Electrified vehicle battery packs with improved thermal interface material distribution

This disclosure details exemplary battery pack designs for use in electrified vehicles. An exemplary battery pack assembly process may include supporting one or more components, such as a heat exchanger plate, of the battery pack against deflection during the assembly process. Supporting the heat exchanger plate to keep the plate relatively flat during the battery pack assembly process improves the flow distribution of a thermal interface material (TIM), thereby achieving improved TIM coverage and improved heat transfer between battery cells and the heat exchanger plate of the battery pack.

TECHNICAL FIELD

This disclosure relates to electrified vehicle battery packs, and more particularly to electrified vehicle battery packs that exhibit improved thermal interface material (TIM) distribution by supporting a heat exchanger plate of the battery pack during the assembly process.

BACKGROUND

The desire to reduce automotive fuel consumption and emissions is well documented. Therefore, vehicles are being developed that reduce or completely eliminate reliance on internal combustion engines. Electrified vehicles are currently being developed for this purpose. In general, electrified vehicles differ from conventional motor vehicles because they are selectively driven by one or more battery powered electric machines. Conventional motor vehicles, by contrast, rely exclusively on the internal combustion engine to propel the vehicle.

A high voltage traction battery pack typically powers the electric machines and other electrical loads of the electrified vehicle. The battery pack includes a plurality of battery cells that store energy for powering these electrical loads. The battery cells generate heat during charging and discharging operations. This heat must be dissipated in order to achieve a desired level of battery performance Heat exchanger plates, often referred to as “cold plates,” may be used for dissipating the heat.

SUMMARY

A method according to an exemplary aspect of the present disclosure includes, among other things, supporting a heat exchanger plate of a battery pack against deflection during an assembly process.

In a further non-limiting embodiment of the foregoing method, the heat exchanger plate is maintained substantially flat during the assembly process.

In a further non-limiting embodiment of either of the foregoing methods, supporting the heat exchanger plate includes positioning a tray of the battery pack against a rigid workstation and positioning the heat exchanger plate against the tray.

In a further non-limiting embodiment of any of the foregoing methods, the rigid workstation includes a convex surface in contact with a bottom of the tray.

In a further non-limiting embodiment of any of the foregoing methods, the convex surface contacts the bottom of the tray near a center of the tray.

In a further non-limiting embodiment of any of the foregoing methods, supporting the heat exchanger plate includes positioning a tray of the battery pack against a rigid workstation, positioning a structural material such as a foam block within the tray, and positioning the heat exchanger plate within the tray such that the foam block is between the tray and the heat exchanger plate.

In a further non-limiting embodiment of any of the foregoing methods, rigidly supporting the heat exchanger plate includes positioning a foam block within a tray of the battery pack, and positioning the heat exchanger plate within the tray such that the foam block is between the tray and the heat exchanger plate.

In a further non-limiting embodiment of any of the foregoing methods, the foam block is constructed of an expanded polymer-based material.

In a further non-limiting embodiment of any of the foregoing methods, the method includes applying a plurality of bead lines of a thermal interface material on the heat exchanger plate, and positioning a battery array against the plurality of bead lines. Moving the battery array into the plurality of bead lines spreads the thermal interface material between the battery array and the heat exchanger plate.

In a further non-limiting embodiment of any of the foregoing methods, applying the plurality of bead lines and moving the battery array into the plurality of bead lines occurs after supporting the heat exchanger plate of the battery pack.

In a further non-limiting embodiment of any of the foregoing methods, the method includes curing the thermal interface material subsequent to moving the battery array into the plurality of bead lines.

In a further non-limiting embodiment of any of the foregoing methods, the heat exchanger plate is substantially rigidly supported during the assembly process.

A battery pack according to another exemplary aspect of the present disclosure includes, among other things, a tray, a structural material positioned against the tray, a heat exchanger plate positioned against the structural material, a thermal interface material disposed on the heat exchanger plate, and a battery array positioned against the thermal interface material.

