Patent Publication Number: US-2021184282-A1

Title: Method for diagnosis of a temperature control means of a battery pack

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method for diagnosis of a temperature control means of a battery pack that comprises a plurality of battery cells and a plurality of temperature sensors for measuring temperatures of the individual battery cells, the battery cells being arranged side by side in the battery pack, in a longitudinal direction of the battery pack, and mechanically connected to each another, and the battery cells being arranged on the temperature control means and mechanically and thermally connected to it. 
     The invention additionally relates to a battery management system that is configured to execute the method according to the invention. 
     The invention also relates to a battery pack that is configured to execute the method according to the invention, and/or that comprises a battery management system according to the invention. 
     The invention furthermore relates to a vehicle that comprises at least one battery pack according to the invention. 
     It is becoming apparent that the use of electrically driven motor vehicles will increase in the future. Rechargeable batteries are used in such electric vehicles, mainly to supply electric drive systems with electrical energy. Lithium-ion battery cells, in particular, are suitable for such applications. Lithium-ion battery cells are characterized by, inter alia, high energy densities, thermal stability and extremely low self-discharge. Lithium-ion battery cells have stringent requirements in respect of functional safety. Improper operation of lithium-ion battery cells can lead to exothermic reactions, including fire and/or release of gas. 
     A battery pack comprises a plurality of such lithium-ion battery cells, which can be electrically connected to each other in series or in parallel. Such a battery pack additionally comprises a management system that monitors the operation of the battery cells and controls it in such a manner that the battery cells are operated safely and sustainably in respect of their service life. 
     To enable the battery cells of a battery pack to be operated sustainably, it is necessary to determine current state variables of the individual battery cells. These state variables comprise various parameters such as, for example, a state of charge, a state of ageing, an internal resistance, a capacity, a temperature and a voltage. 
     SUMMARY OF THE INVENTION 
     A method is proposed for diagnosis of temperature control means of a battery pack. 
     The battery pack in this case comprises a plurality of battery cells, which are arranged side by side in the battery pack, in a longitudinal direction of the battery pack, and mechanically connected to each other. The battery cells are arranged on the temperature control means and are mechanically and thermally connected to it. 
     The battery pack additionally comprises a plurality of temperature sensors for measuring temperatures of the individual battery cells. 
     The battery cells are preferably each realized as a lithium-ion battery cell. The battery cells are each preferably prismatic. 
     In the sequence of the method according to the invention, a reference temperature distribution of the battery pack in the longitudinal direction of the battery pack is first determined. The reference temperature distribution in this case depends on the structure of the battery pack. The reference temperature distribution that is to be expected over the entire service life may be determined, for example, by means of experiments on the battery pack or on the vehicle. It is assumed in this case that the boundary conditions such as, for example, installation and structure of the battery pack, do not change. This temperature distribution over the cells remains constant over the entire service life. For example, a temperature gradient between cells having a highest and lowest temperature is greatest when the battery pack is subjected to high stress. If the battery pack is thermally relaxed, then a temperature gradient is zero. It is assumed that the temperature gradient is in principle always constant, with only an amplitude of the temperature difference changing during operation of the battery pack. 
     The reference temperature distribution in this case may have such a distribution pattern in the longitudinal direction of the battery pack that, when the temperature control means is switched on, the temperature in the battery pack drops from a central region of the battery pack to the respective peripheral regions of the battery pack. In other words, the temperature in the battery pack is from a central region of the battery pack in dependence on a distance between the central region of the battery pack and a position of the individual battery cells. The greater the distance becomes, the lower the temperatures of the individual battery cells becomes. A structure of the battery pack may be used to set this distribution pattern. The temperature gradient between a maximum and minimum cell temperature, i.e. between the central region and the peripheral region of the battery pack, may be minimized, for example, by more intensive cooling in the center of the battery pack. 
     The temperatures of the individual battery cells are then measured during operation of the battery pack when the temperature control means is switched on. An actual temperature distribution of the battery pack, in the longitudinal direction of the battery pack, is formed in this case, on the basis of the measured temperatures of the individual battery cells. 
     After that, the actual temperature distribution is compared with the reference temperature distribution. A profile of a difference between the actual and the reference temperature distribution is formed. 
     An ineffective temperature control state of the temperature control means is then identified if the difference between the actual and the reference temperature distribution exceeds a predefined threshold value. Preferably, the predefined threshold value may be set. The ineffective temperature control state may be caused in this case by ageing of the connection between the battery cells and the temperature control means, high temperatures of the battery cells, an accident or a material defect. The threshold value may be determined, for example, by means of tests in a laboratory or vehicle. Battery packs that are functional, or that have a defect, may be analyzed for this purpose. The functional battery packs may be used to ascertain a normal temperature spread. The defective battery pack may be used to determine the deviation from normal behavior. If a battery cell becomes detached from the cooling system, this battery cell may heat up by, for example, 10° C. more than previously. 
