Abstract:
A method for predicting the usability of a relay or a contactor is described herein. A current flowing through the relay or the contactor and/or a voltage applied to the relay or the contactor is measured repeatedly, and the measured values are transmitted to an observation unit. The observation unit makes a prediction relating to the usability of the relay or of the contactor on the basis of the measured values and a model. Furthermore described are an observation unit and a battery which are configured to carry out the method according to the disclosure.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2011/065097, filed on Sep. 1, 2011, which claims the benefit of priority to Serial No. DE 10 2010 041 998.2, filed on Oct. 5, 2010 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     The present disclosure relates to a method for predicting the usability of a relay or a contactor and to a monitoring unit which is configured to implement the method according to the disclosure. 
     BACKGROUND 
     It will be the case in the future that new battery systems will increasingly be used both in stationary applications (for example in wind power installations) and in vehicles (for example in hybrid and electric vehicles), with very stringent requirements in terms of reliability being placed on these new battery systems. The background for these stringent requirements are that failure of the battery can result in failure of the entire system (for example failure of the traction battery in an electric vehicle) or even in safety-relevant problems (in wind power installations batteries are used, for example, for protecting the installation from impermissible operating states as a result of rotor blade adjustment in the case of a strong wind). 
     In order to enable isolation of the battery system from the connected consumers, for example from an on-board vehicle power supply system, relays (or contactors) are generally provided in series with the two battery terminals. These relays must make it possible for such a system to be disconnected a plurality of times on load. In order to meet these stringent requirements, it needs to be possible to both diagnose and predict the functionality of these relays. 
     In relays and contactors, a switchable electromagnet acts as drive for the actuation of contacts for switching on, switching off and switching over an electrical load. In this case, the contacts are in principle subject to different levels of wear during switching of electrical loads, and this wear increases considerably in the case of loads with medium to high powers. In this case, it may also arise that the contacts are welded to one another and thereby jam, which can result in considerable hazard situations. It is therefore necessary to monitor the state of the switching contacts and to introduce corresponding additional disconnection measures for the electrical load. 
     DE 20310043 U1 describes a monitoring apparatus in which a check element for the coil of the relay is associated with the relay to be monitored. The checking device in this case provides information in respect of whether the coil has been actuated or not. This is made possible by virtue of the fact that the check element comprises a component part which is sensitive to magnetic fields and is arranged in the vicinity of the coil in such a way that, when the coil is switched on, the magnetic field produced by said coil flows through said check element, as a result of which the magnetic field of the coil activates the check element. In this case, a reed contact is provided as check element. One disadvantage with such an arrangement consists in that an additional component part necessarily needs to be provided on the relay to be monitored. 
     SUMMARY 
     The method according to the disclosure for predicting the usability of a relay or a contactor in principle comprises the following steps: a current flowing through the relay or the contactor is measured repeatedly, and the measured values are transmitted to a monitoring unit. The monitoring unit makes a prediction on the usability of the relay or the contactor on the basis of the measured values and a model. 
     Similarly, the method can also be implemented by virtue of the fact that a voltage present at the relay or the contactor is measured since this permits conclusions to be drawn on the current flowing through the relay or the contactor, and vice versa. In principle, this is possible when the (equivalent) resistance of the relay or the contactor in the various phases (during closing and during a closed state) is known. Therefore, either current measured values or voltage measured values can be evaluated by the monitoring unit, wherein the monitoring unit typically calculates the powers acting at the relay or at the contactor from this. It is also possible for measured values of the two physical variables to be evaluated simultaneously by the monitoring unit, which can be advantageous owing to the redundancy associated therewith. 
     The model used by the monitoring unit can be based on characteristics which draw a relationship between, for example, the electrical powers which are switched or the arcs which are produced by the potential differences present and changes at the relay or at the contactor. 
     The method according to the disclosure can be used in particular for calculating aging or for predicting jamming of the relay or the contactor. 
