Abstract:
A method of detecting a leak in a battery pack according to an exemplary aspect of the present disclosure includes, among other things, calculating a predicted amount of thermal energy at a position, measuring an actual amount of thermal energy at the position, and comparing the predicted amount to the actual amount to identify if a battery pack is leaking.

Description:
BACKGROUND 
     This disclosure relates generally to a battery pack for an electric vehicle and, more particularly, to detecting undesirable thermal energy leaks and undesirable fluid leaks in the battery pack. 
     Generally, electric vehicles differ from conventional motor vehicles because electric vehicles are selectively driven using one or more battery-powered electric machines. Conventional motor vehicles, by contrast, rely exclusively on an internal combustion engine to drive the vehicle. Electric vehicles may use electric machines instead of, or in addition to, the internal combustion engine. 
     Example electric vehicles include hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs). Electric vehicles are typically equipped with a battery pack containing battery cells that store electrical power for powering the electric machine. The batteries may be charged prior to use, and recharged during drive by a regeneration brake or engine. 
     Extended exposure to significant thermal energy levels can shorten the useful life of a battery pack. Typically, the battery pack is thus thermally insulated from the surrounding environment. Further, a fan is used to move air through the battery pack. The moving air regulates thermal energy levels. The fan typically draws climate controlled air into the battery pack from a cabin of the vehicle. This air moves through the battery pack and exits to the cabin, the exterior of the vehicle, or trunk, etc. or combined of them. 
     A leak in the battery pack permits undesirable levels of fluid, thermal energy or both to communicate between an interior and an exterior of the battery pack. Insulation breakage during battery pack installation, customized vehicle work, etc. can cause leaks in the battery pack. Leaks lead to increased operating time for the fan, increased vehicle cabin temperatures, increased battery temperatures, reduced vehicle performance, etc. Technicians can undesirably devote considerable time to diagnosing and locating leaks. 
     SUMMARY 
     A method of detecting a leak in a battery pack according to an exemplary aspect of the present disclosure includes, among other things, calculating a predicted amount of thermal energy at a position, measuring an actual amount of thermal energy at the position, and comparing the predicted amount to the actual amount to identify if a battery pack is leaking. 
     In a further non-limiting embodiment of the foregoing method of detecting a leak in a battery pack, the method includes moving a fluid through the battery pack using a fluid movement device. The fluid enters the battery pack at an inlet and exits the battery pack at an outlet. The fluid movement device is positioned downstream from the inlet and upstream from the outlet relative to a direction of flow through the battery pack. 
     In a further non-limiting embodiment of any of the foregoing methods of detecting a leak in a battery pack, the leaking comprises movement of fluid to the battery pack through areas other than the inlet, movement of fluid from the battery pack through areas other than the outlet, or both. The leak, in some examples, is a thermal leak through the insulation layer. Fluid may or may not move through the leak. 
     In a further non-limiting embodiment of any of the foregoing methods of detecting a leak in a battery pack, the fluid movement device is a fan. 
     In a further non-limiting embodiment of any of the foregoing methods of detecting a leak in a battery pack, the method comprises measuring thermal energy of fluid entering through the inlet to provide an inlet fluid amount of thermal energy, and comparing the inlet fluid amount to the predicted amount to identify a location of the leaking. 
     In a further non-limiting embodiment of any of the foregoing methods of detecting a leak in a battery pack, the method includes calculating that the leak is between the inlet and the fluid movement device if the inlet fluid amount of thermal energy is greater than the actual amount of thermal energy at the position. 
     In a further non-limiting embodiment of any of the foregoing methods of detecting a leak in a battery pack, the position is at a battery cell of the battery pack. 
     In a further non-limiting embodiment of any of the foregoing methods of detecting a leak in a battery pack, the position is within the battery pack. 
     In a further non-limiting embodiment of any of the foregoing methods of detecting a leak in a battery pack, the method includes identifying a leak when the actual amount is greater than the predicted amount by at least an established threshold value, for example, three degrees Celsius. 
     In a further non-limiting embodiment of any of the foregoing methods of detecting a leak in a battery pack, the method includes identifying a leak when the predicted amount is greater than the actual amount by at least an established threshold value, for example, three degrees Celsius. 
     In a further non-limiting embodiment of any of the foregoing methods of detecting a leak in a battery pack, the method includes measuring the actual amount of thermal energy using a sensor positioned at or adjacent to battery cell of the battery pack. 
     A leak detection assembly for a battery pack according to an exemplary aspect of the present invention includes, among other things, a sensor to determine an actual amount of thermal energy at an position, and a controller to calculate a predicted amount of thermal energy at the position, and to indicate that a battery pack includes a leak based on a comparison of the predicted amount to the actual amount. 
     In a further non-limiting embodiment of the foregoing leak detection assembly, the assembly includes a fluid movement device to move fluid from an inlet of the battery pack to an outlet of the battery pack. 
     In a further non-limiting embodiment of any of the foregoing leak detection assemblies, the fluid movement device comprises a fan. 
     In a further non-limiting embodiment of any of the foregoing leak detection assemblies, the leak comprises movement of fluid to the battery pack through areas other than the inlet, movement of fluid from the battery pack through areas other than the outlet, or both. The leak may also comprise a thermal leak or movement of thermal energy (and no fluid) from the battery pack through the leak. 
     In a further non-limiting embodiment of any of the foregoing leak detection assemblies, the assembly includes an inlet sensor to measure an amount of thermal energy in fluid entering the battery pack through the inlet, wherein the controller is configured to compare the inlet fluid amount to the predicted amount to identify a location of the leaking. 
     In a further non-limiting embodiment of any of the foregoing leak detection assemblies, the controller indicates a leak when the predicted amount is greater than the actual amount by at least an established threshold value, for example, three degrees Celsius. 
     In a further non-limiting embodiment of any of the foregoing leak detection assemblies, the controller indicates a leak when the actual amount is greater than the predicted amount by at least an established threshold value, for example, three degrees Celsius. 
     In a further non-limiting embodiment of any of the foregoing leak detection assemblies, the position is within the battery pack. 
     In a further non-limiting embodiment of any of the foregoing leak detection assemblies, the position is at or adjacent to a battery cell of the battery pack. 
    
