The present invention relates to a method for balancing the states of charge of battery cells of a battery having at least one battery module string, in which a battery module in the battery module string comprises a coupling unit, and to a battery, in which the method according to the invention is implementable.
It is apparent that, in future, battery systems will increasingly be used both in stationary applications and in vehicles such as hybrid and electric vehicles. In order to be able to meet the demands which are made for a respective application in terms of voltage and power that can be made available, a large number of battery cells are connected in series. Since the current provided by such a battery must flow through all the battery cells, and a battery cell can conduct only a limited current, battery cells are often additionally connected in parallel in order to increase the maximum current. This can be done either by providing a plurality of cell coils within a battery cell housing or by externally interconnecting battery cells. However, one problem in this case is that compensation currents between the battery cells which are connected in parallel may occur on account of cell capacitances and voltages which are not exactly identical.
The arrangement illustrated in
A further disadvantage is that the same current flows through battery cells or modules contained in the system and therefore said battery cells or modules cannot be actuated individually. Therefore, there is no way to influence different states of individual battery cells.
In addition, the prior art has disclosed methods for balancing different states of charge (SOC) between individual battery cells or modules comprising same. The methods often require that an exchange of energy occurs between the battery cells and a connected load. When the electric vehicle is at a standstill, that is to say when no energy is being supplied to the load or taken therefrom, it is not possible to balance the different states of charge using said methods.
According to the invention, a method for balancing the states of charge of battery cells of a battery is therefore provided. The battery comprises at least one battery module string having a plurality of series-connected battery modules. Each of the series-connected battery modules comprises at least one battery cell, at least one coupling unit, a first connection and a second connection and is designed to assume one of at least two switching states on the basis of actuation of the coupling unit. In this case, different switching states correspond to different voltage values between the first connection and the second connection of the battery module. Thus, in each of the switching states a different voltage value can be tapped off between the first connection and the second connection of the battery module.
The method according to the invention comprises the following steps: in a first method step, a first (not necessarily constant) output voltage of the battery module string is provided by suitable actuation of the battery modules in the battery module string and applied to an inductance during a first time interval, such that a current which flows through the inductance is increased. As a result of this, magnetic energy is stored in the inductance according to W=0.5L*I2, wherein L is the self-inductance of the inductance and I is the current which flows through the inductance at the end of the first time interval.
In a second method step, a second (again, not necessarily constant) output voltage of the battery module string is provided by suitable actuation of the battery modules in the battery module string and applied to the inductance during a second time interval. In this case, the second output voltage has opposite polarity with respect to the first output voltage. The battery modules involved in providing the second output voltage are not exclusively the same battery modules as those involved in providing the first output voltage.
During the second method step, the magnetic energy stored in the inductance during the first method step is used to separate charges in the battery modules involved in providing the second output voltage, with the result that, after the second time interval has elapsed, said battery modules have a higher charge state than before.
Since, preferably, battery modules which have a higher charge state than those battery modules involved in providing the second output voltage are involved in providing the first output voltage, energy is shifted from the battery modules with a higher charge state into the battery modules with a lower charge state.
Typically, the second time interval directly follows the first time interval, and the method is periodically repeated.
At least one battery module can be designed either to connect the first connection and the second connection of the battery module or to connect the at least one battery cell between the first connection and the second connection, on the basis of actuation of the coupling unit. Two different switching states are defined hereby. In addition, at least one battery module can be designed to connect the at least one battery cell between the first connection and the second connection, wherein a polarity of the voltage present between the first connection and the second connection is selectable on the basis of actuation of the coupling unit. As a result of this, likewise two switching states emerge, or else three switching states, if the two configurations mentioned are combined with one another.
In a preferred embodiment of the invention, at least one battery module has the last-mentioned three switching states, wherein, in a first switching state, the first connection and the second connection of the battery module are connected, in a second switching state, the at least one battery cell is connected between the first connection and the second connection with a certain polarity (in one example, positive) and, in a third switching state, the at least one battery cell is connected between the first connection and the second connection with the opposite polarity (in the same example, negative).
It is also preferable that the battery module string comprises at least one first and one second battery module having the described three switching states, wherein the first battery module has a higher charge state than the second battery module. The method according to the invention is then implemented hereby such that, during the first time interval, the first battery module is in the second switching state and the second battery module is in the first switching state, while, during the second time interval, the first battery module is in the first switching state and the second battery module is in the third switching state.