     Preferably, one or more battery cells that are temperature-controlled in an ineffective manner by the temperature control means are identified. The battery cells that are temperature-controlled in an ineffective manner by the temperature control means have a higher than expected temperature. These battery cells are identified on the basis of the profile of the difference between the actual and the reference temperature distribution. 
     Preferably, the method according to the invention is executed by means of a battery management system for monitoring and controlling the battery pack. It is also conceivable for the method according to the invention to be executed by another control device such as, for example, a vehicle control device (VCU: vehicle control unit). 
     Preferably, following the identification of an ineffective temperature control state of the temperature control means, and/or following the identification of one or more battery cells that are temperature-controlled in an ineffective manner by the temperature control means, a service procedure is initiated in order to investigate or check, for example, whether connection points between the respective battery cells and the temperature control means are defective, or whether the temperature control means is defective. 
     Preferably, the reference temperature distribution in the longitudinal direction of the battery pack is set by setting a temperature control performance of the temperature control means and/or through design of a structure of the temperature control means. The temperature control performance of the temperature control means may be set, for example, by changing a flow rate of a temperature control medium. The structure of the temperature control means may be designed, for example, in such a manner that a greater quantity of the temperature control medium flows through in a central region of the temperature control means than in the respective peripheral regions of the temperature control means. In this way, a uniform distribution pattern of the reference temperature distribution is achieved. 
     Preferably, the battery cells are mechanically and thermally connected to the temperature control means in a materially bonded manner. The battery cells in this case may be mechanically and thermally connected to the temperature control means by welding, soldering or adhesive bonding. 
     Preferably, the battery cells are each monitored by specifically one temperature sensor. Advantageously, the position of the ineffectively temperature-controlled battery cell can be identified with precision. If a plurality of battery cells is temperature-controlled in an ineffective manner, the positions of the respective battery cells can be identified. However, it is also possible to monitor two battery cells together by means of one temperature sensor. 
     Advantageously, the temperature control means has a channel-type structure, which serves to convey a temperature control medium such as, for example, water. 
     The temperature control means is realized, advantageously, as a Peltier element. A Peltier element is an electrothermal transducer that, based on the Peltier effect, generates a current flow when there is a current flow through a temperature difference or when a temperature difference occurs. Peltier elements may be used both for cooling and—when the current direction is reversed—for heating. 
     Additionally, proposed is a battery management system, which is configured to execute the method according to the invention. 
     Also proposed is a battery pack, which is configured to execute the method according to the invention, and/or which comprises a battery management system according to the invention. 
     Furthermore, proposed is a vehicle, which comprises at least one battery pack according to the invention. 
     Advantages of the Invention 
     The method according to the invention enables an ineffective temperature control state of a temperature control means of a battery pack to be determined in a relatively simple and precise manner. The method according to the invention makes it possible to identify a position at which a thermal connection between a battery cell and the temperature control means is impaired. If the thermal connections are impaired at a plurality of positions, the plurality of positions can also be identified by means of the method according to the invention. With the method according to the invention, it is advantageously possible, following the identification of an ineffective temperature control state of the temperature control means, and/or following the identification of one or more battery cells that are temperature-controlled in an ineffective manner by the temperature control means, to initiate a service procedure in order to investigate or check, for example, whether connection points between the respective battery cells and the temperature control means are defective, or whether the temperature control means is defective. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are explained in greater detail on the basis of the drawings and the following description. 
         FIG. 1  shows a flow diagram of the method according to the invention, and 
         FIG. 2  shows a schematic diagram of a battery pack. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description of the embodiments of the invention, elements that are the same or similar are denoted by the same references, and in individual cases description of these elements is not repeated. The subject-matter of the invention is represented only schematically in the figures. 
       FIG. 1  shows a flow diagram  100  of the method according to the invention for diagnosis of a temperature control means  30  (see  FIG. 2 ) of a battery pack  10  (see  FIG. 2 ), which a plurality of battery cells  20  (see  FIG. 2 ) and a plurality of temperature sensors for measuring the temperatures of the individual battery cells  20 . 
     In a first step  101 , firstly, a reference temperature distribution of the battery pack  10  in a longitudinal direction  12  (see  FIG. 2 ) of the battery pack  10  is determined. 
     Then, in a second step  102 , the temperatures of the individual battery cells  20  during operation of the battery pack  10  are measured, when the temperature control means  30  is switched on. An actual temperature distribution of the battery pack  10  in the longitudinal direction  12  of the battery pack  10  is formed, based on the measured temperatures of the individual battery cells  20 . 
     After that, in a step  103 , the actual temperature distribution is compared with the reference temperature distribution. In this case a profile of a difference between the actual and the reference temperature distribution is formed. 