In a further non-limiting embodiment of the foregoing battery pack, the structural material is configured to maintain the heat exchanger plate in a substantially flat configuration relative to the battery array.

In a further non-limiting embodiment of either of the foregoing battery packs, the structural material is a foam block constructed of an expanded polymer-based material.

In a further non-limiting embodiment of any of the foregoing battery packs, a component of the battery is in direct contact with the thermal interface material.

In a further non-limiting embodiment of any of the foregoing battery packs, the thermal interface material is a compliant and viscous material in an uncured state.

DETAILED DESCRIPTION

This disclosure details exemplary battery pack designs for use in electrified vehicles. An exemplary battery pack assembly process may include supporting one or more components, such as a heat exchanger plate, of the battery pack against deflection during the assembly process. Supporting the heat exchanger plate to keep the plate flat during the battery pack assembly process improves the flow distribution of a thermal interface material (TIM), thereby achieving improved TIM coverage and improved heat transfer between battery cells and the heat exchanger plate of the battery pack. These and other features are discussed in greater detail in the following paragraphs of this detailed description.

FIG. 1schematically illustrates a powertrain10for an electrified vehicle12. 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 (PHEV's), battery electric vehicles (BEVs), fuel cell vehicles, etc.

In an embodiment, the powertrain10is a power-split powertrain system that employs first and second drive systems. The first drive system may include a combination of an engine14and a generator18(i.e., a first electric machine). The second drive system may include at least a motor22(i.e., a second electric machine), the generator18, and a battery pack24. In this example, the second drive system is considered an electric drive system of the powertrain10. The first and second drive systems are each capable of generating torque to drive one or more sets of vehicle drive wheels28of the electrified vehicle12. Although a power-split configuration is depicted inFIG. 1, this disclosure extends to any hybrid or electric vehicle including full hybrids, parallel hybrids, series hybrids, mild hybrids, or micro hybrids.

The engine14, which may be an internal combustion engine, and the generator18may be connected through a power transfer unit30, 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 engine14to the generator18. In a non-limiting embodiment, the power transfer unit30is a planetary gear set that includes a ring gear32, a sun gear34, and a carrier assembly36.

The generator18can be driven by the engine14through the power transfer unit30to convert kinetic energy to electrical energy. The generator18can alternatively function as a motor to convert electrical energy into kinetic energy, thereby outputting torque to a shaft38connected to the power transfer unit30. Because the generator18is operatively connected to the engine14, the speed of the engine14can be controlled by the generator18.

The ring gear32of the power transfer unit30may be connected to a shaft40, which is connected to vehicle drive wheels28through a second power transfer unit44. The second power transfer unit44may include a gear set having a plurality of gears46. Other power transfer units may also be suitable. The gears46transfer torque from the engine14to a differential48to ultimately provide traction to the vehicle drive wheels28. The differential48may include a plurality of gears that enable the transfer of torque to the vehicle drive wheels28. In a non-limiting embodiment, the second power transfer unit44is mechanically coupled to an axle50through the differential48to distribute torque to the vehicle drive wheels28.

The motor22can also be employed to drive the vehicle drive wheels28by outputting torque to a shaft52that is also connected to the second power transfer unit44. In a non-limiting embodiment, the motor22and the generator18cooperate as part of a regenerative braking system in which both the motor22and the generator18can be employed as motors to output torque. For example, the motor22and the generator18can each output electrical power to the battery pack24.

The battery pack24is an exemplary electrified vehicle traction battery. The battery pack24may be a high voltage traction battery that includes a plurality of battery arrays25(i.e., battery assemblies or groupings of battery cells) capable of outputting electrical power to operate the motor22and/or other electrical loads of the electrified vehicle12and are capable of receiving power from the generator18. Other types of energy storage devices and/or output devices could also be used to electrically power the electrified vehicle12, including low voltage batteries.