    
     
       DESCRIPTION OF THE FIGURES 
       The various features and advantages of the disclosed examples will become apparent to those skilled in the art from the detailed description. The figures that accompany the detailed description can be briefly described as follows: 
         FIG. 1  illustrates a schematic view of an example electric vehicle powertrain. 
         FIG. 2  illustrates a schematic view of an example battery pack used within the electric vehicle powertrain of  FIG. 1 . 
         FIG. 3  illustrates the flow of an example method used to identify leaks within the battery pack of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically illustrates a powertrain  10  for an electric vehicle. Although depicted as a hybrid electric vehicle (HEV), it should be understood that the concepts described herein are not limited to HEVs and could extend to other electrified vehicles, including but not limited to, plug-in hybrid electric vehicles (PHEVs) and battery electric vehicles (BEVs). 
     In one embodiment, the powertrain  10  is a powersplit powertrain system that employs a first drive system and a second drive system. The first drive system includes a combination of an engine  14  and a generator  18  (i.e., a first electric machine). The second drive system includes at least a motor  22  (i.e., a second electric machine), the generator  18 , and a battery pack  24 . In this example, the second drive system is considered an electric drive system of the powertrain  10 . The first and second drive systems generate torque to drive one or more sets of vehicle drive wheels  32  of the electric vehicle. 
     The engine  14 , which is an internal combustion engine in this example, and the generator  18  may be connected through a power transfer unit  36 , such as a planetary gear set. Of course, other types of power transfer units, including other gear sets and transmissions, may be used to connect the engine  14  to the generator  18 . In one non-limiting embodiment, the power transfer unit  36  is a planetary gear set that includes a ring gear  40 , a sun gear  44 , and a carrier assembly  48 . 
     The generator  18  may be driven by engine  14  through the power transfer unit  36  to convert kinetic energy to electrical energy. The generator  18  can alternatively function as a motor to convert electrical energy into kinetic energy, thereby outputting torque to a shaft  52  connected to the power transfer unit  36 . Because the generator  18  is operatively connected to the engine  14 , the speed of the engine  14  can be controlled by the generator  18 . 
     The ring gear  40  of the power transfer unit  36  may be connected to a shaft  56 , which is connected to vehicle drive wheels  32  through a second power transfer unit  60 . The second power transfer unit  60  may include a gear set having a plurality of gears  64 . Other power transfer units may also be suitable. The gears  64  transfer torque from the engine  14  to a differential  68  to ultimately provide traction to the vehicle drive wheels  32 . The differential  68  may include a plurality of gears that enable the transfer of torque to the vehicle drive wheels  32 . The second power transfer unit  60  is mechanically coupled to an axle  72  through the differential  68  to distribute torque to the vehicle drive wheels  32 . 
     The motor  22  (i.e., the second electric machine) can also be employed to drive the vehicle drive wheels  32  by outputting torque to a shaft  78  that is also connected to the second power transfer unit  60 . In one embodiment, the motor  22  and the generator  18  cooperate as part of a regenerative braking system in which both the motor  22  and the generator  18  can be employed as motors to output torque. For example, the motor  22  and the generator  18  can each output electrical power the battery pack  24  through a high voltage bus  82 . 
     The battery pack  24  may be a high voltage battery that is capable of outputting electrical power to operate the motor  22  and the generator  18 . Other types of energy storage devices and/or output devices can also be used with the electric vehicle. 
     Referring now to  FIG. 2 , the example battery pack  24  includes a plurality of individual battery cells  86  and a fan  90 . A battery pack housing  94  holds the battery cells  86  and the fan  90 . The battery pack housing  94  includes an insulative layer  96  to thermally insulate the battery pack  24 . The battery pack housing  94  can include other layers in addition to the insulative layer  96 . 
     The battery pack  24  includes a flowpath F extending from an inlet  98  to an outlet  102 . The battery cells  86  are positioned within the flowpath F. The fan  90  moves a fluid along the flowpath F from the inlet  98  to the outlet  102 . The fluid, in this example, is drawn from a passenger cabin  110  of the vehicle. The fluid moves from the outlet  102  back to the passenger cabin  110 . 
     In another example, the fluid moves from the outlet  102  to an area outside of the passenger cabin  110 , such as through an outlet  102 ′. The fluid may move through the outlet  102 ′ instead of, or in addition to, moving through the outlet  102 . 
