Abstract:
An electrochemical energy accumulator ( 100 ) and a method for balancing a multiplicity of sections (S 1,  S 2,  S 3 ) of electrochemical accumulator modules ( 10 ) connected in parallel with one another by means of at least one UniBB module are proposed. In this context, the following steps are carried out: detecting (S  100 ) a first state of charge (I) of a first accumulator module (M 1 ), detecting (S 200 ) a second state of charge (II) of a second accumulator module (M 2 ), wherein the second accumulator module (M 2 ) is a UniBB module, and operating (S 300 ) the second accumulator module (M 2 ) as a power source in order to adapt the first stage of charge (I) and the second state of charge (II) to one another.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to an electrochemical energy store and to a method for balancing a multiplicity of strings of electrochemical storage modules of an electrochemical energy store connected in parallel with one another. Electrochemical energy stores consist of individual structural units (“modules”) in order to be able to provide the high currents which are required for some applications and which can no longer be provided by a single module. For said application it is necessary to connect modules in parallel. This is in turn not readily possible with standard modules since compensation currents that can lead to undesired operating states can flow between the modules. The modules of present rechargeable battery cells typically contain fluids based on lithium chemistry. These cells are sensitive both to overcharging and to deep discharge. An overcharging above approximately 4.2 V per cell or module results in exothermic processes that can lead to the destruction of the cell. A deep discharge below approximately 2.5 V per cell or module leads to permanent degradation of the energy storage and current carrying capacity of the cell. If cells are connected in series, then they are charged and discharged only jointly. In other words, the charge changes of each cell of the string are identical. Differences in the cell constitution then have the effect that the state of charge (SOC) of the cells is no longer identical with increasing use. This may have the effect that during charging individual cells may be overcharged, while other cells are not yet fully charged. The latter alternative entails the risk that during discharging individual cells may be subjected to deep discharge, even though other cells are not yet fully discharged. For this reason, the state of the rechargeable battery cells is monitored permanently and charging or discharging is possibly prevented. A process of charge equalization—also referred to as “balancing”—between the cells of a rechargeable battery pack (“electrochemical energy store”) pursues the aim of all the cells having the same SOC. The balancing of the cells within a module is ensured by an integrated measuring and balancing circuit designed for this purpose. 
         [0002]    Moreover, the prior art discloses so-called UniBB (universal buck-boost) modules comprising an electrochemical energy store (e.g. lithium ion based or lithium polymer based) and an electrical circuit by means of which they can realize different terminal characteristics. By way of example, a UniBB module can be used both as voltage source and as current source. The electrical circuit within the UniBB module could also be referred to as “coupling unit” which contains an inductance. The construction and the functioning of a UniBB module is known for example from the prior art. 
         [0003]    CN 102496970 discloses a balancing system for an electrochemical energy store of an electrically operated means of locomotion. The electrochemical energy store contains two storage strings connected in parallel with one another. Each of the storage strings contains individual storage cells. Via a resistance and a capacitance, an SOC value of the storage strings is determined and energy of individual cells is shifted to other cells having a lower SOC. 
         [0004]    US 2011/025258 discloses an interconnected system of electrochemical storage cells in which balancing is carried out whilst avoiding overcharging or deep discharge of the cells. The document discloses a control unit that monitors the SOC of the cells and groups the cells into dischargeable and chargeable cells on the basis of their SOC. Depending on their group association the cells are charged either individually or jointly. 
         [0005]    US 2009/208824 discloses an interconnected system of electrochemical storage cells which are connected in parallel with one another. The cells have a respective controller, which can be configured as a buck-boost controller. If it is recognized that the cells have different states of charge, the energy drawn from the individual cells is varied. This prevents different cells from having different states of charge for a long period of time. 
         [0006]    It is an object of the present invention to improve balancing of electrochemical stores connected in parallel. 
       SUMMARY OF THE INVENTION 
       [0007]    The abovementioned object is achieved according to the invention by means of an electrochemical energy store and a method for balancing a multiplicity of strings of electrochemical storage modules connected in parallel with one another by means of at least one UniBB module. The method comprises identifying a first state of charge of a first storage module. In addition, the state of charge of a second storage module is identified. The first and second storage modules are arranged in different strings. The second storage module is configured as a UniBB module and is operated as a current source in order to adapt the first state of charge and the second state of charge to one another. By virtue of the fact that the UniBB module is operated as a current source, charge can be shifted between the first storage module and the second storage module in a controlled manner, without impermissibly high compensation currents flowing on account of great differences in the state of charge and/or great differences in the terminal voltage. 
