Patent Application
20150249351
The present invention relates to a method for generating a periodic AC voltage using a battery module string and a battery management system for a battery module string. The battery module string comprises battery modules which are electrically connectable, individually or in series, to poles of the battery module string and which are electrically disconnectable individually, each battery module having at least one battery cell, for example a lithium-ion battery cell, as is used, for example, in battery systems of electric or hybrid motor vehicles, i.e. in motor vehicles having at least partially or temporarily electrically operated drives. Therefore, the invention also relates to a motor vehicle and a battery system.
Owing to improved storage capacity, more frequent rechargeability and increased energy densities, batteries for example batteries based on lithium-ion cells or nickel metal hydride batteries, are used in ever broader applications. Batteries having a lower energy storage capacity are used, for example, for small portable electronic appliances such as mobile telephones, laptops, camcorders or the like, while batteries with a high capacity are used as energy source for the drive of motors of hybrid or electric vehicles, etc. or as stationary batteries.
Batteries can be formed, for example, by interconnecting battery modules in series, wherein there are sometimes also parallel interconnections of the battery modules, and the battery modules for their part can also consist of battery cells connected in series and/or parallel.
For driving motors of hybrid or electric vehicles, in particular polyphase battery systems are suitable, which are also referred to as battery direct inverters (BDIs) and are described in DE 10 2010 027 864. BDIs comprise battery modules and coupling units assigned to the modules. The battery modules are individually electrically connectable and electrically disconnectable by means of the coupling units, optionally with the possibility of choosing between a positive and a negative connection.
Therefore, the polyphase battery system can be used by corresponding connection and disconnection of the modules to generate a plurality of AC voltages which are phase-shifted with respect to one another. Given a corresponding design, for example, three AC voltage profiles which approximate sinusoidal curves and are phase-shifted with respect to one another can be generated, as a result of which a rotating field which can be used directly for driving electric or hybrid motors is generated. The voltage is in this case selected such that the alternating current which is set brings about the torque which is required for operating the motor.
Battery management systems are used for battery management, for example for basic actuation of modules, for increasing the safety of batteries, for increasing efficiency and for extending the life of battery modules and battery systems comprising battery modules. One function of battery management systems is to record current intensity and/or voltage profiles over time with an accuracy and sampling frequency that is necessary for determining a present state of charge and/or a present state of ageing, an internal impedance, a temperature value and/or the value of another characteristic variable of the individual battery modules.
Although the production of the battery cells is standardized and excellent production methods are implemented by the cell manufacturer, differences in the electrical properties, for example with respect to voltage, capacitance or internal resistance, are observed. In addition to these differences which arise owing to the production process, ageing processes cause a further change in the electrical properties of the battery cells. In a battery module comprising one or more cells, the individual battery cells are therefore never identical in respect of the electrical power. In particular in the case of a series interconnection of the cells in a module, the response of the module is determined by the electrical characteristic of the weakest cell. Therefore, the individual battery modules likewise have individual electrical characteristics.
A balancing method between the modules of a battery is described in the present application.
In particular, the invention describes a battery management system according to claim 1 for a battery module string comprising battery modules which are electrically connectable, individually or in series, to poles of the battery module string and are electrically disconnectable individually.
In this case, the battery module string is suitable for generating an AC voltage by disconnection and connection of the battery modules, the battery management system is characterized in that it is configured to assign a respective time period during which the battery module is electrically connected within a half-cycle of the AC voltage to each battery module depending on its position in a battery module list.
This enables dynamic management of the loading of the battery modules by dynamic management of the battery module list.
In a possible embodiment, the battery management system comprises an apparatus for receiving transmitted voltage profiles of the battery modules and means for using the measured profile for determining a state of wear of the respective battery module. In this case, the battery management system is designed to use the determined states for arranging the battery modules in the battery module list so that the battery modules in the battery module list are arranged according to increasing wear. Furthermore, the battery management system is configured to assign a time period to a battery module which has been subjected to less wear, which time period is longer than a time period assigned to a battery module which has been subjected to comparatively greater wear.