In an embodiment, the electrified vehicle12has two basic operating modes. The electrified vehicle12may operate in an Electric Vehicle (EV) mode where the motor22is used (generally without assistance from the engine14) for vehicle propulsion, thereby depleting the battery pack24state of charge up to its maximum allowable discharging rate under certain driving patterns/cycles. The EV mode is an example of a charge depleting mode of operation for the electrified vehicle12. During EV mode, the state of charge of the battery pack24may increase in some circumstances, for example due to a period of regenerative braking. The engine14is generally OFF under a default EV mode but could be operated as necessary based on a vehicle system state or as permitted by the operator.

The electrified vehicle12may additionally operate in a Hybrid (HEV) mode in which the engine14and the motor22are both used for vehicle propulsion. The HEV mode is an example of a charge sustaining mode of operation for the electrified vehicle12. During the HEV mode, the electrified vehicle12may reduce the motor22propulsion usage in order to maintain the state of charge of the battery pack24at a constant or approximately constant level by increasing the engine14propulsion. The electrified vehicle12may be operated in other operating modes in addition to the EV and HEV modes within the scope of this disclosure.

FIGS. 2 and 3schematically depict a battery pack24that can be employed within an electrified vehicle. For example, the battery pack24could be part of the powertrain10of the electrified vehicle12ofFIG. 1.FIG. 2is a cross-sectional view of the battery pack24, andFIG. 3is an exploded view of the battery pack24(without cover62).

The battery pack24houses a plurality of battery cells56that store energy for powering various electrical loads of the electrified vehicle12. The battery pack24could employ any number of battery cells56within the scope of this disclosure. Therefore, this disclosure is not limited to the exact configuration shown inFIGS. 2-3.

The battery cells56may be stacked side-by-side to construct a grouping of battery cells56, sometimes referred to as a “cell stack” or “cell array.” In an embodiment, the battery cells56are prismatic, lithium-ion cells. However, battery cells having other geometries (cylindrical, pouch, etc.), other chemistries (nickel-metal hydride, lead-acid, etc.), or both could alternatively be utilized within the scope of this disclosure.

The battery cells56, along with any support structures (e.g., array frames, spacers, rails, walls, plates, bindings, etc.), may collectively be referred to as a battery array. The battery pack24depicted inFIG. 2includes a first battery array25A and a second battery array25B that is positioned adjacent to the first battery array25A. Although the battery pack24ofFIG. 2is depicted as having two battery arrays, the battery pack24could include a greater or fewer number of battery arrays within the scope of this disclosure. In addition, the battery arrays25A,25B are shown as being positioned end-to-end. However, the battery arrays25A,25B could alternatively be positioned side-by-side or in any other configuration relative to one another. Unless stated otherwise herein, when used without any alphabetic identifier immediately following the reference numeral, reference numeral “25” may refer to either battery array25A or battery array25B.

An enclosure assembly58houses each battery array25of the battery pack24. In an embodiment, the enclosure assembly58is a sealed enclosure that includes a tray60and a cover62that is secured to the tray60to enclose and seal each battery array25of the battery pack24. In another embodiment, the battery arrays25are positioned within the tray60of the enclosure assembly58, and the cover62may then be received over the battery arrays25. The enclosure assembly58may include any size, shape, and configuration within the scope of this disclosure.

Each battery array25of the battery pack24may be positioned relative to a heat exchanger plate64, sometimes referred to as a cold plate, such that the battery cells56are in close proximity to the heat exchanger plate64. In an embodiment, the battery arrays25A,25B share a common heat exchanger plate64. However, the battery pack24could employ multiple heat exchanger plates within the scope of this disclosure.

The heat exchanger plate64may be part of a liquid cooling system that is associated with the battery pack24and is configured for thermally managing the battery cells56of each battery array25. For example, heat may be generated and released by the battery cells56during charging operations, discharging operations, extreme ambient conditions, or other conditions. It may be desirable to dissipate the heat from the battery pack24to improve capacity, life, and performance of the battery cells56. The heat exchanger plate64may be configured to conduct the heat out of the battery cells56. For example, the heat exchanger plate64may function as a heat sink for removing heat from the heat sources (i.e., the battery cells56). The heat exchanger plate64could alternatively be employed to heat the battery cells56, such as during extremely cold ambient conditions, for example. Although shown as a separate component from the tray60, the heat exchanger plate64could be integrated with the tray60as a single component.