         [0008]    Preferably, the first storage module is also a UniBB module and is operated as a voltage source during the balancing described above. In particular, all further storage modules within the electrochemical energy store are also configured as UniBB modules. In this case, all storage modules with the exception of the second storage module are operated as a voltage source. In this way, it is possible to control the balancing according to the invention according to each of the storage modules as a current source. 
         [0009]    Preferably, the second state of charge is lower than the first state of charge. In other words, that storage module having the lower state of charge is switched as a current source. The latter then controls a current flow by means of which its own SOC is raised. Alternatively, the first state of charge may also be higher, such that the first storage module is switched as a current source and increases its own SOC. It goes without saying that it is also possible to operate the respective storage module in the current source mode in such a way that the storage module in the current source mode decreases its own SOC. 
         [0010]    By way of example, the first storage module may be operated in a first operating mode from the group “regulated”, “unregulated”, “buck” or “boost”. Correspondingly, the second storage module which is arranged in a different string than the first storage module, is operated in a second operating mode of the same group, wherein the second operating mode is preferably not identical to the first operating mode. This configuration avoids impermissible operating states and in particular also an increased energy expenditure and wear during balancing. 
         [0011]    With further preference, the first storage module may be operated unregulated and the second storage module may be operated in the boost current source operating mode. Alternatively, the first storage module may be operated in the buck charging mode and the second storage module may be operated in the current source mode. This configuration constitutes a preferred and particularly advantageous combination of two operating states in different strings of arranged storage modules. 
         [0012]    With further preference, the multiplicity of storage modules within the electrochemical energy store comprises further electrochemical storage modules. The latter are put into a blocking mode if they belong to strings not involved in the balancing, and are put into a bypass mode if they belong to strings involved in the balancing. In this case, the blocking mode constitutes an interruption of the electric circuit, while the bypass mode corresponds to an external short circuit of the respective storage module, such that a current flow can take place past the latter without being impeded. A simple and efficient selection of the strings participating in the balancing is possible in this way. 
         [0013]    If only one storage module is contained per string, a storage module having a highest state of charge may be operated as a voltage source and the other electrochemical storage modules not put into a blocking mode may be operated in a current source mode. However, if more than one storage module is contained per string (in other words if series-connected storage modules are present within the electrochemical energy store) a storage module having a highest state of charge is operated in a current source mode and the other electrochemical storage modules not put in a blocking mode or bypass mode are operated in a voltage source mode. In this way, the balancing can be performed with the least possible electrical loss. 
         [0014]    With further preference, a charging current impressed into the multiplicity of electrochemical storage modules is controlled depending on a respective state of charge of the storage modules. The charging current is preferably controlled in such a way that a predefined, in particular complete state of charge of the storage modules is substantially attained at an identical point in time. In this way, the charging process can be effected in a time-optimized manner and the power loss can be reduced further. 
         [0015]    With further preference, electrochemical storage modules within the electrochemical energy store are put in a blocking mode as soon as they have attained a predefined maximum voltage. This is carried out in particular in such a case in which the electrochemical storage module is the only storage module in its string. If a second module has to be charged in the string while a first storage module has attained its predefined maximum voltage, the fully charged electrochemical storage module is to be put into a bypass mode in order still to enable a charging process of the remaining storage module. It is only if the last storage module within a respective string has attained its maximum voltage that at least one of the storage modules enters a blocking mode in order not to produce any unnecessary electrical losses during the further balancing process for remaining strings. 
         [0016]    It goes without saying that balancing may also be performed during the operation of the multiplicity of electrochemical storage modules of an electrochemical energy store. For this purpose, the strings, depending on a respective average state of charge, are involved in a total current to be supplied in order to assume a substantially identical state of charge. A string-by-string control of the discharge current may constitute a variant of the balancing according to the invention with particular reduction of losses, since, for the balancing, all modules take up energy (charging process) or emit energy (discharging process), without charge being shifted solely for balancing purposes. 