     The fan  90  is a type of fluid movement device. A fan motor  104  drives a shaft  108  that extends through the battery pack housing  94  to drive the fan  90 . The fan  90  is downstream from the inlet  98  and upstream from the outlet  102  relative to a general direction of flow along the flowpath F. 
     Some of the battery cells  86   u  are positioned upstream from the fan  90  relative to a general direction of flow through the battery pack  24 . Other battery cells  86   d  are positioned downstream from the fan  90  relative to the general direction of flow through the battery pack  24 . 
     Fluid moving along the flowpath moves across the battery cells  86 , which regulates the amounts of thermal energy in the battery cells  86 . In one example, fluid moves across the battery cells  86  to carry thermal energy away from the battery cells  86  and thereby lower a temperature of the battery cells  86 . 
     A plurality of thermal energy sensors  118  extend into the battery pack  24 . The sensors  118  collect thermal energy measurements from the battery pack  24 . The temperatures of the battery cells  86  within the battery pack  24  can range, for example, from −40 to 65 degrees Celsius, so the example thermal energy sensors  118  are able to collect thermal energy measurements across at least this range. 
     In this example, some of the sensors  118   b  measure thermal energy amounts at ten separate locations that are within or near the groups of battery cells  86 . Five of the sensors  118   b  measure thermal energy at locations within the battery cells  86   u  that are upstream from the fan  90 . Five of the sensors  118   b  measure thermal energy amounts at locations within the battery cells  86   d  that are downstream from the fan  90 . 
     The readings from the sensors  118   b  reveal an actual amount of thermal energy at the locations. When the location is on (or sufficiently close to) one or more of the battery cells  86 , the readings from the sensors  118   b  represent an actual amount of thermal energy in those battery cells  86 . 
     Another of the sensors  118   i  measures an amount of thermal energy in the flow moving through the inlet  98 . 
     The sensors  118  are operably coupled to a controller  122  that collects the thermal energy readings from the sensors  118 . 
     Leaks can develop in the battery pack housing  94 . Leaks permit undesirable fluids, thermal energy, or both to move to or from the battery pack housing  94  at a location other than the inlet  98  and the outlet  102 . 
     The example battery pack  24  includes upstream leaks L u  that are upstream relative to the fan  90 . The upstream leaks L u  permit fluid or thermal energy outside the passenger cabin  110  to be drawn into the battery pack  24 , rather than fluid from the passenger cabin  110 . 
     The example battery pack  24  includes downstream leaks L d  that are downstream relative to the fan  90 . The downstream leaks L d  permit fluid or thermal energy within the battery pack  24  to escape from the battery pack  24  though a location other than the outlet  102 . 
     The upstream leaks L u  offer little resistance to fluid moving into the battery pack  24  compared to the relatively restricted flow from the passenger cabin  110 . The upstream leaks L u  can cause an amount of thermal energy in the battery cells  86  to increase or decrease depending on the ambient temperature of the battery pack  24  and the temperature of the fluid in the environment that enters the battery pack  24  via the upstream leaks L u  Fluid entering the battery pack  24  via the upstream leaks L u  has not been conditioned within the passenger cabin  110 . 
     In hot weather, for example, fluid moving into the battery pack  24  through the upstream leaks L u  could be very hot, which could cause some areas of the battery pack  24  near the upstream leaks L u  to heat up disproportionately to other areas. In cold weather, fluid moving into the battery pack  24  through the upstream leaks L u  could be very cold and cause some areas of the battery pack  24  near the downstream leaks L d  to cool disproportionately to other areas. 
     In an example embodiment of this disclosure, the controller  122  identifies that the battery pack  24  has leaks using, in part, readings from the sensors  118   b , the sensor  118   i , or both. If the controller  122  calculates that the battery pack  24  includes leaks, a technician can then inspect and repair the battery pack  24 . If a leak is found, the leak can be repaired to ensure that flow across the battery cells  86  is not influenced by the leak. 
     Referring now to  FIG. 3  with continuing reference to  FIG. 2 , to detect leaks, an example method  200 , at a step  210 , calculates a predicted amount of thermal energy at a position within the battery pack  24 . The method  200  also measures, at a step  220 , the actual amount of thermal energy at the position. The method  200  may use, for example, readings of thermal energy from the sensors  118   b  to measure the actual amount of thermal energy at the position. 
     At a step  230 , the method  200  compares the predicted amount of thermal energy to the actual amount of thermal energy. The method  200  detects leaks in the battery pack  24  based on this comparison. 
     The method  200  may rely, in part, on a battery pack temperature estimation equation, which has been reproduced below as equation (1). 
     
