Abstract:
A battery includes a cell element that is disposed in a housing, and the housing is sealed with a top cover made primarily of plastic. The top cover may include a layer of metallic foil, which may make the top cover more impermeable to moisture. The top cover may also include a vent, which may or may not utilize the metallic foil to determined the primary opening force of the vent. The top cover may also have one or more stiffening ribs that extend downwardly from a bottom portion of the top cover to contact the cell element, so as to limit movement of the cell element within the housing. In addition, the top cover may have one or more conductive terminals that are at least partially overmolded by the plastic of the top cover.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Non-Provisional Application of U.S. Provisional Patent Application No. 61/593,247, entitled “Plastic Cover for Lithium-ion Cells,” filed Jan. 31, 2012, which is herein incorporated by reference in its entirety for all purposes. 
     
    
     BACKGROUND 
       [0002]    This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
         [0003]    The present disclosure relates generally to the field of batteries and battery systems. More specifically, the present disclosure relates to batteries and battery systems that may be used in vehicle applications to provide at least a portion of the motive power for the vehicle. 
         [0004]    Vehicles using electric power for all or a portion of their motive power (e.g., electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and the like, collectively referred to as “electric vehicles” (xEVs)) may provide a number of advantages as compared to more traditional gas-powered vehicles using internal combustion engines. For example, electric vehicles may produce fewer undesirable emission products and may exhibit greater fuel efficiency as compared to vehicles using internal combustion engines. In some cases, such vehicles may eliminate the use of gasoline entirely, as is the case of certain types of EVs. 
         [0005]    As electric vehicle technology continues to evolve, there is a need to provide improved power sources (e.g., battery systems or modules) for such vehicles. For example, it is desirable to increase the distance that such vehicles may travel without the need to recharge the batteries. It is also desirable to improve the performance of such batteries and to reduce the cost associated with manufacturing and/or using the battery systems. 
       SUMMARY 
       [0006]    Certain aspects commensurate in scope with certain embodiments disclosed herein are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain embodiments and that these aspects are not intended to limit the scope of the disclosure or claims. Indeed, the disclosure and claims may encompass a variety of aspects that may not be set forth below. 
         [0007]    A battery includes a top cover to enclose a cell element and other internals within a housing. In certain embodiments, the top cover may include a molded plastic material that hermetically seals the battery internals. Plastic is relatively light and may have a lower manufacturing cost and improved mechanical properties compared to certain other materials. The positive and/or negative terminals of the battery may be overmolded by the top cover to further simplify manufacturing and reduce cost. Furthermore, a metal foil may be disposed within the plastic cover to reduce the overall water permeability of the battery and/or to provide for efficient venting of the battery. 
         [0008]    With regard to venting, the cell element of the battery may produce gases that build up within the housing, and these gases can increase the internal pressure of the battery. Accordingly, it may be desirable to vent these gases to decrease the internal pressure of the battery. To this end, the top cover may include a vent that is designed to create an opening in the top cover, thereby enabling the gases to escape. In particular, the vent may be designed to deploy from a central point or axis of the vent. In other words, the peripheral portion of the vent may remain fixed (e.g., hinged) to the top cover, whereas an inner portion of the vent extends in a direction transverse to the top cover. In addition, the vent may be thinner than the remainder of the top cover to facilitate deployment of the vent while retaining the structural integrity of the top cover. 
         [0009]    Furthermore, the battery may include one or more stiffening ribs internally coupled to the top cover. In certain embodiments, the top cover of the battery may be substantially flat, whereas the cell element within the housing has a prismatic shape with a rounded or arcuate edge. Due to their respective geometries, a gap may be produced between the top cover and the cell element. In embodiments where the stiffening ribs are employed, the stiffening ribs serve to occupy the gap between the top cover and the cell element, thereby forming an abutment surface between the top cover and the cell element. The abutment surface helps to support the cell element within the housing and to reduce movement of the cell element within the housing. 