         [0017]    A second aspect of the present invention proposes an electrochemical energy store comprising a multiplicity of strings of electrochemical storage modules connected in parallel with one another, and a processing unit for carrying out a method in accordance with the first-mentioned aspect of the invention. In this case, each string may comprise a single electrochemical storage module or a multiplicity of electrochemical storage modules connected in series. The storage modules may all be configured as a UniBB module according to the first-mentioned aspect of the invention, the mode of operation being controlled or co-ordinated by the superordinate processing unit. In an alternative configuration, the processing unit is contained in an electrochemical storage module which (at times) performs a master function by carrying out or coordinating the method in accordance with the first-mentioned aspect of the invention. In principle, each storage module may comprise a processing unit, as a result of which it is designed for performing the master function mentioned above. The functioning of the electrochemical energy store according to the invention is evident according to that of the first-mentioned aspect of the invention, and so, for description of the features, feature combinations and advantages, reference is made to the above explanations in order to avoid repetitions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    Exemplary embodiments of the invention are described in detail below with reference to the accompanying drawings. In the drawings: 
           [0019]      FIG. 1  shows a circuit diagram of one exemplary embodiment of a UniBB module that is useable according to the invention; 
           [0020]      FIG. 2  shows a circuit diagram of an alternative exemplary embodiment of a UniBB module that is useable according to the invention; 
           [0021]      FIG. 3  shows a schematic circuit diagram regarding the parallel operation of three electrochemical storage modules; 
           [0022]      FIG. 4  shows a schematic circuit diagram regarding the discharge of electrochemical storage modules connected in parallel; 
           [0023]      FIG. 5  shows a schematic circuit diagram regarding the balancing of electrochemical storage modules connected in parallel; 
           [0024]      FIG. 6  shows a schematic circuit diagram regarding the balancing of electrochemical storage modules connected in parallel in a different operating state; 
           [0025]      FIG. 7  shows a schematic circuit diagram regarding the operation of parallel strings of electrochemical storage modules; 
           [0026]      FIG. 8  shows a schematic circuit diagram regarding the balancing of parallel-connected strings of electrochemical storage modules in a first operating state; 
           [0027]      FIG. 9  shows a schematic circuit diagram regarding the balancing of parallel-connected strings of electrochemical storage modules in a second operating state; 
           [0028]      FIG. 10  shows a schematic circuit diagram regarding the balancing of parallel-connected strings of electrochemical storage modules in a third operating state; 
           [0029]      FIG. 11  shows a flow diagram illustrating steps of one exemplary embodiment of a method according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]      FIG. 1  shows the circuit diagram of one exemplary embodiment of a UniBB module  10 . Via a first terminal  11  and a second terminal  12  the UniBB module  10  is designed to be interconnected with further UniBB modules to form a string. Four semiconductor switches T 1 , T 2 , T 3 , T 4 , which are preferably configured as MOSFETs or IGBTs, with associated freewheeling diodes D 1 , D 2 , D 3 , D 4  are arranged between the first terminal  11  and the second terminal  12 . The semiconductor switches T 1  to T 4  may be combined with the respective freewheeling diodes D 1  to D 4  as electrical two-terminal networks ZP 1  to ZP 4 . In this case, a first terminal of the first two-terminal network ZP 1  is connected to a positive terminal of the energy store  7 . A second terminal of the first two-terminal network ZP 1  is connected firstly to a first terminal of the fourth two-terminal network ZP 4  and then secondly via an inductance L to a first terminal of the second two-terminal network ZP 2  and also a second terminal of the third two-terminal network ZP 3 . A first terminal of the third two-terminal network ZP 3  is connected to the first terminal  11  of the UniBB module  10 . A first terminal of a capacitance C is also connected to the first terminal  11 , the second terminal of said capacitance being connected to the second terminal  12  of the UniBB module  10 . The second terminals of the second two-terminal network ZP 2  and of the fourth two-terminal network ZP 4 , respectively, are connected firstly to the second terminal  12  of the UniBB module  10  and secondly to a second terminal of the electrical energy store  7 . The energy store  7  supplies a module voltage U M . The UniBB module  10  illustrated has a positive pole (plus pole) at the first terminal  11  and a negative pole (minus pole) at the second terminal  12 . Control lines for driving the semiconductor switches T 1  to T 4  are not illustrated for the sake of clarity. The same applies to current sensors. The electrical energy store  7  may consist of one or a plurality of electrochemical cells  1  or other electrical energy storage units, which in this case jointly provide the module voltage U M . The UniBB module  10  may assume a plurality of different operating states, depending on how the semiconductor switches T 1  to T 4  are actuated. In particular, a bypass, a voltage source in the buck or boost mode, a current source in the buck or boost mode, a charging circuit and a blocking may be realized. For details, reference is made to the prior art. 