The heat exchanger plate64may include a plate body66and a coolant circuit68formed inside the plate body66. The coolant circuit68may include one or more passageways70that extend inside the plate body66. In an embodiment, the passageways70establish a meandering path of the coolant circuit68.

A coolant C may be selectively circulated through the passageways70of the coolant circuit68to thermally condition the battery cells56of the battery pack24. The coolant C may enter the coolant circuit68through an inlet72and may exit from the coolant circuit68through an outlet74(seeFIG. 3). The inlet72and the outlet74may be in fluid communication with a coolant source (not shown). The coolant source could be part of a main cooling system of the electrified vehicle12or could be a dedicated coolant source of the battery pack24. Although not shown, the coolant C may pass through a heat exchanger before entering the inlet72.

In an embodiment, the coolant C is a conventional type of coolant mixture, such as water mixed with ethylene glycol. However, other coolants, including gases, are also contemplated within the scope of this disclosure.

In use, heat from the battery cells56is conducted into the plate body66of the heat exchanger plate64and then into the coolant C as the coolant C is communicated through the coolant circuit68. The heat may therefore be carried away from the battery cells56by the coolant C.

In an embodiment, the heat exchanger plate64is an extruded part. In another embodiment, the heat exchanger plate64is made of aluminum. However, other manufacturing techniques and materials are also contemplated within the scope of this disclosure.

A thermal interface material (TIM)76may be positioned between the battery arrays25and the heat exchanger plate64such that exposed surfaces of the battery cells56are in direct contact with the TIM76. In an embodiment, downwardly facing bottom surfaces of the battery cells56are in direct contact with the TIM76. In another embodiment, thermal fins that are positioned between adjacent battery cells56of the battery arrays25are in direct contact with the TIM76. The TIM76maintains thermal contact between the battery cells56and the heat exchanger plate64and increases the thermal conductivity between these neighboring components during heat transfer events.

In an embodiment, the TIM76includes an epoxy resin. In another embodiment, the TIM76includes a silicone based material. Other materials, including thermal greases, may alternatively or additionally make up the TIM76.

Referring now primarily toFIG. 3, a plurality of bead lines78may be applied on the heat exchanger plate64during assembly of the battery pack24. Once cured, the bead lines78establish the TIM76between the battery cells56of the battery arrays25and the heat exchanger plate64. As the battery arrays25are moved (i.e., pushed down) onto the bead lines78or the bead lines78are moved (i.e., pushed up) into the battery arrays25during the assembly process, the bead lines78attempt to spread-out or distribute themselves evenly between the battery arrays25and the heat exchanger plate64. Prior to curing, the beads lines78are generally viscous and compliant; however, the bead lines78also exhibit some degree of resilience and therefore may provide a resistance force to the distribution of the TIM76. The resistance force may be transmitted to the heat exchanger plate64and cause the heat exchanger plate64to deflect downwardly (i.e., toward the tray60) during assembly. Deflection of the heat exchanger plate64may result in poor distribution of the bead lines78, thereby reducing the thermal effectiveness of the TIM76.

It is therefore desirable to substantially eliminate the deflection of the heat exchanger plate64during the battery pack assembly process in order to maximize the spreading distribution of the TIM76. Exemplary techniques for substantially eliminating deflection of the heat exchanger plate64during the assembly process are further discussed below.

FIG. 4, with continued reference toFIGS. 1-2, schematically illustrates an exemplary battery pack assembly process according to a first embodiment of this disclosure. The heat exchanger plate64may be maintained substantially flat (i.e., little to no bending) during the assembly process in order to prevent its deflection and maximize coverage of the TIM76. In an embodiment, a combination of supporting the tray60and incorporating a foam block80(i.e., a structural material) into the battery pack24substantially eliminates deflection of the heat exchanger plate64.