In this or another embodiment, the battery management system can be configured to determine the number and duration of interruptions for dividing the respective time period into partial time periods separate from one another, wherein the number and duration of the interruptions are dependent on a voltage made available by the respective battery module and a desired voltage profile of the AC voltage between a start time and an end time of the respective time period.
Furthermore, a battery system as claimed in claim 4 is proposed for generating at least one AC voltage. The battery system comprises at least one battery module string comprising battery modules which are electrically connectable, individually or in series, to poles of the battery module string and are electrically disconnectable individually, and a battery management system according to the invention.
Also described is a motor vehicle which comprises an at least temporarily or partially electrically operable drive unit and a battery system as claimed in claim 4.
Furthermore, a method for generating a periodic AC voltage is proposed, wherein the AC voltage is generated at poles of a battery system comprising battery modules. In this case, the battery system makes it possible for each of the battery modules to be connected electrically individually to the poles or disconnected electrically from the poles. The AC voltage is generated by changing electrical connection states of the battery modules to the poles, wherein, in a half-cycle, each battery module is electrically connected for different lengths of time. The method is characterized by the fact that a respective time period during which a respective battery module is intended to be electrically connected during the half-cycle is assigned at least to a position of the respective battery module in a battery module list. That is to say that the respective time period during which the respective battery module is intended to be electrically connected during the half-cycle is dependent at least on a position of the respective battery module in a battery module list.
In one possible embodiment, the method comprises, for each of the battery modules, the steps of measuring a profile of at least one voltage of the respective battery module and using the measured profile for determining a state of wear of the respective battery module. In the possible embodiment, the method furthermore comprises using the determined states for arranging the battery modules in the battery module list so that the battery modules in the battery module list are arranged according to increasing wear, wherein the respective time period is longer the less wear the respective battery module has been subjected to.
In this case, the battery module which was connected for a longest time period in a half-cycle directly proceeding the half-cycle can be connected for a shortest time period in the half-cycle.
The respective time period can consist of partial time periods separated from one another by at least one interruption. The number and duration of the interruptions can then be selected depending on a voltage made available by the respective battery module and a desired voltage profile of the AC voltage between a start time of a temporally first of the partial time periods and an end time of a temporally last of the partial time periods, with the result that, by means of pulse width modulation, the desired voltage profile is approximated more effectively.
Exemplary embodiments of the invention will be explained in more detail with reference to the drawings and the description below. In the drawings:
In a first exemplary embodiment of the invention, a balancing method between modules of a polyphase battery system, i.e. a battery system with BDI, is described. The balancing method comprises prioritization of the battery modules for the generation of the sinusoidal voltage profile. In this case, a module is awarded higher priority for use in the generation the higher its performance capability. This can be determined, for example, on the basis of the state of charge. As an alternative or in addition, the state of wear of the module can be taken into consideration, wherein a higher priority is awarded for use in the generation the lower the state of wear.
The state of wear of a module can be estimated, for example, on the basis of the total time period for which the module has been used. In addition or as an alternative, the determination of the state of wear can be based on one or more measurements at the module, for example an impedance measurement and/or a capacitance measurement.
The prioritization limits the use of the modules with a weaker performance which are weaker owing to the level of wear or the present state of charge than other modules and instead causes increased utilization of the modules with a greater performance. Further weakening and/or ageing of the weaker modules is thus limited. This causes an extension of the usability of these modules and therefore longer and improved usability of the overall system.
In battery module systems with direct converters (BDIs), modules can be connected and disconnected individually. In the process, in addition it is possible to select between positive and negative connection. As a result, approximately sinusoidal AC voltage profiles can be generated with a battery module system with a direct converter.
