Plastic battery container having reduced end wall deflection

A plastic battery container for a recombinant sealed lead-acid battery having reduced end wall deflection, the end wall comprising a base portion from which extends a series of integrally molded ribs disposed at approximately +/-45.degree. to the horizontal, substantially all intersection points of the ribs with the base portion having a rounded character to enhance processing characteristics.

FIELD OF THE INVENTION
 This invention relates generally to battery containers for lead acid
 batteries, and more particularly to battery container designs for
 minimizing container end panel distortion in recombinant sealed batteries.
 BACKGROUND OF THE INVENTION
 Lead-acid batteries and cells have been known for a substantially long
 period of time and have been employed commercially in a relatively wide
 variety of applications. Such applications have ranged from starting,
 lighting and ignition for automobiles, trucks and other vehicles (often
 termed "SLI batteries") to marine and golf cart applications and to
 various stationary and motive power source applications (sometimes termed
 "industrial battery" applications).
 The lead-acid electrochemical system provides a reliable energy source
 which is capable of being manufactured in automated production while
 providing acceptable quality. At this time, battery containers are
 generally manufactured in large volumes as injection molded plastic parts.
 As the battery container includes five of the six sides of the exterior of
 the battery, this component is largely responsible for the final
 dimensions of the battery, as well as its cosmetic appearance. Beyond the
 appearance of the battery, the dimensions of the upper opening of the
 container must be sufficiently precise to permit a seal between the
 container and the lid of the battery in order to ensure proper operation
 and prevent leakage.
 During use, however, lead-acid batteries may develop or be exposed to
 extremely high operating temperatures and pressures. The electrochemical
 reactions within the cells of a lead-acid battery, particularly in a
 recombinant sealed battery, result in the development of high pressures,
 as well as high temperatures. While the exact parameters reached will vary
 based upon the particular battery design, the internal pressure of a
 battery, for example, may reach on the order of three to six pounds per
 square inch (3-6 p.s.i.), while the temperature may reach over 200.degree.
 F.
 These high pressures and temperatures within the battery may cause the
 battery container to deflect and distort. This deflection may be
 restrained along the side walls of the container inasmuch as the
 partitions between the cells extend crosswise through the battery from
 side wall to side wall. Accordingly, the bulk of such deflection occurs on
 the end walls of the container where there are no interior partitions to
 restrain the deflection. In tests of a Group 27 battery of the assignee of
 the present invention, the end wall of the container having vertical
 ribbing was measured to deflect 0.085 inch at 1 p.s.i., 0.236 inch at 3
 p.s.i., and 0.342 inch at 5 p.s.i.
 This deflection may adversely affect the performance of the battery as well
 as the cosmetic appearance. As the end walls deflect, the cells expand,
 allowing the plates to separate and pull apart. This reduction in cell
 compression results in a corresponding reduction in battery performance.
 Further, the deflection of the end walls increases the effective length of
 the battery and decreases the overall attractiveness thereof. It has
 further been observed that in severe cases, the plastic container may
 crack at points of high deflection and stress, resulting in leaks.
 This problem may be exacerbated by the environmental conditions of the
 battery. For example, current vehicles, particularly automobiles,
 emphasize aerodynamic styling and are equipped with a variety of driver
 comfort features and safety devices. These features have resulted in such
 vehicles operating in many situations with very high underhood engine
 temperatures. The battery may be located in the front of the underhood
 compartment, where there is little air movement, or where the engine fan
 blows hot air directly onto the battery. Accordingly, during stop-and-go
 driving, or while the engine of the vehicles is idling, there is typically
 very little air or wind movement, causing the underhood air temperatures
 to often exceed 200.degree. F. in some parts of the United States. Thus,
 these increased temperatures may further contribute to distortion of the
 battery container during operation.
 In the early part of the twentieth century and up to the sixties, battery
 containers were constructed of molded hard rubber, sometimes using coal as
 a filler. On occasion, the molded rubber container was surrounded by a
 wooden box in order to permit easy handling or restrain the walls of the
 container. Further, because the container was made of molded rubber, it
 could readily be molded to a thicker dimension in order to minimize any
 deflection thereof. Recombinant sealed batteries, however, were not
 developed and did not come into common use until the late 1970's and early
 1980's. Accordingly, the high internal pressures associated therewith were
 not typically even a problem with batteries which utilized molded hard
 rubber containers prior to the advent of the plastic battery container.