     It is preferred for the monitoring unit to estimate a present temperature of the relay or the contactor on the basis of the measured values and the model. Owing to the fact that the present temperature of the relay or the contactor is calculated in the monitoring unit, there is now no need to measure the temperature using a temperature detection apparatus arranged on the relay or the contactor. Instead, the disclosure manages in principle without the use of temperature detection apparatuses. However, the estimation of the present temperature of the relay or the contactor can be improved by virtue of the fact that an ambient temperature of the relay or the contactor is measured and the ambient temperature measured value is transmitted to the monitoring unit. However, it is sufficient here to provide a temperature detection apparatus which is arranged outside the relay or the contactor and not anywhere near said relay or contactor. For example, when using the method in a battery, the battery temperature which is detected in any case by a battery management unit can be transmitted to the monitoring unit. 
     The current flowing through the relay or the contactor or the voltage present at the relay or the contactor can be measured in different phases: during a switching operation of the relay or the contactor in which the relay or the contactor changes from an open state to a closed state (or vice versa) or else during the closed state. When the estimated temperature of the relay or the contactor exceeds a temperature threshold value, a warning signal can be generated. As a result of this or as a result of similar measures, jamming of the relay or the contactor can be avoided, which represents an advantage in terms of safety and reliability. 
     A further aspect of the disclosure relates to a monitoring unit which is configured to implement the method according to the disclosure, to a battery, preferably a lithium-ion battery, with the monitoring unit according to the disclosure and to a motor vehicle with the battery according to the disclosure. 
     In addition to its use in a battery system of a motor vehicle, the disclosure can also advantageously be used in other systems, for example motor stop-start systems, in order to predict the life of relays or contactors. In motor stop-start systems, a relay for activating the starter is provided, which relay is connected upstream of a starter motor. In this case, too, high currents flow during a switch-on operation, and these high currents may damage the contacts of the relay. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the disclosure will be explained in more detail with reference to the drawings and the description below. In the drawings: 
         FIG. 1  shows an exemplary embodiment of the battery according to the disclosure, and 
         FIG. 2  shows an exemplary embodiment of the method according to the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a battery denoted overall by  100  in a block circuit diagram. A multiplicity of battery cells  10  are connected in series and optionally additionally in parallel in order to achieve a high output voltage and battery capacity desired for a respective application. A charging and isolating device  16  is connected between the positive pole of the battery cells and a positive battery terminal  14 . In addition, an isolating device  17  is connected between the negative pole of the battery cells  10  and a negative battery terminal  15 . The isolating and charging device and the isolating device  17  each comprise a contactor  18  and  19 , respectively, which are provided for disconnecting the battery cells from the battery terminals  14 ,  15  in order to switch the battery terminals to zero potential. Owing to the high DC voltage of the series-connected battery cells  10 , there is a considerable potential risk for maintenance personnel or the like otherwise. In addition, a charging contactor  20  with a charging resistor  21  connected in series with the charging contactor  20  is provided in the charging and isolating device  16 . The charging resistor  21  limits a charging current when the battery is connected to a DC voltage intermediate circuit (not illustrated). For this purpose, first the contactor  18  is left open and only the charging contactor  20  is closed. When the voltage at the positive battery terminal  14  reaches the voltage of the battery cells  10 , the contactor  19  can be closed and possibly the charging contactor  20  can be opened. The contactors  18 ,  19  and the charging contactor  20  do not considerably increase the costs for a battery  100  since stringent requirements are placed on the reliability of said contactors and on the currents to be conducted through said contactors. 
     Every switch-on and connection process at the contactors  18 ,  19 ,  20  can age said contactors. It is therefore necessary to estimate the life of said contactors in order to be able to prevent failures and to provide a diagnosis as to when one of the contactors  18 ,  19 ,  20  needs to be replaced. The method according to the disclosure is designed with this in mind, with it being possible for said method to be applied to any of the contactors  18 ,  19 ,  20 , but in this case this is only explained in respect of the (negative) contactor  19 . 
     The battery  100  or a battery management unit (not illustrated in any more detail) comprises a current measurement unit  11 , which is configured to measure a battery current and therefore also a current flowing through the contactor  19 , a voltage apparatus  12 , which is configured to measure a voltage present at the contactor  19 , and a temperature measurement apparatus  13 , which is configured to measure a battery temperature. In order to implement the method according to the disclosure, measurements either by the current measurement apparatus  11  or by the voltage apparatus  12  are sufficient, with the result that the respective other measurement apparatus and the temperature measurement device  13  merely provide redundant information, or information which fine-tunes the calculations. 
