Abstract:
A device for measuring a maximum cell voltage among cell voltages of a plurality of battery cells connected in series includes a plurality of ohmic resistors connected in series. The device is configured to be connected to a plurality of battery cells connected in series in such a way that a respective battery cell is associated with each ohmic resistor according to the series connections. Each ohmic resistor, with the exception of a first ohmic resistor, is configured to conduct the larger of (i) a current that corresponds to the cell voltage of the associated battery cell, and (ii) the current that is conducted by the preceding ohmic resistor in the series connection.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2012/063353, filed on Jul. 9, 2012, which claims the benefit of priority to Serial No. DE 10 2011 079 360.7, filed on Jul. 19, 2011 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     The present disclosure relates to a device and a method for measuring the maximum cell voltage among the cell voltages of a multiplicity of series-connected battery cells, and to a battery management unit having such a device, to a battery having such a device or such a battery management unit and to a motor vehicle having such a battery. 
     BACKGROUND 
     It is likely that, in future, new battery systems on which very high demands in terms of reliability are placed will increasingly be used both in static applications, for example in wind power installations, and in vehicles such as hybrid and electric vehicles. The reason behind said high demands is that failure of the battery can lead to failure of the entire system or even to a safety-related problem. Thus, in wind power installations, for example, batteries are used in order to protect the installation against inadmissible operating states in a high wind by virtue of rotor blade adjustment. 
     Usually nowadays, lithium ion batteries involve the voltage of each cell being monitored individually, in order to be able to protect said cells from overloading during the charging process. Typically, a non-generic integrated circuit having multiplexers and analog/digital converters is used for this purpose, which integrated circuit communicates with a control unit. This solution is complex and expensive. 
       FIG. 1  illustrates the principle of such monitoring of an individual cell in accordance with the prior art. A battery management unit  10  comprises an integrated circuit  14  that is electrically connected to each of the battery cells  12   a ,  12   b , . . .  12   n  of a battery. The integrated circuit  14  comprises a multiplexer and an analog/digital converter and is connected to a control unit  18  by way of a communication bus  16 . 
     A charging circuit for battery cells is known from DE 10 2006 033 171 A1, in which in each case a bypass is assigned to the individual battery cells, via which bypass a charging current is conducted to the individual battery cells depending on the charging state thereof. The charging process can be terminated if all of the battery cells or a predetermined number of battery cells have reached the maximum permissible voltage value thereof. 
     SUMMARY 
     The disclosure provides a device for measuring the maximum cell voltage among the cell voltages of a multiplicity of series-connected battery cells, wherein the device comprises a multiplicity of series-connected nonreactive resistors, wherein the device is connectable to a multiplicity of series-connected battery cells in such a way that a battery cell is assigned to each nonreactive resistor in accordance with the series connections, wherein a first nonreactive resistor, which precedes all of the other nonreactive resistors in the series connection, is designed to conduct a current which corresponds to the cell voltage of the assigned battery cell, and wherein each nonreactive resistor except the first nonreactive resistor is designed to conduct the greater of a current which corresponds to the cell voltage of the assigned battery cell and the current which is conducted by the preceding nonreactive resistor in the series connection. 
     Preferably, each of the nonreactive resistors has the same resistance value R. Preferably, the current corresponding to a cell voltage U is given by (U−ΔU)/R, wherein ΔU is a predetermined voltage. 
     In a preferred embodiment, the device also comprises, for each of the multiplicity of nonreactive resistors an associated diode, an associated transistor and an associated reference voltage source, wherein in each case the anode of each diode is connectable to the positive pole of a battery cell assigned to the associated nonreactive resistor, the cathode of each diode is connected to a first connection of the associated nonreactive resistor, a second connection of each nonreactive resistor is connected to a first connection of the associated transistor, the control connection of each transistor is connected to the positive pole of the associated reference voltage source, the negative pole of each reference voltage source is connectable to the negative pole of a battery cell assigned to the associated nonreactive resistor, and the first connection of each nonreactive resistor except the first nonreactive resistor is connected to a second connection of the transistor associated with the preceding nonreactive resistor in the series connection. Preferably, the reference voltage sources are designed to provide in each case the same reference voltage, and the predetermined voltage AU is given by said reference voltage. 
     In a further preferred embodiment, the device also comprises for each of the multiplicity of nonreactive resistors an associated first transistor, an associated second transistor and an associated third transistor, wherein in each case the control connection of each first transistor is connected to a first connection of the associated second transistor, to the control connection of the associated second transistor and to the control connection of the associated third transistor, a first connection of each third transistor is connectable to the positive pole of the battery cell assigned to the associated nonreactive resistor, a second connection of each second transistor is connectable to the negative pole of the battery cell assigned to the associated nonreactive resistor, the control connections, which are connected together, of the first, second and third transistors are in each case connected via an associated additional nonreactive resistor to the first connection of the associated third transistor, a first connection of the first nonreactive resistor is connected to the first connection of the associated third transistor, a first connection of each nonreactive resistor except the first nonreactive resistor is connected to a second connection of the first transistor associated with the preceding nonreactive resistor in the series connection, a second connection of each nonreactive resistor is connected to a first connection of the associated first transistor and the second connection of each first transistor is connected to the second connection of the associated third transistor. 
