BATTERY SYSTEM AND METHOD FOR CONTROLLING A BATTERY SYSTEM

Abstract

A battery system including a number of first battery modules each including a number of battery cells, and a number of second battery modules each including a number of battery cells. Each second battery module includes a power electronics unit having a DC/DC converter. The first and second battery modules are connected in series. The first battery modules are connected directly in series and the second battery modules are connected via their power electronics units.

Claims

1. A battery system, comprising: a number of first battery modules each comprising a number of battery cells; and a number of second battery modules each comprising a number of battery cells, wherein each second battery module comprises a power electronics unit having a DCDC converter, and wherein the first and second battery modules are connected in series, wherein the first battery modules are connected directly in series and the second battery modules are connected via their power electronics units.

2. The battery system according to claim 1, wherein the first battery modules are comprised of high-energy cells and the second battery modules are comprised of high-power cells.

3. The Battery system according to claim 1, wherein the DCDC converters are operable in buck mode, boost mode, path-through mode, bypass mode and/or off-mode.

4. The battery system according to claim 1, wherein the power electronics units comprise means to switch their respective module to a bypass mode, in which the module is not connected in series to other modules.

5. The battery system according to claim 1, further comprising a smart battery management system for controlling the power electronics units, the smart battery management system comprising means for determining a state of charge and/or a temperature and/or a voltage drop across the modules; means for determining an optimum output voltage for each DCDC converter; and means for controlling the power electronics units by switching the DCDC converter in order to set its output voltage to the determined optimum output voltage.

6. The battery system according to claim 1, wherein each first battery module comprises a cell supervision circuit for battery balancing.

7. An electric vehicle with a battery system according to claim 1.

8. A method for controlling a battery system according to claim 1, comprising: determining a state of charge and/or a temperature and/or a voltage drop across the module; determining an optimum output voltage for each DCDC converter; and controlling the power electronics units by switching the DCDC converter in order to set its output voltage to the determined optimum output voltage.

9. The method according to claim 8, comprising: if the determined optimum output voltage for a battery module is 0 V, controlling the respective power electronics unit by switching the DCDC converter into a bypass mode.

10. The method according to claim 8, wherein all cells of a first battery module are balanced among each other and among the cells of other first battery modules, while the cells of a second battery module are only balanced among each other.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Embodiments of the present invention are described with reference to schematic figures.

[0033] FIG. 1 shows a battery system according to an embodiment of the present invention and

[0034] FIG. 2 shows a part of the battery system according to FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] FIG. 1 shows a battery system 1. The battery system 1 comprises a number of first battery modules 2 and a number of second battery modules 3. In the embodiment shown in FIG. 1, there are only two first battery modules 2 and a number N of second battery modules 3. However, a smaller or larger number of first battery modules 2 may be provided in alternative embodiments.

[0036] In the following the battery system is disclosed as a battery system for an electric vehicle. It is noted, that the battery system can also be used for stationary power storage. For instance, the battery system may be used for a photovoltaic system.

[0037] The battery modules 2, 3 are connected in series to provide the DC link voltage between the main contactors 7, 9 of the battery system 1.

[0038] The first battery modules 2 are comprised of battery cells 4, which are connected in series and parallel to form a first battery module 2. Furthermore, each first battery module 2 has a cell supervision circuit 13 which supervises cell voltage and cell temperatures. The cell supervision circuit 13 communicates with a battery management system 11 of the battery system 1 to transmit voltage and temperature measurements to the battery management system 11.

[0039] The second battery modules 3 are also comprised of battery cells 5 connected to each other in series and parallel. The second battery modules 3 also comprise a cell supervisor circuit 13 for the supervision of cell voltage and temperature. In addition, the second battery modules 3 each comprise a power electronics unit 14 having a DCDC converter.

[0040] The first battery modules 2 may comprise a different kind of battery cells 4 as the second battery modules 3. In particular, the battery cells 4 may be high-energy cells while the battery cells 5 may be high-power cells.

[0041] The DCDC converters can be used to set the output voltage of the respective second battery module 3 to a predetermined value. To achieve this, the DCDC converters of the power electronics units 14 are operable in buck mode, boost mode and preferably also in the path-through mode and in a bypass mode.

[0042] The first battery modules 1 are directly connected to each other in series, which is indicated by the connection 18. The second battery modules 3 are in contrast connected to each other in series via their power electronics units 14, which is indicated by the connection 19. In operation of the electric vehicle, i.e. while charging or while drawing current from the battery, the battery management system 11 which communicates with a vehicle control unit 12 receives all voltage measurements and temperature measurements from the first battery modules 2 and the second battery modules 3 and then estimates the state of charge of each cell and the average state of charge for each module 2, 3. For a certain requested DC link voltage from the vehicle control unit 12, the battery management system 11 decides which second battery modules 3 should contribute to the DC link voltage and therefore be connected to the first battery modules 2. The reference voltage for the DCDC converters for each of these modules 3 is calculated. The DCDC converters of the second battery modules 3 are controlled accordingly to deliver the desired output voltage or to bypass the certain second battery module 3.

[0043] Thus, the power electronics units 14 comprising the DCDC converters decouple the second battery modules 3 from each other and from the first battery modules 2.

[0044] From the voltage and temperature measurements, the battery management system 11 can calculate the differences in state of charge between first battery modules 2 and the difference in state of charge between each of the second battery modules 3. Consequentially, the battery management system 11 can control the cell supervisor circuits 13 to balance the battery cells 4 of the first battery modules 2, which are dependent on each other. The cells 5 of the second battery modules 3 need to be balanced only on the module level as the second battery modules 3 are decoupled from each other by the power electronics units 14.

[0045] The battery system 1 further comprises a battery junction box 6 comprising two main contactors 7 and 9 for connection to the battery modules 2, 3. The battery junction box 6 further comprises a DC charge fuse 16 and a main fuse 17 as well as terminals 15 for DC charge +, AC charger +, DC link +, AC charger -, DC link - and DC charge -.

[0046] Usually, the battery management system 11 is arranged inside a housing of the battery junction box 6. It may also be arranged outside the battery junction box 6.

[0047] FIG. 2 shows one second battery module 3 of the battery system 1 according to FIG. 1 and its power electronics unit 14. The cell supervision circuit is not shown in FIG. 2. According to this embodiment, the power electronics unit 14 comprises a DCDC converter and additional switches to control different modes of operation of the power electronics unit 14. In a buck/boost mode, switch S3 and S4 are on and switches S1 and S2 are switching. In the buck/boost mode, the voltage drop between terminals 20 and 21 can be set to a predetermined voltage according to a state of charge of the second battery module 3 and according to a demand from the vehicle control unit 12.

[0048] In a bypass mode, switches S1, S2 and S4 are on and switch 3 is off, so that the second battery module 3 is bypassed. This mode can be chosen, when a certain second battery module 3 is not required to contribute to the operation of the electric vehicle or when a certain second battery module 3 is defective.

[0049] In a path-through mode, switches S1, S3 and S4 are on and switch 2 is off. In this mode, the second battery module 3 is operated in a conventional mode as the first battery modules 2 without adjusting the output voltage.

[0050] Furthermore, the power electronics unit 14 can be operated in a standby mode, where switches S1 and S2 are off and switches S3 and S4 are on, and in an open circuit mode, in which all switches are off and no high voltage is present.

[0051] The switches S1, S2, S3 and S4 are controlled by the battery management system 11.