         [0031]      FIG. 2  shows a circuit diagram of an alternative UniBB module  10  to the UniBB module  10  illustrated in  FIG. 1 . Compared with the arrangement illustrated in  FIG. 1 , the capacitance C and transistor T 4  (but not the diode D 4  thereof) are omitted. Moreover, only a single storage cell  1  is contained for the sake of clarity. 
         [0032]      FIG. 3  shows a circuit diagram of an arrangement  100  in which parallel-connected storage modules  10  (individually identified by M 1 , M 2  and Mn) are arranged between a charger L and a load V. The charger L is connectable to the parallel circuit of the modules  10  via a charging contactor SL, and the load V is connectable to the parallel circuit of the modules  10  via a load contactor SV. An arrow P is used to indicate that further modules  10  could be supplementarily connected. The modules  10  are connected by their negative terminal in the direction of an electrical ground and by their positive terminal to a common busbar  2 . The different switching states will be discussed in greater detail in association with the subsequent figures. 
         [0033]      FIG. 4  shows the circuit  100  illustrated in  FIG. 3  in a state in which the charger L, on account of an opened charging contactor SL, has no effect on the functioning and is therefore not illustrated. The load V is not connected to the modules  10  either, since the load contactor SV has been opened. The arrangement illustrated represents for example a case in which an electrically drivable vehicle is not connected to a charging station and is at a standstill. While the module Mn has a full state of charge SOC I, the module M 1  has a second state of charge SOC II that is lower than the state of charge I. A balancing between the modules M 1 , Mn takes place by means of the current I B . For this purpose, either the module M 1  may be operated as a voltage source and the module Mn as a current source, or vice versa. In both configurations, a plurality of combinations of operating states from the group “regulated”, “unregulated”, “boost”, “buck” are possible. By way of example, when the module M 1  is operated as a voltage source and the module Mn as a current source, the module M 1  may be unregulated. The module Mn must then operate in the boost current source mode since its voltage must indeed be higher than the terminal voltage of the module M 1 . Alternatively, it is possible for the module M 1  to operate in the buck charging mode (wherein the voltage present is lower than the terminal voltage of the module Mn). In this case, the module Mn is operated in the current source mode. The method that generates the least loss will be used in practice. The corresponding considerations are familiar to the person skilled in the art for voltage converters, for which reason reference is made to the relevant technical literature for further considerations. The module M 2  forms a string S 0  not involved in the balancing method. It is put into a blocking mode in which a current flow through the string S 0  cannot take place. In other words, an electrical connection between ground and the busbar  2  is interrupted within the string S 0 . If more than two modules, for example three or more modules, are intended to interact during the balancing, then it is assumed that the module having the highest SOC (for example the module Mn) operates as a voltage source and the modules to be charged (the modules M 1 , M 2 ) operate in the current source mode. An internal balancing within a module generally takes place by means of the resistive method. That means that the state of charge of cells  1  having a high SOC is brought to the state of cells  1  having a lower SOC by resistive loading. This process takes place periodically as soon as the vehicle is at a standstill or the load V is disconnected from the modules  10  by means of the load contactor SV. Consequently, the SOC of modules  10  also varies over time even if they are not included in the balancing method described above. An iterative module balancing taking account of all modules  10  in the manner described above is necessary for this reason. 