     
    
     
       DRAWINGS 
         [0010]    The disclosure will become more fully understood from the following detailed description, taken in conjunction with the accompanying figures, wherein like reference numerals refer to like elements, in which: 
           [0011]      FIG. 1  is a perspective view of an embodiment of a vehicle having a battery system to provide power for various components of the vehicle; 
           [0012]      FIG. 2  is a cutaway schematic view of an embodiment of the vehicle and the battery system of  FIG. 1 ; 
           [0013]      FIG. 3  is a perspective view of an embodiment of a battery that may be used within the battery system of  FIG. 1 ; 
           [0014]      FIG. 4  is an exploded view of an embodiment of the battery of  FIG. 3 ; 
           [0015]      FIG. 5  is a cross-sectional view of an embodiment of the battery of  FIG. 3 , taken along lines  5 - 5 , illustrating overmolded terminals and different thicknesses of a vent and a remaining portion of a top cover of the battery; 
           [0016]      FIG. 6  is a top view of an embodiment of the battery of  FIG. 3 , illustrating features of the vent; 
           [0017]      FIG. 7  is a schematic view of an embodiment of the vent of  FIG. 6 , illustrating a rectangular shape; 
           [0018]      FIG. 8  is a schematic view of an embodiment of the vent of  FIG. 6 , illustrating an arcuate shape; 
           [0019]      FIG. 9  is a schematic view of an embodiment of the vent of  FIG. 6 , illustrating a square shape; 
           [0020]      FIG. 10  is a side view of an embodiment of the battery of  FIG. 3 , taken along lines  5 - 5 , illustrating one or more stiffening ribs that may abut against a cell element; and 
           [0021]      FIG. 11  is a side view of an embodiment of the stiffening rib, taken along lines  11 - 11  of  FIG. 10 , illustrating an arcuate shape that may abut against the cell element. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    One or more specific embodiments of the present disclosure will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
         [0023]    For the purposes of the present disclosure, it should be noted that the presently disclosed embodiments are particularly directed toward applications for electric vehicles. In particular, the term “xEV” may be used herein to describe any vehicle that derives at least a portion of its motive power from an electric power source (e.g. a battery system). As will be appreciated by those skilled in the art, hybrid electric vehicles (HEVs) combine an internal combustion engine propulsion and high voltage battery power to create traction. The term HEV may include any variation of a hybrid electric vehicle, such as micro-hybrid and mild hybrid systems, which disable the internal combustion engine when the vehicle is idling and utilize a battery system to continue powering the air conditioning unit, radio, or other electronics, as well as to kick-start the engine when propulsion is desired. The mild hybrid system may apply some level of power assist to the internal combustion engine, whereas the micro-hybrid system may not supply power assist to the internal combustion engine. A plug-in electric vehicle (PEV) is any vehicle that can be charged from an external source of electricity, such as wall sockets, and the energy stored in the rechargeable battery packs drives or contributes to drive the wheels. PEVs are a subcategory of electric vehicles that include all-electric or battery electric vehicles (BEVs), plug-in hybrid vehicles (PHEVs), and electric vehicle conversions of hybrid electric vehicles and conventional internal combustion engine vehicles. An electric vehicle (EV) is an all-electric vehicle that uses for its propulsion one or more motors powered by electric energy. The term “xEV” is defined herein to include all of the foregoing or any variations or combinations thereof that include electric power as a motive force. 
         [0024]    In accordance with presently disclosed embodiments, provided herein are batteries with improved mechanical properties and reduced manufacturing costs. In particular, a top cover of the battery may include a molded plastic. A vent may be disposed on the top cover has a lesser thickness than the remainder of the top cover, thereby facilitating deployment of the vent to allow effluent gases to escape from the battery while retaining the structural integrity of the top cover. In addition, the top cover may include an internal metal foil to reduce the water permeability of the battery and/or to work in conjunction with the vent to facilitate venting of battery gases. Furthermore, the top cover may also include one or more stiffening ribs that extend into the housing to reduce the movement of a cell element within the battery. It should be noted that the plastic top cover, the vent, the internal metal foil, and the stiffening ribs may be used separately or in some combination with one another. 
         [0025]    Turning now to the figures,  FIG. 1  is a perspective view of an embodiment of a vehicle  10  (e.g., an xEV) in the form of an automobile (e.g., a car) having a battery system  12  for providing power to various components of the vehicle  10 . For example, the battery system  12  may provide all or a portion of the motive power for the vehicle  10 . Such a vehicle  10  may be an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), or another type of vehicle using electric power for some or all of its motive power. 