     Preferably, the first transistors have a first base-emitter voltage, the second and third transistors have a second base-emitter voltage, and the predetermined voltage ΔU is given by the sum of the first base-emitter voltage and the second base-emitter voltage. The first base-emitter voltage and the second base-emitter voltage may be the same or different. 
     The device can be connected to a multiplicity of series-connected battery cells in such a way that a battery cell is assigned to each nonreactive resistor in accordance with the series connections. Preferably, the battery cells are lithium ion battery cells. 
     The disclosure also provides a battery management unit having a device according to the disclosure, a battery having a device according to the disclosure or a battery management unit according to the disclosure, and a motor vehicle, in particular an electric motor vehicle, having a battery according to the disclosure. 
     The disclosure further provides a method for measuring the maximum cell voltage among the cell voltages of a multiplicity of series-connected battery cells, wherein a first current which corresponds to the cell voltage of a first battery cell is induced, said first battery cell preceding all of the other battery cells in the series connection, and wherein further currents are induced, wherein each further current is assigned to a battery cell and wherein each further current is the greater of a current which corresponds to the cell voltage of the assigned battery cell and the current assigned to the battery cell which precedes the assigned battery cell in the series connection. 
     Advantageous developments of the disclosure are specified in the dependent claims and described in the description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the disclosure are described in more detail with reference to the drawings and the following description. In the drawings: 
         FIG. 1  shows a circuit diagram of a device for individually monitoring the voltage of battery cells in accordance with the prior art, 
         FIG. 2  shows a diagram for representing the principle of the disclosure, 
         FIG. 3  shows a first exemplary embodiment of a device according to the disclosure for measuring a maximum cell voltage, and 
         FIG. 4  shows a second exemplary embodiment of a device according to the disclosure for measuring a maximum cell voltage. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows the principle according to which in accordance with the disclosure the maximum cell voltage among the cell voltages of a multiplicity of series-connected battery cells is ascertained. The battery to be monitored comprises a series of battery cells  22   a ,  22   b ,  22   c , . . . . In a comparison step  24   a , the cell voltage of the battery cell  22   a  is compared with a voltage that has been input at  26   a , and the larger of the two voltages is output at  26   b . Likewise, in a comparison step  24   b , the voltage input at step  26   b  is compared with the cell voltage of the battery cell  22   b , and the larger of the two voltages is output at  26   c . The largest of the cell voltages of all battery cells  22   a ,  22   b ,  22   c , . . . is determined by continuing this method for all battery cells. 
       FIG. 3  shows a first exemplary embodiment of a device according to the disclosure for measuring the maximum cell voltage among the cell voltages of a multiplicity of series-connected battery cells. The cell voltages U C1 , U C2 , U C3 , . . . are tapped via diodes D 1 , D 2 , D 3 , . . . that are considered as idealized diodes having infinitesimal forward voltage drop. By means of nonreactive resistors R 1 , R 2 , R 3 , . . . and transistors T 1 , T 2 , T 3 , . . . the cell voltages in each case are compared with reference voltage sources U ref,1 , U ref,2 , U ref,3 , . . . , wherein the transistors T 1 , T 2 , T 3 , . . . are regarded as idealized transistors, for example PNP transistors having infinitesimal base-emitter voltage drop. The term ‘base-emitter voltage of a transistor’ is understood to refer to the relevant voltage that is present between the base and the emitter of the transistor when a current flows between the emitter and the collector; this voltage is approximately independent of the current flowing between the emitter and the collector. Ideally, all of the nonreactive resistors R 1 , R 2 , R 3 , . . . have the same resistance value R, and all of the reference voltage sources U ref,1 , U ref,2 , U ref,3 , . . . provide the same voltage U ref . 
     For the purposes of the following consideration, the cell having the cell voltage U c3  is assumed to be the first cell of the battery; however, the results correspondingly apply to any number of cells. In the case of the described idealized properties of the components, the voltage U c3 -U ref  is applied to the nonreactive resistor R 3 , with the result that a current I 3 =(U c3 −U ref )/R flows. This current flows onwards through the nonreactive resistor R 2 , with the result that at said resistor at least the voltage R I 3 =U c3 −U ref  drops off. If this voltage is greater than the difference U c2 −U ref , then the diode D 2  becomes nonconductive, and only the current I 2 =I 3  flows through the nonreactive resistor. However, if the voltage is smaller, in other words, if U c3 −U ref &lt;U c2 −U ref  and therefore U c2 &gt;U c3 , then the diode D 2  becomes conductive, and the voltage U c2 −U ref  is present at the nonreactive resistor R 2 , with the result that a current I 2 =(U c2 −U ref )/R flows. In both cases, a current I 2  consequently flows, which current corresponds to the greater of the two voltages U c2  and U c3 , in other words I 2 =(max (U c2 , U c3 )−U ref )/R. 