In a first embodiment of the invention, sketched by way of example in
A most closely weaker or second most powerful module 11-2 of the n modules is connected positively during the time period t1 for a time period t2≦t1. During the time period t2, the second most powerful module 11-2 makes available a voltage V2 where |V2|≦|V1. Again, fast-switching-based pulse width modulation can be used instead of permanent connection in order to make available the voltage V2 on average. The modules are connected positively in series, so that, during the time period t2, at least one voltage with a magnitude of greater than or equal to |V1|+|V2| is available.
An i-th most powerful module 11-i, 1<i<n, is connected during the time period t(i−1) for a time period ti≦t(i−1). During the time period ti, the i-th most powerful module 11-i makes available a voltage Vi where |Vi|≦|V(i−1)|. Again, pulse width modulation can be used to make available the voltage Vi on average. The modules are connected positively in series, with the result that, during the time period ti, at least one voltage with a magnitude of greater than or equal to |V1|+|V2|+ . . . +|V(i−1)|+|Vi| is available.
A weakest module 11-n is connected during the time period t(n−1) for a time period tn≦t(n−1). During the time period tn, the weakest module 11-n makes available a voltage Vn where |Vn|≦|V(n−1)|. The modules are connected positively in series, with the result that, during the time period tn, at least one voltage is available whose magnitude is |V1|+|V2|+ . . . +|V(n−1)|+|Vn|=Vmax. Vmax is then the amplitude of the AC voltage. Again, pulse width modulation can be used in order to make available the voltage Vn on average. This variant of the first embodiment is illustrated by way of example in
The intervals ti, t2, . . . , tn are centered around the time t0+T/4, wherein the half-cycle begins at t0, for the generation of the first half-cycle of a voltage profile approximating a sinusoidal curve.
In this first embodiment, the assignment of interval to module is kept in the second half-cycle. That is to say that, in the second half-cycle, the most powerful module weakest module 11-1 is again connected for a time period ti, the i-th most powerful module 11-i, 1<i<n, is again connected for a time period ti≦tj, for all 1≦j<i, and the weakest module 11-n is again connected for a time period tn≦tj for all 1≦j≦n. In the second half-cycle, the connection is negative, however. The magnitudes of the generated voltages are the same as in the first half-cycle. Again, the intervals ti, t2, . . . tn are centered, but around the time t0+3*T/4 in the second half-cycle.
The hierarchization of the modules according to how powerful they are can be performed using calculated and/or measured variables. Thus, for example, the total use duration of the modules in the BMS can be tracked using the connection durations and can be used for the hierarchization according to power level. In addition or alternatively, a present state of charge of the modules can be determined on the basis of measured values such as voltage, current and/or capacitance, and the hierarchization of the modules can take place according to state of charge.
In this case, it is advantageous to update the hierarchization after each period so that actual states of charge, wear and/or ageing are used for the hierarchization.
In a second embodiment, sketched by way of example in
In the second embodiment, the loading of the modules within a period is identical or approximately identical. In particular, the loading of the modules which are connected for the longest in a half-cycle and for the shortest in the other half-cycle is identical.
If the duration ti and the voltage Vi are selected, however, such that different total loads per period result for different i, it is possible to hierarchize the different i according to the total loads per period. The weakest module can then be connected for the duration ti in the first half-cycle for which the total load is at the lowest in the two half-cycles. Correspondingly, more powerful modules can be subjected to greater loads and the most powerful module can be subjected to the greatest load.
In a third embodiment, the connection is not performed according to power, but according to voltage made available. In order to increase the magnitude of the voltage by a magnitude |Vi|, the module whose voltage made available is greater than or equal to |Vi| in magnitude and whose magnitude has the smallest discrepancy in relation to |Vi| is then connected. Thus, a particularly good approximation to a sinusoidal profile can be achieved.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102012216158.9 | Sep 2012 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/065139 | 7/18/2013 | WO | 00 |