In another preferred embodiment of the invention, at least one inductance of an electric motor connected to the battery is used as inductance. As a result of this, either a movement of the electric motor during the implementation of the method can be blocked or else, during a movement of the electric motor, the first and/or the second time interval can be selected such that the current which flows through the inductance of the electric motor in the first and/or second time interval does not contribute to a torque in the electric motor, as a result of which the magnetic energy stored in the inductance is not converted into kinetic energy, rather used only for charge separation. The invention thus provides a method which can be implemented both during operation of the motor and while a system driven by the motor is in the idle state (that is to say without a flow of energy).
A further aspect of the invention relates to a battery which comprises at least one battery module string having the properties described above. The battery is connectable to an inductance and is designed to implement the method according to the invention. In addition, it can be connectable to an inductance of an electric motor. The control device which is additionally required for full implementation of the method can be part of the battery, although this is not essential. The battery is preferably a lithium-ion battery.
Furthermore, a motor vehicle having an electric drive motor for driving the motor vehicle and a battery according to the invention connected to the electric drive motor is specified.
Exemplary embodiments of the invention are explained in more detail with reference to the drawings and the following description, wherein identical reference signs indicate identical or functionally identical components. In the drawings:
A control device 60 shown in
By suitable actuation of the plurality of battery modules 40-1, . . . , 40-n in m battery module strings 50-1, 50-2, . . . , 50-m, m sinusoidal output voltages can thus be produced which actuate the electric motor 13 in the desired form without the use of an additional pulse-controlled inverter.
In another embodiment, it is provided that the battery modules 40-1, . . . , 40-n used in one of the m battery module strings 50-1, 50-2, . . . , 50-m are designed to connect their battery cells 41 between the first connection 42 and the second connection 43 such that a polarity of the voltage present between the first connection 42 and the second connection 43 is selectable on the basis of actuation of the coupling unit.
The method according to the invention for balancing the states of charge of battery cells of a battery will be described in the following text with reference to
Said embodiment of the battery module 40 is, as explained above, designed to take selectively one of at least three switching states on the basis of actuation of the coupling unit. In a first switching state, the first connection 42 and the second connection 43 of the battery module 40 are connected. In a second switching state, the plurality of battery cells 41 is connected between the first connection 42 and the second connection 43 with a positive polarity. Finally, in a third switching state, the plurality of battery cells 41 is connected between the first connection 42 and the second connection 43 with a negative polarity.
The battery module string 50 illustrated in
Before the start of the method according to the invention, there is no current flowing through the inductance L. The first battery module 40-1 has a higher charge state than the second battery module 40-2.
Now, as illustrated in
During the second time interval Δt2, as illustrated in
At the end of the second time interval Δt2, the first battery module 40-1 thus has a lower charge state than at the start of the method, and the second battery module 40-2 has a higher one.
The method according to the invention can be applied without problems to the case in which the battery module string 50 comprises a greater number of battery modules 40. In this case, preferably, those battery modules which have a higher charge state than the battery modules involved in providing the second output voltage are involved in providing the first output voltage during the first time interval Δt1. As a result of this, there is an overall exchange of charge between the battery cells of the different battery modules, and the different states of charge of the battery modules are balanced.
Under idealized preconditions, the process illustrated in
In an exemplary embodiment of the invention which is not shown in more detail, an inductance of the electric motor 13, which is connected to the battery 10, for example a permanent magnet synchronous motor, is used as inductance L. Since in practice most of all the motors used are three-phase motors, in this case the arrangement can be as illustrated in
In order to ensure that the magnetic energy stored in the inductance L is not converted into kinetic energy but rather used only for charge separation, the drive system should be in the idle state. More precisely, the drive system must be firmly braked, that is to say the torque arising during the implementation of the method according to the invention is not permitted to exceed the breakaway torque necessary for a movement of the motor. (In the case of an asynchronous machine, there is no danger of this as no torque occurs here).
Alternatively, the method according to the invention can also be implemented in the event of a movement of the drive system. When describing synchronous and asynchronous machines, it is common to use a rotating coordinate system. The axes of said coordinate system are designated d-q and rotate with the speed of the magnetic field, wherein the d axis is oriented, by definition, in the direction of the field. In the case of a symmetrical synchronous machine, the current running in the d direction does not contribute to torque generation. Thus, the method described above can be implemented by the build-up and decrease of a current in this direction. Only the rotation of the current space vector should be taken into account when selecting the battery module to be activated. Only a particular angle range in which the current can be built up is available for a given battery module. Likewise, only a particular angle range is available for a battery module by means of which the current is to be decreased again.
Number | Date | Country | Kind |
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10 2011 082 973. | Sep 2011 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2012/066659 | 8/28/2012 | WO | 00 | 10/29/2014 |