Abstract:
The disclosure describes a method for checking the proper method of operation of a current sensor which is designed to measure a battery current. During a precharge phase which begins at a time at which at least one electrical component is connected to a battery and during which the electrical component is precharged by a precharge current, at least one measured value provided by the current sensor is compared with an expected current value determined from an expected temporal profile of the precharge current. The disclosure also describes a computation unit and a battery which are designed to carry out the method according to the disclosure.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2011/064256, filed on Aug. 19, 2011, which claims the benefit of priority to Serial No. DE 10 2010 041 275.9, filed on Sep. 23, 2010 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     The present disclosure relates to a method for checking the proper method of operation of a current sensor, a computation unit that is designed to perform the method in accordance with the disclosure, a battery having the computation unit in accordance with the disclosure and also a motor vehicle having the battery in accordance with the disclosure. 
     PRIOR ART 
     Nowadays, batteries that are derived from lithium ion or nickel metal hydride technologies are installed in particular in hybrid vehicles and electric vehicles, which batteries comprise a large number of electrochemical battery cells that are connected in series. A battery management unit is installed for the purpose of monitoring the battery and in conjunction with safety monitoring is intended to ensure an operating life that is as long as possible. For this purpose, the voltage of each individual battery cell together with the battery current and the battery temperature is measured and the condition is estimated (by way of example of the charge condition or of the ageing condition of the battery). In order to maximize the operating life, it is helpful to know the actual prevailing maximum capacity of the battery, in other words the maximum electrical power that can be discharged or received. If this capacity is exceeded, the ageing of the battery will be drastically accelerated. 
     In order to be able to obtain a precise measurement of the battery current, it is necessary to monitor the proper method of operation of the current sensors that are used in the battery management unit. In particular, it is necessary to calibrate an amplification factor and an offset of each current sensor that is used. In addition, the quality of the measurement data supplied by each current sensor can be assessed and consequently checked for plausibility. 
     Furthermore, methods for limiting a start-up current in an electrical network that is connected to a battery are known from the prior art, in which network the battery is pre-charged during a start-up operation, so that a start-up current that flows through a battery and a connected consumer is reduced. A pre-charging of this type can by way of example be conducted by way of a pre-charge resistor. If in a system of this type the system voltage is adapted to the battery voltage, the pre-charge resistor is short-circuited and consequently deactivated. 
     SUMMARY 
     In accordance with the disclosure, a method for checking the proper method of operation of a current sensor is provided. The current sensor is designed for the purpose of measuring a battery current. Typically, the battery current is a total current that flows through a plurality of individual battery cells that are connected in series. During a pre-charge phase at least one measured value that is supplied by the current sensor is compared with an expected current value that is determined from an expected progression with respect to time of a pre-charge current. In this case, the pre-charge phase commences at a point in time at which at least one electrical component is connected to the battery. During the pre-charge phase, the electrical component is charged by way of the pre-charge current. By virtue of the fact that the pre-charge current that is flowing through the battery during the pre-charge phase is known in its progression with respect to time, the resultant calculated expected current values can be reconciled with the values actually measured by the current sensor. 
     The method in accordance with the disclosure represents an extremely cost-effective method; since it renders it possible to check for plausibility or rather calibrate the current sensors that are used in a battery without having to provide additional operating components and only by way of providing suitable software in a control unit of the battery management unit. 
     It is preferable that the electrical component is pre-charged during the pre-charge phase by way of a pre-charge resistor, the resistance value of which is generally known. Additionally, it is preferred that the electrical component is a capacitor and the expected progression with respect to time of the pre-charge current during the pre-charge phase consequently corresponds to an exponential function. If the resistance value of the pre-charge resistor and the capacity of the capacitor are known, it is even possible to calculate the exponential progression of the pre-charge current and then to assess the quality of the measured values that are supplied by the current sensor. 
     In a further embodiment of the disclosure, the electrical component is pre-charged during the pre-charge phase using a pre-charge current that is constant with respect to time. A uniform pre-charging of this type can be implemented by way of a connection that supplies a current that is constant with respect to time and the magnitude of which is exactly known. 