         [0026]    Although the vehicle  10  is illustrated as a car in  FIG. 1 , the type of vehicle may differ according to other embodiments, all of which are intended to fall within the scope of the present disclosure. For example, the vehicle  10  may be a truck, bus, industrial vehicle, motorcycle, recreational vehicle, boat, locomotive, airplane or any other type of vehicle that may benefit from the use of electric power generated by a battery system. However, it should be appreciated that the techniques described herein could also be used in battery systems in a wide variety of non-vehicular applications as well, such as generator sets, turbines, wind farms, etc. 
         [0027]    To use electrical power to propel the vehicle  10 , the vehicle  10  may include various internal components, such as a motor, a transmission system, and the like. The various internal components of the vehicle  10  are illustrated in greater detail with respect to  FIG. 2 , which illustrates a cutaway schematic view of the vehicle  10 . The battery system  12  is provided toward the rear of the vehicle  10  proximate a fuel tank  14 . It should be noted that the battery system  12  may be located in various areas within the vehicle  10 , such as immediately adjacent the fuel tank  14  or in a separate compartment of the vehicle  10 . The battery system  12  is used to provide power to an electric motor  16 , which, in turn, may provide all or a portion of the motive power for the vehicle  10 . An internal combustion engine  18  may also be used to provide a portion of the motive power for the vehicle  10 . 
         [0028]    As shown, the electric motor  16  and the engine  18  are coupled to a transmission system  20  to provide motive power for the vehicle. The transmission system  20  provides a controlled application of power from the electric motor  16  and the engine  18  to a plurality of wheels  22 . As noted earlier, the type of the vehicle  10  may differ, and the number of wheels may also differ accordingly. For example, the vehicle  10  may have 2 wheels (e.g., a motorcycle), 3 wheels (e.g., an all-terrain vehicle), 4 wheels (e.g., a car), or 5 or more wheels (e.g., a truck, bus, and the like). The electric motor  16  is powered by a plurality of electrochemical cells or batteries  24  within the battery system  12 . That is, the batteries  24  supply electrical energy to the electric motor  16 , which converts the electrical energy into mechanical energy to rotate the wheels  22 . 
         [0029]    To illustrate the external structure of the battery  24 ,  FIG. 3  is a perspective view of an embodiment of the battery  24  used to supply power to the electric motor  16 . As shown in  FIG. 3 , the battery  24  includes a housing  26  having a generally flat top cover  28 , a bottom portion  30 , and a side portion  32  extending therebetween. The shape of the housing  26  enables multiple batteries  24  to be placed adjacent to one another with minimal void space, such as in a battery pack of the battery system  12 . However, the shape of the housing  26  may vary according to implementation-specific designs, and may be, for example, cylindrical, prismatic, polyhedral, or any other suitable shape. 
         [0030]    The top cover  28  includes a positive terminal  34  and a negative terminal  36  that enable the transfer of energy from the battery  24  to a load. The positive terminal  34  and negative terminal  36  may be made of conductive material, such as steel, aluminum or copper. Moreover, since the top cover  28  of the battery  24  is made of a plastic or polymer material in this embodiment, the positive terminal  34  and negative terminal  36  may be overcoated with such material. In other words, during manufacture of the plastic top cover  28 , the conductive materials used to form the positive terminal  34  and the negative terminal  36  may be placed in the mold so that the plastic is formed around the conductive materials, as will be described in greater detail with respect to  FIG. 5 . Such a process fixes the positive terminal  34  and negative terminal  36  in place, thus reducing manufacturing time, steps and cost. 
         [0031]    The top cover  28  includes a vent  38  that is designed to open along a center or axis  40  in response to a pressure buildup within the battery  24 . That is, gases may accumulate within the housing  26  as a result of chemical reactions occurring within the battery  24 . Buildup of these gases may increase the pressure within the housing  26 , thereby reducing the operability of the battery  24 . If the pressure becomes great enough, it may be is desirable to release the gases through the vent  38  to relieve the pressure within the battery housing  26 . The vent  38  will be described in more detail below with respect to  FIGS. 6-9 . 
