim 9 is configured so as to carry out a method according to one embodiment of the invention. The system according to claim 10 comprises a control unit according to one embodiment of the invention and the battery unit. Further features and advantages of the present invention become apparent from the following description of preferred exemplary embodiments, with reference to the appended illustrations. The same reference numerals are used for the same or similar components and for components having the same or similar functions. The following are shown: Fig. 1 a schematic circuit diagram of a system according to one embodiment of the invention; Fig. 2 a schematic circuit diagram of an embodiment of a battery module; and Fig. 3 a schematic circuit diagram of an alternative embodiment of a battery module. The system is a component of a motor vehicle and comprises a battery unit having a plurality of battery modules U1, U2, UN, V1, V2, VN, W1, W2, and WN, a drivetrain M, and a low-voltage tap LV. Via the low-voltage tap LV, the battery modules U1, U2, UN, V1, V2, VN, W1, W2, and WN are electrically connected to an on-board network of the motor vehicle. The on-board network has a lower voltage than the voltage supplied by the battery modules. The battery modules U1, U2, UN, V1, V2, VN, W1, W2, and WN are also electrically connected to the drivetrain M and are configured so as to provide electrical energy to the drivetrain M. The drivetrain M is configured so as to drive the motor vehicle. Figs. 2 and 3 show possible embodiments of the battery modules U1, U2, UN, V1, V2, VN, W1, W2, and WN. They each comprise a voltage source 1 and a plurality of switching elements 2. The switching elements 2 can in particular be semiconductor C\J switching elements, for example metal-oxide-semiconductor field-effect transistors (MOSFETs). By means of suitable circuitry of the switching elements 2, the battery modules U1, U2, UN, V1, V2, VN, W1, W2, and WN can be connected to one another in different ways. If the motor vehicle is not used for an extended period of time, the voltage sources 1 discharge at different rates, such that they have differently high voltages upon powering-on of the motor vehicle. When the motor vehicle is put into service with relatively large differences in voltages of the voltage sources 1, there is a risk of large equalizing currents that can damage the battery modules or other components. Therefore, it is contemplated to transition the battery unit into a powered-on state with battery modules U1, U2, UN, V1, V2, VN, W1, W2, and WN connected to one another in parallel when the battery unit has been in a powered-off state for a period of time that exceeds a time threshold. When the battery modules U1, U2, UN, V1, V2, VN, W1, W2, and WN are connected in parallel to one another, the voltages are matched to one another. The parallel circuitry of the battery modules can be achieved by a suitable circuitry of the switching elements 2. CM In this way, an adjustment of the state of charges of the battery modules U1, U2, UN, V1, V2, VN, W1, W2, and WN and thus also of their voltages can be achieved without additional measurements. It is particularly advantageous when the powered-on state with the battery modules U1, U2, UN, V1, V2, VN, W1, W2, and WN connected in parallel is a mode already provided in the control unit with battery modules U1, U2, UN, V1, V2, VN, W1, W2, and WN connected in parallel. This mode can be, for example, an insulation measurement.

Claims

1. A method for controlling a battery unit of a motor vehicle, wherein the battery unit is configured so as to provide electrical energy to an electric drivetrain of the motor vehicle, wherein the electric drivetrain is configured so as to drive the motor vehicle, wherein the battery unit comprises a plurality of battery modules, wherein the method comprises the following steps:- monitoring whether the battery unit is in a powered-off state;- measuring a period of time during which the battery unit is in the powered-off state; and- transitioning the battery unit from the powered-off state into a powered-on state when the period of time exceeds a time threshold, wherein the battery modules are electrically connected in parallel to one another in the powered-on state;wherein the time threshold is changed by monitoring electrical voltages present on the battery modules in the powered-on state and, if it has been detected that a voltage differential between a highest one of the voltages and a lowest one of the voltages is greater than a first battery module threshold, the time threshold is decreased, wherein, after powering the unit on, an insulation measurement is carried out with the battery modules connected in parallel, wherein the insulation measurement is a measurement of the insulation resistance of the circuit to which the battery unit is electrically connected.

2. The method according to any one of the preceding claims, wherein the battery unit is configured so as to output an AC voltage.

3. The method according to any one of claim 1, wherein the battery unit is configured so as to output a DC voltage.

4. The method according to any one of the preceding claims, wherein, after transitioning the unit into the powered-on state, a voltage on each of the battery modules is measured, wherein the parallel circuitry of the battery modules andthe powered-on state are maintained until a difference between a highest of the voltages and a lowest of the voltages is less than a voltage threshold.

5. The method according to any one of the previous claims, wherein a respective state of charge of the battery modules is determined after powering the unit on.

6. A control unit for a battery unit of a motor vehicle, wherein the control unit is configured so as to carry out a method according to any one of the preceding claims.

7. A system comprising a control unit according to the previous claim and the battery unit.CM

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