     On the basis of the comparison of the measured value supplied by the current sensor with the expected current value, it is possible to check for plausibility further measured values that are supplied by the current sensor. In particular, it can be decided whether the current sensor is defective. 
     In addition, the current sensor can be calibrated on said basis. In particular, the amplification factor and/or the offset of the current sensor can be corrected. 
     Before the commencement of the pre-charge phase a measured value that is supplied by the current sensor for monitoring the proper method of operation can be used since at this point in time a negligibly low battery current and consequently a measured value that is different from zero can be taken as cause to perform a corresponding correction of the offset of the current sensor. 
     A further aspect of the disclosure relates to a computation unit that is designed for the purpose of performing the method in accordance with the disclosure. 
     A further aspect of the disclosure relates to a battery having a computation unit in accordance with the disclosure as well as a motor vehicle that comprises a computation unit in accordance with the disclosure or a battery in accordance with the disclosure. The battery in accordance with the disclosure is preferably a lithium ion battery. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the disclosure are further explained with reference to the drawings and the description hereinunder. In which: 
         FIG. 1  illustrates an exemplary embodiment of a battery in accordance with the disclosure, and 
         FIG. 2  illustrates the progression with respect to time of a battery current I and a voltage U that is present at an electric component that is charged during a pre-charge phase. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an exemplary embodiment of a battery in accordance with the disclosure, designated as a whole by  100 . The battery  100  comprises a plurality of battery cells  10  that are connected in series, in particular lithium ion battery cells. An electric motor vehicle (not illustrated) comprises the battery  100 , to which an inverter  20  can be connected during the operating mode, which inverter converts a direct current, which is supplied by the battery  100 , into an alternating current that drives an electric motor  22 . A smoothing capacitor  24  is connected in parallel to the inverter  20  and the electric motor  22 , which smoothing capacitor is used for the purpose of smoothing a high frequency voltage ripple that is generated by the inverter  20 . A first positive relay  12  and a second negative relay  14  are provided on the terminals of the battery  100  and said relays can be opened in order to disconnect the electric motor vehicle from the voltage supply if said motor vehicle is parked by a user. A third relay  16  and a pre-charge resistor  18  are connected parallel to the first relay  12 . At the commencement of a driving cycle, the relays  12 ,  14 ,  16  are connected in a predefined circuit sequence. 
     If the pre-charge resistor  18  were not provided in the battery  100 , a high start-up current would flow through the battery  100  upon connecting the electrical components  20 ,  22 ,  24 , said high start-up current being due to the fact that the smoothing capacitor  24  is being charged. In order to limit the start-up current, the pre-charge resistor  18  is connected by way of closing the third relay  16  upstream of the electrical components  20 ,  22 ,  24 . If the pre-charge current has reduced or rather the voltage at the smoothing capacitor  24  has been adjusted to the battery voltage, the third relay  16  is opened and the first relay  12  is closed, as a consequence of which the pre-charge resistor  18  is short circuited. Any further charging of the smoothing capacitor  24  is limited in this phase only by the internal resistance of the battery  100 . The pre-charging of the battery by way of the pre-charge resistor  18  prevents damage to the first relay  12 . 
     The battery  100  comprises current sensors  26 ,  28  that are designed for the purpose of measuring the battery current that is flowing through the battery cells  10 . A first current sensor  26  comprises a shunt-resistance across which a voltage drop is measured. A second current sensor  28  is provided in the form of a Hall sensor that supplies an output voltage that is proportional to the product of the magnetic field strength of a magnetic field that penetrates said Hall sensor and of the battery current that is to be measured. The current sensors  26 ,  28  can have an unknown offset and an unknown amplification factor at a predefined point in time, which by way of an example in the case of the Hall sensor  28  can be as a result of the effect of a hysteresis. The measured values of the battery current that is supplied by the current sensors  26 ,  28  are fed into a computation unit in the form of a microcontroller  36  that together with the current sensors  26 ,  28  is a part of a battery management unit that is not illustrated. In addition the microcontroller  36  controls the relays  12 ,  14 ,  16  (not illustrated in  FIG. 1 ). 