     The current measurement apparatus  11 , the voltage apparatus  12  and the temperature measurement apparatus  13  transmit their measured values to a control device  22 , which acts as monitoring unit, which implements the method according to the disclosure, or which comprises such a monitoring unit. 
     Therefore, the monitoring unit is provided in  FIG. 1  as part of the battery  100 , in particular a battery management unit. However, the monitoring unit can also be arranged outside the battery  100  and can be included in a main control device of a motor vehicle, for example. 
     It is not necessary to arrange the temperature measurement apparatus  13  in the vicinity of the contactor  19  to be monitored. The temperature measurement apparatus  13  does not directly measure the temperature of the contactor  19 , but merely an ambient temperature, for example a temperature in the vicinity of the battery cells. 
       FIG. 2  shows an exemplary embodiment of the method according to the disclosure for predicting the usability of a contactor, which method is implemented in the battery  100  illustrated in  FIG. 1 . During the entire process, the current measurement apparatus  11 , the voltage apparatus  12  and the temperature measurement apparatus  13  detect measurement data and transmit these to the monitoring unit  22 . Likewise during the entire process, a present temperature of the contactor  19  is calculated or estimated by the monitoring unit  22 . For the case in which no current is flowing through the contactor  19  for a relatively long period of time, the estimated temperature of the contactor  19  is equated with the ambient temperature. 
     In method step S 11 , the contactor  19  is in an open state. No current is flowing through the contactor  19 , with the result that no further heating of the contactor  19  can take place. 
     In method step S 12 , the contactor  19  is closed. During the switching operation, a sparkover takes place, which heats the contacts of the contactor  19 . The current pulse during the sparkover is measured and the temperature rise associated therewith in the contactor  19  is calculated by the monitoring unit  22 . 
     Primarily on repeated opening and closing of the contactor  19  within a short period of time, the temperature of said contactor can increase such that the contacts jam owing to a welding process. Therefore, in step S 13 , the estimated temperature of the contactor is compared with a predetermined temperature threshold value. If the estimated temperature of the contactor  19  is greater than the predetermined temperature threshold value, a warning signal is generated in step S 14  by virtue of, for example, a replacement recommendation being expressed, which can result in the avoidance of malfunction of the contactor  19 . The estimation of the increase in temperature of the contactor  19  is in this case implemented on the basis of a temperature model which draws a relationship between a current pulse and the temperature rise associated therewith. 
     When the estimated temperature of the contactor  19  is no greater than the predetermined temperature threshold value, the profile of the current flowing through the contactor  19  during continuous operation, when the contacts are closed, is recorded in method step S 15 . In the process, the contactor  19  is heated by the continuing operating current. This heating of the contactor  19  is predicted by the model used by the monitoring unit  22 . In this case, consideration is also given to the fact that the heating of the contacts is greater the greater the amount of damage already existing on the contacts. The aging of the contactor  19  can be predicted from the heating of the contacts, more precisely from the development of the estimated temperature of the contactor  19  over time. 
     In method step S 16 , the contactor  19  is opened again, which in turns results in a sparkover, as in method step S 12 . Again for the case (S 17 ) in which the temperature rise which is caused by the arc produced is too high, a warning signal can be generated (S 14 ). Otherwise, the branch returns to the beginning of the method in step S 11 , with this step corresponding to a closed state of the contactor  19 . 
     The model used by the monitoring unit  22  can be fine-tuned by look-up tables, which are provided by the manufacturer of the contactor and generally indicate how long the life (measured as the number of switching operations) of a relay or a contactor is depending on a constant load current. In contrast to these figures, however, the model used by the monitoring unit  22  is designed to be able to make an aging prediction even in the case of variable currents and therefore to be able to take into consideration, for example, the case in which a first switching operation is implemented at a specific load current and then a second is implemented at a load current which is ten times higher. 
     The predictions in respect of the aging and the jamming of the contactor  19  are communicated by the monitoring unit  22  via a data link, in particular via a bus system, to other control devices, for example the main control device of the motor vehicle.