     The same applies for the diode D 1  and the nonreactive resistor R 1 , with the result that the current I 1 =(max (U c1 , max (U c2 , U c3 ))−U ref ) R=(max (U c1 , U c2 , U c3 )−U ref )/R flows through the nonreactive resistor R 1  and the voltage max (U c1 , U c2 , U c3 )−U ref  drops off at said nonreactive resistor. As a result, the sought maximum cell voltage max (U c1 , U c2 , U c3 ) can be tapped at U out . 
       FIG. 4  shows a second exemplary embodiment of a device according to the disclosure for measuring the maximum cell voltage among the cell voltages of a multiplicity of series-connected battery cells, wherein the non-ideal properties of the components are taken into account. Here, instead of the diodes D 1 , D 2 , D 3 , . . . , in each case two additional transistors T a,n , T a,n-1 , . . . and T b,n , T b,n-1 , . . . and an additional nonreactive resistor R′ n , R′ n-1 , . . . are provided for each battery cell. In this case, all of the transistors T a,n , T a,n-1 , . . . and T b,n , T b,n-1 , . . . are ideally constructionally identical to one another and at the same temperature, with the result that they have identical characteristics; likewise, the transistors T n , T n-1 , . . . are constructionally identical to one another. 
     The collectors and bases of the transistors T a,n , T a,n-1 , . . . are connected in each case and the cell voltages U c,n , U c,n-1 , . . . are applied to the transistors T a,n , T a,n-1 , . . . via the additional nonreactive resistors R′ n , R′ n-1 , . . . . Thus, the transistors T a,n , T a,n-1 , . . . are conductive independently of the currents flowing in the nonreactive resistors R n , R n-1 , . . . , and in each case the base-emitter voltage U BE  drops between base and emitter. In addition, the collectors and bases of the transistors T a,n , T a,n-1 , . . . are in each case connected to the bases of the transistors T n , T n-1 , . . . . As a result of this, the transistors T n , T n-1 , . . . are also conductive as long as the sum of two base-emitter voltages does not exceed the cell voltage. 
     Thus, the voltage U n =U c,n −(U′ BE +U BE ) is present across the nonreactive resistor R n , wherein U′ BE  is the base-emitter voltage of the transistors T a,n , T a,n-1 , . . . and T b,n , T b,n-1 , . . . and U BE  is the base-emitter voltage of the transistors T n , T n-1 , . . . . Correspondingly, the current U n /R flows through the nonreactive resistor R n . This current flows onward through the nonreactive resistor R n-1 , with the result that at least the voltage U n  drops across said nonreactive resistor R n-1 . 
     The voltage U n-1  across the resistor R n-1  results from the sum of the cell voltage U c,n-1  and the voltages which are present in each case between base and emitter of the transistors T a,n , T b,n , T n-1  and T a,n-1 . In this case, the voltages associated with T a,n  and T a,n-1  cancel each other out, since said transistors are conductive and constructionally identical and the associated voltages come into the sum with opposite mathematical signs. If U c,n-1 &lt;U c,n  and, as a result, U c,n-1 &lt;R I n +(U′ BE +U BE ), then the transistor T b,n  becomes nonconductive and only the current I n-1 =I n  flows through the nonreactive resistor R n-1  at a corresponding voltage U n-1 =U n . Conversely, if U c,n-1 &lt;U c,n , then the transistor T b,n  becomes conductive and the voltage U n-1 =U c,n-1 −(U′ BE +U BE ) drops across the nonreactive resistor R n-1 . Therefore, in both cases, U n-1 =max (U c,n-1 , U c,n-1 )−(U′ BE +U BE ). This correspondingly applies for the further stages of the circuit, such that the voltage max (U c,n-1 , U c,n-1 , . . . , U c1 )−(U′ BE +U BE )+(U′ BE +U BE )=max (U c,n-1 , U c,n-1 , U c1 ), that is to say the sought maximum cell voltage, is present between the ground and the upper connection of the nonreactive resistor R 1  (not shown), it being possible for said maximum cell voltage to be tapped there and used for monitoring the battery. 
     The above-described device can be used as part of a battery management unit which monitors the maximum cell voltage of the battery cells of a battery and protects the battery cells against overloading. A battery management unit of this type can be used as part of a battery, in particular a battery used in a motor vehicle.