       
         
           
             
               
                 
                   
                     
                       
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     In this example, C p, cell  represents the heat capacity of the battery cell  86  and harnesses associated with the battery cell  86 . E cell  represents the electrical energy in the battery cell  86 . T air  represents a temperature of the air into the battery pack  118   i , T cell  represents a temperature of a battery cell  86 , and h represents the heat transfer coefficient. V cell  represents the terminal voltage of the cell  86 . I represents the pack current. In the current sign convention, a positive value is for a charge current. Thus, equation (1), in this example, sets the thermal and electrical energy increase in the battery pack  24  to be equal to both the heat transfer into battery pack  24  and the power generated by the battery pack  24 . 
     To calculate the estimated temperature at a position within the battery pack  24 , such as a selected battery cell  86 , the method  200  may determine heat transfer out (HT out ) of battery cell  86  using equation (2), which has been reproduced below:
 
HT out =( h   0 +fan_air_flow_rate× h   fan   _   on )×( T   air   −T   cell )  Equation (2)
 
     The heat transfer coefficient h 0  and the fan on heat transfer coefficient h fan   _   on  are constants and can be obtained with fan off and fan on battery pack temperature test data. A person having skill in this art and the benefit of this disclosure would be able to obtain these coefficients. 
     The heat generated by the battery cell is, essentially, the electrical energy consumed. The heat generated by the battery cell is represented by BP HG  and can be calculated using equation (3), which has been reproduced below:
 
BP HG   =I   2   ×R   cell   Equation (3)
 
     In Equation (3), I is the electrical current through the battery pack  24  and R cell  is the total electrical resistance (both cell and harnesses) of the battery cell  86 . 
     The battery cell temperature T cell  can then be estimated using Equation (4), which has been reproduced below:
 
BP HG −HT out   =C   p,cell ×( d ( T   cell )/ dt )  Equation (4)
 