         [0034]      FIG. 5  shows the circuit  100  illustrated in  FIG. 3  in an operating state of a balancing during charging. In the example illustrated, the module M 1  is again assumed to have a lower SOC II than the module Mn (which has a higher SOC I). The charger L is assumed to act as a voltage source since the charging current regulation of the modules  10  is controlled such that the charging current I 1  into the module M 1  is somewhat higher than the charging current In into the module Mn. By means of suitable setting of the charging currents I 1 , In, both modules  10  are fully charged in the same time. This principle can be generalized to all modules  10 . Modules  10  having a lower SOC are charged to a greater extent than those having a higher SOC. Modules in which the storage cell  1  (e.g. a rechargeable battery) has attained the maximum voltage are put into the blocking mode. If appropriate, internal (resistive) balancing then begins after the end of which the module  10  as a whole can again enter the charging mode in order to maximize its SOC. For the sake of simplicity, the load contactor SV has been opened, such that the circuit  100  is in a pure charging state. 
         [0035]      FIG. 6  shows the circuit  100  illustrated in  FIG. 3  in a balancing method during the discharging. In other words, the charger L has been disconnected from the modules  10  by the opening of the charging contactor SL while the load V is connected to the modules  10  via the busbar  2  by virtue of a closed load contactor SV. The states of charges I, II correspond to the relationships discussed in association with  FIGS. 4 and 5  for the sake of comparability. With an identical SOC of all the modules  10 , it is desirable for the discharge currents I 1 , I 2 , In for all the modules  10  to be identical. However, if different SOCs (different states of charge I, II) are present, then it is desirable for those modules  10  having a higher SOC to be discharged to a greater extent, and modules having a lower SOC to be discharged to a lesser extent. In  FIG. 6 , I 1  less than In would have to prevail for this purpose. Since the modules  10  are in the current source mode anyway, such a regulation can be carried out in a simple manner. The regulated variable for the current intensity is the respective SOC of the modules  10 . The charger L is not involved in the balancing as a result of a charging contactor SL having been opened, such that a pure discharging process is illustrated. 
         [0036]      FIG. 7  shows the circuit  100  illustrated in  FIG. 3  for the case where the modules  10  have been supplemented by further modules respectively connected in series (specifically identified by M 1 - 1  to M 3 - n ). The modules M 1 - x  form a first string S 1 . The modules M 2 - x  form a second string S 2 . The modules M 3 - x  form a third string S 3 . It goes without saying that further parallel strings may be provided in reality. 
         [0037]      FIG. 8  shows the energy store  100  illustrated in  FIG. 7  during balancing at a standstill (without charging). This is the state in which the machine or the application (for example a vehicle or an automobile as load V) which is driven by the electrochemical energy store  100  illustrated is deactivated, without involvement of the charger L, for example by the charging contactor SL and the load contactor SV being opened. In the simplest type of embodiment, only two modules  10  in different strings S 1 , S 2 , S 3  interact with one another. It is assumed in  FIG. 8 , for example, that the module M 1 - 2  has a lower SOC II than the module M 2 - n  (which has a higher SOC I). Firstly, the string contactors SS 1  and SS 2  are closed, and unaffected modules M 3 - 1  to M 3 - n  are put into the bypass mode e.g. by virtue of the closing of the string contactor SS 3 . In order to balance the SOCs I, II, the balancing current I B  must flow from the module M 2 - n  in the direction of the module M 1 - 2 . Consequently, either the module M 1 - 2  may be switched as a voltage source and the module M 2 - n  as a current source, or vice versa. In both cases, a plurality of combinations of operating states from the group “regulated”, “unregulated”, “buck”, “boost” are possible. By way of example, when the module M 1 - 2  is operated as a voltage source, the module M 1 - 2  may be unregulated. The module M 2 - n  must then operate in the boost current source mode since its voltage must indeed be higher than the terminal voltage of the module M 1 - 2 . Alternatively, it is also possible for the module M 1 - 2  to operate in the buck charging mode (wherein the voltage present is lower than the electrical voltage of the storage cell  1  of the module M 1 - 2 ), and the module M 2 - n  to operate in the current source mode, etc. The method that generates the least loss will be used in practice. Unaffected modules  10  are put into the bypass mode, in which they permit a current flow through the respective string without impediment. If more than two modules  10 , for example three (or more) modules  10 , are intended to interact, then it is assumed that the module  10  having the highest SOC operates as a current source, while the modules  10  to be charged operate in the voltage source mode. The module  10  having a higher SOC must be situated in a different string than the two modules  10  having a weaker SOC, which must be situated in series in the same string S 1 , S 2 , S 3 . If the modules  10  to be balanced with regard to their state of charges are situated in the same string S 1 , S 2 , S 3 , for example the module M 1 - 2  having a lower SOC II and the module M 1  -n having a higher SOC I, a direct balancing between the affected modules is not possible. Firstly, a module  10  in an adjacent string S 1 , S 2 , S 3  must be used as a charge source for the weaker module M 1 - 2 , for example the module M 3 - 1 . Afterward, the module M 3 - 1  can then be charged again by the stronger module M 1 - n . By means of this intermediate step, it is possible to balance two arbitrary modules  10  even in the same string S 1 , S 2 , S 3  with regard to their state of charge SOC. Internal balancing within a module  10  generally takes place by means of the resistive method. This means that the state of charge SOC of cells  1  having a high SOC are brought to the state of cells  1  having a lower SOC by resistive loading. This process takes place periodically as soon as the vehicle is at a standstill or the load V as well as the charger L are not connected to the modules  10 . Consequently, the SOC of modules  10  also varies over time even if they are not included in the above balancing method. An iterative module balancing taking account of all modules  10  according to the manner described above is necessary for this reason. 