         [0032]    To illustrate the internal components of the battery  24 ,  FIG. 4  is an exploded view of an embodiment of the battery  24  of  FIG. 3 . A cell element  46  is disposed within the housing. The cell element  46  includes a positive electrode  48  and a negative electrode  50  (e.g., a cathode and an anode), as well as an electrolyte (e.g., lithium, nickel-metal-hydride, lead, and the like) that stores chemical potential energy that may be later be converted into electrical energy for the electric motor  16  of  FIG. 2 . The positive electrode  48  is coupled to the positive terminal  34  of the battery  24  via a positive current collector  52 , and the negative electrode  50  is coupled to the negative terminal  36  of the battery  24  through a negative current collector  54 . Thus, when an electrical connection is made between the positive and negative terminals  34  and  36  of the battery  24 , a complete electrical circuit is formed between the positive and negative electrodes  48  and  50 , thereby allowing current to flow from the battery  24 . 
         [0033]    In embodiments where the positive and negative terminals  34  and  36  are not overmolded with the plastic top cover  28 , the battery  24  may also include insulators  56  and  58  that electrically insulate the current collectors  52  and  54  and the terminals  34  and  36  from the housing  26 . In a similar manner, electrically insulating films  60  and  62  (e.g., polymer, polyimide, etc.) may surround the cell element  46  when the battery  24  is assembled, thereby insulating the cell element  46  from the housing  26 . Accordingly, the insulators  56 ,  58 ,  60 , and  62  may reduce the possibility of charge or electrical current flowing through the housing  26 , which is typically made of a conductive material, such as metal. However, in certain embodiments, the insulating films  60  and/or  62  may not be used. For example, if all or a portion of the housing  26  is constructed from non-conductive materials, e.g., non-metallic materials such as plastic, to decrease the weight and manufacturing cost of the battery, then the housing  26  itself may provide adequate electrical insulation. Alternatively, if the housing  26  is made of a conductive material, such as metal, the insulating films  60  and/or  62 , as well as one of the insulators  56  or  58 , may not be used if it is desired that the housing  26  be negatively or positively charged. 
         [0034]    However, in embodiments where the positive and negative terminals  34  and  36  are overmolded with the plastic top cover  28 , the battery  24  may generally be depicted as illustrated in  FIG. 5 , which is cross-sectional view of the battery  24  taken along lines  5 - 5  of  FIG. 3 . In certain embodiments, the plastic top cover  28  may be formed by, for example, over-molding or insert-molding plastic over the terminals  34  and  36 . In other words, portions  63  of the top cover  28  may be molded around the terminals  34  and  36  so that they may extend through the plastic top cover  28 . As depicted, a portion of the terminals  34  and  36  may extend above the portions  63  of the top cover  28  so that the terminals  34  and  36  can be easily coupled to a load, and a lower portion of the terminals  34  and  36  are disposed inside the housing  26  and respectively coupled to the negative and positive current collectors  52  and  54 . Overmolding the terminals  34  and  36  with the plastic top cover  28  not only simplifies manufacturing and reduces costs, but it also provides a hermetic seal around the terminals  34  and  36  to facilitate operability of the battery  24 . 
         [0035]    There are a variety of ways that the top cover  28  may be coupled to the housing  26  of the battery  24 . In the depicted example, the housing  26  may be made of metal, the plastic top cover  28  may be supported by a ledge  64  and substantially surrounded by extensions  66 . The extensions  66  of the housing  26  may be folded over the plastic top cover  28  to secure the plastic top cover  28  in place. In embodiments, where the housing  26  is made of another material, such as plastic, the extensions  66  may also be made of plastic and molded over the top cover  28 . Of course, it should be appreciated that the top cover  28  may be coupled to the housing  26  in any suitable manner, such as by using bolts, welding, adhesive, etc. 
         [0036]    In the depicted embodiment, the plastic top cover  28  includes a thin metallic foil  68 , which may decreases the water permeability of the top cover  28 . As will be appreciated, the introduction of water into the housing  26  may reduce the efficiency of the battery  24  by interfering with chemical reactions within the cell element  46 . Plastics are generally more water-permeable than metals. However, the inclusion of the metal foil  68  enables the top cover  28  to be formed from plastic while still retaining the lower water permeability of metals. 