     Then, in a step  104 , an ineffective temperature control state of the temperature control means  30  is identified if the difference between the actual and the reference temperature distribution exceeds a predefined threshold value. Preferably, the predefined threshold value may be set. 
     After that, in a step  105 , there is identification one or more battery cells  20  that are temperature-controlled in an ineffective manner by the temperature control means  30 . The battery cells  20  that are temperature-controlled in an ineffective manner by the temperature control means  30  have a higher than expected temperature. These battery cells  20  are identified by the profile of the difference between the actual and the reference temperature distribution. 
     Preferably, the method according to the invention is executed by means of a battery management system for monitoring and controlling the battery pack  10 . It is also conceivable for the method according to the invention to be executed by another control unit such as, for example, a vehicle control device (VCU: vehicle control unit). 
     Preferably, following the identification of an ineffective temperature control state of the temperature control means  30 , and/or following the identification of one or more battery cells  20  that are temperature-controlled in an ineffective manner by the temperature control means  30 , a service procedure is initiated in order to investigate or check, for example, whether connection points between the respective battery cells  20  and the temperature control means  30  is defective, or whether the temperature control means  30  is defective. 
     A battery pack  10  is represented schematically in  FIG. 2 . 
     The battery pack  10  in this case comprises a plurality of battery cells  20 .  FIG. 2  shows nine battery cells  20 , namely, as viewed from the right, a first battery cell  21  to a ninth battery cell  29 . The battery cells  20 ,  21  to  29  are in this case arranged side by side in the battery pack  10 , in a longitudinal direction  12  of battery pack  10 , and are mechanically connected to each other. The battery cells  20 ,  21  to  29  are arranged on a temperature control means  30 , and are mechanically and thermally connected to it by an adhesive layer  40 . 
     The battery cells  20 ,  21  to  29  in this case may also be mechanically and thermally connected to the temperature control means  30  by welding or soldering. 
     The battery cells  20 ,  21  to  29  are each realized as a lithium-ion battery cell and are prismatic. 
     The temperature control means  30  has a channel-type structure (not represented), which serves to convey a temperature control medium such as, for example, water. The temperature control means  30  in this case has a supply connection  31  for supplying the temperature control medium, and has a discharge connection  32  for discharging the temperature control medium. 
     It is also conceivable for the temperature control means  30  to be realized as a Peltier element. 
     For the battery pack  10  represented in  FIG. 2 , a reference temperature distribution is determined, which has such a distribution pattern that the temperature in the battery pack  10  drops from a central region of the battery pack  10  to respective peripheral regions  14  of the battery pack  10  when the temperature control means  30  is switched on. In the present embodiment, the fifth battery cell  25 , which is arranged in a central region of the battery pack  10 , has a highest temperature in the battery pack  10 . 
     The battery pack  10  additionally comprises a plurality of temperature sensors (not represented) for measuring the temperatures of the individual battery cells  20 ,  21  to  29 . Preferably, the battery cells  20 ,  21  to  29  are each monitored by specifically one temperature sensor. 
     During operation of battery pack  10 , the temperatures of the individual battery cells  20 ,  21  to  29  are measured when the temperature control means  30  is switched on. Based on the measured temperatures of the individual battery cells  20 ,  21  to  29 , an actual temperature distribution of battery pack  10  in the longitudinal direction  12  of battery pack  10  is formed. 
     A profile of a difference between the actual and the reference temperature distribution is then formed by comparison of the actual the and reference temperature distribution. 
     It can be seen from  FIG. 2  that the adhesive layer  40  has a defect  50  in an region between the third battery cell  23  and the temperature control means  30 , which results in an impairment of a thermal connection between the third battery cell  23  and the temperature control means  30 . The defect  50  may in this case be caused by ageing of the adhesive layer  40 , high temperatures of the battery cells  20 ,  21  to  29 , an accident or a material defect. In this case, less heat is transferred from the third battery cell  23  to the temperature control means  30 , or from the temperature control means  30  to the third battery cell  23 , and the third battery cell  23  has a higher than expected temperature. 
     A difference between the measured temperature of the third battery cell  23  and a temperature resulting from the reference temperature distribution of the battery pack  10  is represented by means of the profile of the difference between the actual and the reference temperature distribution. 
     If the difference between the measured temperature of the third battery cell  23  and a temperature resulting from the reference temperature distribution of the battery pack  10  exceeds a given threshold value, an ineffective temperature control state of temperature control means  30  is identified. 
     If the battery cells  20 ,  21  to  29  are each monitored by specifically one temperature sensor, a position at which the thermal connection between the third battery cell  23  and the temperature control means  30  is impaired can be identified. 
     The invention is not limited to the design exemplary embodiments described here and to the aspects highlighted therein. Rather, within the range indicated by the claims, a large number of variations are possible, which are within the scope of professional practice.