         [0038]      FIG. 9  shows the circuit  100  presented in association with  FIG. 7  during a balancing in the course of a charging process. By way of example, the module M 1 - 2  is assumed to have a lower SOC II than the module M 2 - n  (which has a higher SOC I). The charger L is assumed to act as a voltage source and the charging current regulation of the strings S 1 , S 2  is controlled such that the charging current I 1  in the first string S 1  is somewhat higher than the charging current I 2  in the second string S 2 . Both modules M 1 - 2 , M 2 - n  are intended to be fully charged in the same time in this way. Strings S 1  having a lower SOC II are charged to a greater extent than strings S 2  having a higher SOC I. Modules  10  in which the storage cell  1  (e.g. a rechargeable battery) has attained the maximum voltage are put into the bypass mode. In the latter, a current I 3  may be routed past the modules M 3 - 1  to M 3 - n  without being impeded. If the last module  10  of a string is fully charged, all the modules  10  are brought to the blocking mode or the corresponding string contactor SS 1 , SS 2 , SS 3  is opened. 
         [0039]      FIG. 10  shows the circuit  100  presented in  FIG. 7  during a balancing in the course of discharging. Given an identical SOC of all the strings S 1 , S 2 , S 3 , it is desirable for the discharge currents I 1 , I 2 , I 3  of all the strings S 1 , S 2 , S 3  to be identical. However, if different SOCs I, II are present, as is illustrated in  FIG. 10 , then it is desirable for a string S 2  having a higher average SOC Ito be discharged to a greater extent and a string S 1  having a lower average SOC II to be discharged to a lesser extent. In  FIG. 10 , it would then be the case that I 1 &lt;I 2 . Since the strings S 1 , S 2 , S 3  are in the current source mode anyway in the course of discharging, such a regulation can be carried out in a simple manner. In this case, the regulated variable is the average SOC of the strings S 1 , S 2 , S 3 . If permitted by the power requirements of the load V, weak modules M 1 - 2  may temporarily be put completely into the bypass mode. The role of the weak module M 1 - 2  may then be taken on by a module having a higher SOC in the boost mode in the same string S 1 . Module balancing within a string S 1 , S 2 , S 3  may take place as a result of this mechanism. 
         [0040]      FIG. 11  shows a flow diagram illustrating steps of one exemplary embodiment of a method according to the invention. In step S 100 , a first state of charge of a first storage module  10  is identified. In step S 200 , a second state of charge of a second storage module  10  is identified. The second storage module  10  is a UniBB module. The latter is operated as a current source in step S 300  in order to adapt the first state of charge SOC I and the second state of charge SOC II to one another. In other words, a balancing between the first storage module  10  and the second storage module  10  is carried out by the operation of the UniBB module as a current source. 
         [0041]    Even though the aspects and advantageous embodiments according to the invention have been described in detail on the basis of the exemplary embodiments explained in association with the companying figures of the drawings, modifications and combinations of features of the exemplary embodiments illustrated are possible for the person skilled in the art, without departing from the scope of the present invention, the scope of protection of which is defined by the appended claims.