For example, the tray60may be positioned against a rigid workstation82. The workstation82supports a bottom84of the tray60. In an embodiment, the workstation82substantially rigidly supports the bottom84of the tray60. In this disclosure, the phrase “substantially rigidly supports” means that deflection of the tray60and/or heat exchanger plate64is reduced by at least 50% during the assembly process compared to a tray/heat exchanger plate that is not substantially rigidly supported during the assembly process.

The foam block80may then be positioned within the tray60followed by the heat exchanger plate64. The foam block80may therefore be positioned between the tray60and the heat exchanger plate64and substantially rigidly supports a bottom86of the heat exchanger plate64.

The combination of the rigid workstation82and the relatively stiff foam block80maintains the heat exchanger plate64flat during the subsequent assembly steps in which the battery array(s)25are positioned within the tray60and moved into contact with the bead lines78of the TIM76. Coverage of the TIM76relative to the heat exchanger plate64can therefore be maximized. In another embodiment, the foam block80or the rigid workstation82can be utilized alone to rigidly support the heat exchanger plate64during the assembly process.

The foam block80may be constructed of an expanded polymer-based material. Exemplary expanded polymer-based materials can include, but are not limited to, expanded polypropylene, expanded polystyrene, and expanded polyethylene. Generally, these polymer-based materials are considered relatively structural and stiff foamed polymer-based materials. By considering the design space that is available, the density of the foam block80may be chosen such that it offers the required stiffness for maintaining flatness of the heat exchanger plate64during assembly.

In an embodiment, the foam block80is maintained within (i.e., not removed from) the battery pack24after positioning the battery arrays25. The foam block80is therefore an integral structural component of the battery pack24upon completion of the battery pack assembly process.

FIG. 5schematically illustrates another exemplary battery pack assembly process. In this embodiment, the tray60may be positioned against a rigid workstation82during the assembly process. The workstation82substantially rigidly supports the bottom84of the tray60. In an embodiment, the workstation82includes a protruding surface88. The protruding surface88may be a convex surface, in an embodiment. The protruding surface88may be loaded against the tray60to force the tray60into the heat exchanger plate64near a center of the heat exchanger plate64as the heat exchanger plate64is positioned within the tray60. The protruding surface88maintains the heat exchanger plate64flat during the subsequent assembly steps in which the battery array(s)25are positioned within the tray60and moved into contact with the bead lines78of the TIM76. Coverage of the TIM76relative to the heat exchanger plate64can therefore be maximized. Although not shown, the foam block80could additionally be used to rigidly support the heat exchanger plate64in combination with the protruding surface88during the battery pack assembly process ofFIG. 5.

FIGS. 6A and 6Billustrate a side-by-side comparison of the TIM76coverage for a heat exchanger plate64-U that was unsupported during the battery pack assembly process (FIG. 6A) and for a heat exchanger plate64-S that was supported (such as in the manner shown inFIG. 4 or 5) during the battery pack assembly process (FIG. 6B). The unsupported heat exchanger plate64-U exhibits relatively poor distribution and coverage of the TIM76as evidenced by the relatively large gaps G1that extend between adjacent bead lines78of the TIM76. In contrast, the supported heat exchanger plate64-S exhibits significantly improved distribution and coverage of the TIM76as evidenced by the reduced gaps G2between the adjacent bead lines78of the TIM76. In an embodiment, the TIM76coverage for the supported heat exchanger plate64-S is up to 40% higher than the unsupported heat exchanger plate64-U.

The electrified vehicle battery pack designs of this disclosure utilize one or more stiff materials to provide a distributed response load from the workstation surface, to the battery pack tray, and then to the battery pack heat exchanger plate in order to promote a more complete flow distribution of the liquid TIM. The thermal effectiveness of the TIM is thereby improved during the life of the battery pack.