     Solving the above equations provides T cell , which is an estimate of the thermal energy in the battery cell  86  associated with the position. To compensate the delay of sensor measured temperature, filter(s) and/or pure delay can be used to simulate the real sensor reading. In the example method, if temperature of the battery cell  86  (measured by one or more of the sensors  118   b ) is significantly different than estimated temperature of the battery cell T cell , the battery pack  24  is considered to have a leak. 
     In some examples, the predicted amount of thermal energy and the actual amount of thermal energy are represented as temperatures measured in degrees Celsius. If the predicted amount equals or exceeds an established threshold value, the method  200  indicates that the battery pack  24  is leaking. For example, if the established threshold value is an absolute value of three degrees Celsius, and the predicted amount is three or more degrees Celsius greater than the measured amount, the method  200  indicates that the battery pack  24  is leaking. In addition, if the predicted amount is three or more degrees Celsius less than the actual amount of thermal energy, the method  200  indicates that the battery pack  24  is not leaking. In this example, the threshold value is calibratable and can be adjusted based on specific requirements, such as battery pack  24  size. 
     In one example, the battery pack  24  is only to be considered leaking if, in addition to the predicted amount varying three or more degrees from the actual amount, a fault associated with the sensor  118   b  has not also been detected within a certain timeframe, say this drive cycle. An example fault is a sensor  118   b  that has failed and does not provide a reading, for example. 
     A further example embodiment of this disclosure can determine the general positions of leaks within the battery pack  24  to assist the technician inspecting the battery pack  24 . For example, in hot weather, the air outside the battery pack  24  is relatively hot, and the thermal energy levels of the battery cells  86  upstream from the leak will be higher than those downstream from the leak. The controller  122  calculates the location within the battery pack  24  where the temperature transitions from a lower to a higher temperature. This location is then flagged as the likely location of the leak. 
     The technician begins a search for the leak in the flagged location, which can help reduce inspection time. The controller  122  uses information collected by the sensors  118   b , the sensor  118   i , or both to calculate the location of the temperature transition. 
     Identifying the general location of the leak could involve calculating whether the leak is on, for example, the driver or passenger side of the battery pack  24 . Identifying the general location of the leak could be more specific, such as a specific battery cell  86  that is closest to the leak. 
     The controller  122  can identify that the leak is an upstream leak L u , in one example, when the temperature of fluid moving out of the outlet  102  into the passenger cabin  110  is elevated. This may be due to the flow from the passenger cabin  110  mixing with the hotter fluid entering the flowpath F through the upstream leak L u  The fluid moving out of the outlet will cause the temperature measured by the inlet sensor  118   i  to be higher than the temperature measured by the sensors  118   b  upstream from the upstream leak L u . 
     In one example, the controller  122  can identify that the leak is a downstream leak L d . Fluid can move out of the battery pack  24  through the downstream leak L d  instead of through the outlet  102 . The fluid moves through the path of the downstream leak L d  rather than through the outlet  102  due to less air resistance along the path provided by the downstream leak L d  versus the resistance associated with the outlet  102 . The resistance of the outlet  102  is typically higher than the resistance at the location of the leak due to a pipe  130  and the structure of the outlet  102  providing a relatively open path to the passenger cabin  110 . 
     The controller  122  calculates an area having the leak recognizing that areas of the battery pack  24  downstream from the leak will have elevated temperatures relative to areas of the battery pack  24  upstream from the leak. The relative elevated temperatures are due to less air movement across the areas downstream from the leak. 
     In some examples, if the battery pack  24  includes leaks both upstream and downstream from the fan  90 , fluid moving through the leak paths will be cycled out of the battery pack  24 . The battery pack  24  will then use flow from the area around the pack instead of passenger cabin  110  and the battery pack  24  will be heated very quickly. Fan speed will have little impact on the ability of the fan  90  to reduce the temperature of the battery cells  86  within the battery pack  24 . 
     Although the different non-limiting embodiments are illustrated as having specific components or steps, the embodiments of this disclosure are not limited to those particular combinations. It is possible to use some of the components or features from any of the non-limiting embodiments in combination with features or components from any of the other non-limiting embodiments. Further, unless otherwise specified, the steps may be performed in any order. 
     The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. Thus, the scope of legal protection given to this disclosure can only be determined by studying the following claims.