     During a pre-charge phase, during which the first relay is opened and the two other relays  14 ,  16  are closed, the smoothing capacitor  24  is charged by way of the pre-charge resistor  18 , wherein a progression with respect to time of the pre-charge current that is flowing through the battery  100  and in particular through the pre-charge resistor  18  is known and can be calculated on the basis of the known resistance value of the pre-charge resistor  18  and the capacity of the smoothing capacitor  24 . The expected progression with respect to time of the pre-charge current is stored in the microcontroller  36 . During the pre-charge phase the microcontroller  36  compares the two measured values that are supplied by the current sensors  26 ,  28  at a predefined point in time with the expected current value that corresponds to said point in time, which expected current value is determined from the progression with respect to time that is calculated and stored in the microcontroller  36 . Where necessary, at a later point in time, the microcontroller  36  initiates a further comparison of the current values. The microcontroller  36  calibrates the current sensors  26 ,  28  on the basis of the said comparison and corrects in particular the amplification factors and/or offsets of the current sensors  26 ,  28 . If the current sensors  26 ,  28  do not undergo a calibration process, then further measured values that are supplied by the current sensors  26 ,  28  can be subjected to at least one plausibility check. In this case, the microcontroller  36  decides whether the deviation of the measured values that are supplied by the current sensors  26 ,  28  from the calculated expected current value is below a predefined threshold, by way of example a percentage tolerance threshold. If the deviation exceeds the predefined threshold, the microcontroller  36  generates a warning signal that indicates that at least one of the current sensors  26 ,  28  is defective. 
       FIG. 2  illustrates an expected progression with respect to time of a battery/pre-charge current I and also a voltage U that is present at the smoothing capacitor  24 . In a disconnected phase  30  that precedes the pre-charge phase  32 , all three relays  12 ,  14 ,  16  of the battery  100  are open so that there is no current I flowing through the battery. A plausibility check can be performed on the current sensors  26 ,  28  to ensure that during the proper method of operation they measure that there is no current. In particular, it is possible to calibrate the offset of the two current sensors  26 ,  28  in a simple manner in the disconnected phase  30 . 
     The pre-charge phase  32  commences at a point in time  34  at which the electrical components  20 ,  22 ,  24  are connected to the battery  100  by virtue of the fact that at this point in time the second, negative relay  14  and the third relay  16  are closed. In the pre-charge phase  32 , the smoothing capacitor  24  is charged by way of an exponentially increasing pre-charge current I. Since both the capacity value of the smoothing capacitor  24  and also the resistance value of the pre-charge resistor  18  are known, the expected progression with respect to time of the pre-charge current I can be calculated in the pre-charge phase  32  and stored in the microcontroller  36 . This similarly applies for the expected progression with respect to time of the voltage U that is present at the smoothing capacitor  24 . Alternatively, the expected progression with respect to time of the pre-charge current I can be calculated by way of a comparison of the measured voltages that are present at the smoothing capacitor  24  (U c ) or rather on the plurality of battery cells  10  (U B ), in accordance with the formula I=(U B −U c )/R, wherein R is the resistance value of the pre-charge resistor  18 . Expected current values are determined for different times by the microcontroller  36  from the expected progression with respect to time of the battery current I. 
     The current measured values that are actually supplied by the current sensors  26 ,  28  are compared with the corresponding expected current values by the microcontroller  36  at different points in time during the pre-charge phase  32 , as a consequence of which the quality of each individual sensor  26 ,  28  can be assessed. At a point in time  38  at which the pre-charge current I (measured or calculated) is below a predefined threshold, the first relay  12  is closed, as a consequence of which a peak of the battery current I is generated since the charging of the smoothing capacitor  24  is only limited by the internal resistance of the battery  100 . If the progression of the current peak at the point in time  38  is sufficiently known, then the method in accordance with the disclosure can also be performed in this phase that follows the pre-charge phase  32 .