         [0037]    The shape of the metallic foil  68  may be based on the location of the terminals  34  and  36 . For example, if the metallic foil  68  is in electrical contact with both of the terminals  34  and  36 , an undesirable short circuit could be formed through the metallic foil  68  and the cell element  46 . Accordingly, the metallic foil  68  may be designed to be in electrical contact with at most one (i.e., zero or one) of the terminals  34  and  36 . In some configurations, the design of certain systems (e.g., a current-interrupt system) may be based on having an electrically-charged housing  26 . The electrically-charged housing  26  may be enabled by having the metallic foil  68  in contact with one of the terminals  34  or  36 . Alternatively, a neutral-charged housing  26  may be obtained by ensuring that the metallic foil  68  does not contact either of the terminals  34  or  36 . 
         [0038]    As explained earlier, the battery  24  includes the vent  38  that may be used to relieve effluent gases within the housing  26 , thereby decreasing the internal pressure of the battery  24 . In certain embodiments, the vent  38  may be integrally formed in the top cover  28 . For example, grooves and/or other features of the vent  38  may be molded, stamped, scribed or otherwise formed into the top cover  28 . These grooves or other features are designed to provide an area of the top cover  28  for the vent  38  that is weaker than the remainder of the top cover  28 , so that the vent  38  will open once the build up of gases within the housing  26  reaches a certain point. Furthermore, as explained in greater detail with respect to  FIGS. 6-9 , these grooves or other features may be designed to cause the vent  38  to open in a certain manner. 
         [0039]    To maintain the structural integrity of the top cover  28  while providing the vent  38 , the top cover  28  may have a variable thickness across its length. More specifically, the plastic in the area  70  of the vent  38  is thinner than the plastic in the remaining area  72  of the top cover  28 . The thinner plastic  70  enables the vent  38  to open with less resistance, while the thicker plastic  72  maintains the structural integrity of the top cover  28 . 
         [0040]    The thickness of the plastic in the area  70  of the vent  38  may be designed based on a desired operating force needed to deploy the vent  38 . For example, the plastic in the area  70  may be made thicker if a greater operating force is desired or thinner if a lesser operating force is desired. Furthermore, the thickness of the plastic in the area  70  may depend on various other factors, such as the chemistry of the cell element  46  or the application of the battery system  12 . The desired operating force may also affect other factors in designing the vent  38 , such as the size, shape, and geometry of the vent  38 , as discussed below with respect to  FIGS. 6-9 . 
         [0041]    Generally speaking, however, the thickness of the plastic in the area  70  of the vent  38  may not contribute significantly in the determination of how much force is required to open the vent  38  in embodiments in which the top cover  28  includes the metallic foil  68 . Indeed, the thickness and/or composition of the metallic foil  68  may be selected such that the metallic foil  68  provides the primary barrier between the internals of the battery  24  and the environment outside of the battery  24 . In such embodiments, the plastic in the area  70  of the vent  38  may provide little additional resistance to forces that may build up inside of the battery  24 . However, even if the plastic in the area  70  provides little resistance with regard to the force required to open the vent  38 , the design of the grooves and/or other features of the plastic in the area  70  may provide some control as to the manner in which the vent  38  opens or deploys. Furthermore, the plastic in the area  70  provides protection for the metallic foil  68  to prevent corrosion or mechanical damage, e.g., scratches. 
         [0042]    In one example, the vent  38  may be circular, as illustrated in  FIG. 6 . In this embodiment, a circular groove  78  defines an outer periphery of the vent  38 , and crosswise grooves  82  and  84  extend from the circular groove  78  through the center or axis  40  of the vent  38  to create four flaps or subsections  86 . When the vent  38  deploys, the vent  38  separates along the grooves  82  and  84  such that the subsections  86  open away from the center or axis  40  of the vent  38 . When the subsections  86  are extended, the circular groove  78  tends to act as a hinge so that outer periphery of the vent  38  remains fixed to the top cover  28 . Hence, the grooves  78 ,  82  and  84  facilitate a relatively controlled and predictable manner in which the vent  38  opens. 
         [0043]    In certain embodiments, the number, size, and/or shape of the vents  38  may vary. For example, the top cover  28  may include a single vent  38  or multiple vents  38 , and the vents  38  may be square, circular, polygonal, arcuate, or another suitable shape. Furthermore, the size, location, and number of the grooves may vary. Examples of alternative designs are illustrated in  FIGS. 7-9 , but it should be noted that these embodiments are given by way of example, and are not intended to be limiting. 
         [0044]    In one alternative embodiment,  FIG. 7  illustrates that the vent  38  has its outer periphery defined by a rectangular groove  80  with crosswise grooves  80  and  82  defining the flaps or subsections  86 . Like the circular design described above, when the vent  38  deploys, the vent  38  separates along the grooves  82  and  84  such that the subsections  86  open away from the center or axis  40  of the vent  38 . When the subsections  86  are extended, the rectangular groove  88  tends to act as a hinge so that outer periphery of the vent  38  remains fixed to the top cover  28 . In another alternative embodiment,  FIG. 8  illustrates that the vent  38  has its outer periphery defined by an elliptical groove  96  with the crosswise grooves  82  and  84  defining the flaps or subsections  86 . When the vent  38  deploys, the vent  38  separates along the grooves  82  and  84  such that the subsections  86  open away from the center or axis  40  of the vent  38 . When the subsections  86  are extended, the elliptical groove  96  tends to act as a hinge so that outer periphery of the vent  38  remains fixed to the top cover  28 . However, if the grooves  82  and  84  do not extend to an outer edge  96  of the vent  38 , the subsections  86  may be more resistant to separation and may not separate entirely during deployment of the vent  38 . Furthermore, the operating force needed to deploy the vent  38  may increase. Lastly,  FIG. 9  illustrates a vent  38  having its outer periphery defined by a square groove  98  with a single groove  100  extending through the center  40  of the vent  38  to define two flaps or subsections  86 . When the vent  38  deploys, the vent  38  separates along the groove  100  such that the subsections  86  open away from the center or axis  40  of the vent  38 . When the subsections  86  are extended, at least some portion of the square groove  98  tends to act as a hinge so that outer periphery of the vent  38  remains fixed to the top cover  28 . 
         [0045]    The top cover  28  may also include other features to improve the operability and life of the battery  24 . For example, as illustrated in  FIG. 10 , the bottom portion of the top cover  28  may include a structure designed to hold the top portion of the cell element  46  in place to reduce or eliminate axial movement of the cell element  46  within the housing  26 . In the illustrated embodiment, this structure may take the form of one or more stiffening ribs  102  coupled to the bottom of the top cover  28 . The stiffening ribs  102  extend downwardly within the housing  26  to contact the top of the cell element  46 . Because the top of the cell element  46  has a generally arcuate shape due to the winding or folding of the plates used to form the cell element  46 , the portion of the stiffening ribs  102  that contact the top of the cell element  46  may have a complimentary arcuate shape, as shown in  FIG. 11 . In particular, the length, shape and number of the stiffening ribs  102  are selected to substantially eliminate any gap between the top cover  28  and the cell element  46  so that movement of the cell element  46  within the housing  26  is reduced or eliminated. 
         [0046]    The stiffening ribs  102  may be formed of plastic, and in certain embodiments, may be integrally formed with the plastic top cover  28 . In addition, the stiffening ribs  102  may be disposed in a variety of locations along the plastic top cover  28 . For example, as shown in  FIG. 10 , the stiffening ribs  100  are not disposed directly underneath the vent  38  to facilitate deployment of the vent  38 . However, in certain embodiments, the stiffening ribs  102  may be employed continuously across the length of the top cover  28 . Furthermore, the number of stiffening ribs  102  may vary. For example, the top cover  28  may include a plurality of equally-spaced stiffening ribs  102 , as shown. Alternatively, the top cover  28  may include a single stiffening rib or surface extending across the length of the top cover  28 . The number and size of stiffening ribs  102  may be influenced by several factors, such as a desired weight of the battery, manufacturing cost, and other mechanical properties. 
         [0047]    While the disclosure may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the disclosure is not intended to be limited to the particular forms disclosed. Rather, the disclosure is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure as defined by the following appended claims.