 Accordingly, deflection of the end walls due to the batteries developing
 high internal pressures or temperatures during use was not typically a
 design consideration with molded rubber containers. Molded rubber
 containers also had certain disadvantages. Due to the thick, dense walls
 of the container, they are relatively heavy. Additionally, such containers
 were relatively fragile.
 While molded plastic containers are advantageous in view of size and
 weight, molding of plastic presents certain processing and design
 limitations, particularly in recombinant sealed batteries. In particular,
 molded plastic components exhibit different shrinkage factors depending
 upon the geometry and part thickness. As a result, and contrary to the
 design of molded hard rubber containers, the thickness of the end walls of
 a battery container may not be disparately greater than the thickness of
 the side walls or the partitions between the cell of the container without
 incumbent molding difficulties. Accordingly, battery designers have
 sometimes incorporated vertical and horizontal ribbing in the battery
 container in order to reduce container wall deflection. This design
 feature, however, has met with limited success.
 OBJECTS OF THE INVENTION
 Accordingly, it is the primary object of the invention to provide a
 container for a recombinant sealed lead-acid battery wherein the end walls
 demonstrate reduced deflection over those of conventional end walls. It is
 a more specific object of the invention to provide a lead-acid battery
 container that substantially retains the desired dimensions during use.
 It is a further object of the invention to provide a battery container for
 a recombinant sealed lead-acid battery that may be economically
 manufactured. It is a more specific object to provide a recombinant sealed
 lead-acid battery that may be molded in a reduced molding time using
 conventional plastic materials and conventional injection molding
 techniques. An additional object of the invention is to provide a
 container for a recombinant sealed lead-acid battery that requires reduced
 plant labor for processing thereof.
 A related object is to provide a container for a recombinant sealed
 lead-acid battery wherein the design displays good material
 characteristics.
 SUMMARY OF THE INVENTION
 In accomplishing these and other objects of the invention, there is
 provided a plastic container for a recombinant sealed lead-acid battery
 wherein the structure of at least one of the end walls includes a base
 portion having a series of ribs integrally molded therewith. The ribs are
 disposed on the order of +/-45.degree. to the horizontal. Preferably, at
 least four or more substantially parallel ribs are provided in a grid-like
 arrangement. The ribs thus define an arrangement of diamond-shaped flat
 sections along the base portion.
 All edges associated with the ribs are preferably rounded, that is, the
 crest of each rib, as well as the lines at which the sides of the rib meet
 the edges of the diamonds are rounded. The corners of the diamonds are
 likewise rounded, and round up into the intersections of the ribs.
 It will thus be appreciated that the ribs increase the effective strength
 and thickness of the end walls without the thickness of the end wall being
 uniformly increased. Moreover, the rounded character of the ribs provides
 many processing advantages that minimize fabrication costs. First, during
 molding, there is increased plastic mold flow, resulting in a part with
 less porosity and quicker molding time. Further, a container molded
 according to the inventive design may be easily demolded, decreasing cycle
 time and minimizing the opportunity for scrap parts. Additionally, during
 further processing, the rounded corners of the end wall retain a minimal
 amount of water, further reducing plant labor.
 While rib edges having square corners as opposed to rib corners to having a
 rounded character will likewise operate to reduce outward expansion of the
 end walls, the processing characteristics of a battery container having
 end walls with diagonal ribs having square corners are less advantageous
 than those having rounded edges. For example, containers having ribs with
 such square edges are not as easily demolded as those having round edges.
 Further, chipping can occur at these sharp corners when battery containers
 contact during the manufacturing processing and handling. This not only
 detracts from the aesthetic appearance of the battery container, but can
 actually cause serious damage to the container, which may consequently
 result in performance problems.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 Turning now to the drawings, there is shown in FIG. 1, a perspective view
 of a battery 10, which includes a top 12 and a container 14. The
 embodiment illustrated is a top terminal battery and, accordingly, the top
 12 includes the battery terminals 16, 18 and a venting structure 20. An
 alternate terminal structure may be provided, however, wherein the
 terminals are along a side of the battery (as in a side terminal battery),
 or along both the top and a side of the battery (as in a dual terminal
 battery).
 The battery container 14 is divided into a series of internal cells by cell
 partitions 22. As illustrated, six cells, which contain the chemical
 components of the battery 10, are provided for a twelve volt battery, as
 is of course customary for automotive SLI batteries, for a six volt
 battery, only three cell will be used, and the like. The cell partitions
 22 are preferably integrally molded with the container 14, extending
 between the container side walls 26 and the container bottom 28 (see FIG.
 7). It will thus be appreciated that during use, the cell partitions 22
 restrain the side walls 26 as well as the bottom 28 to limit any bulging
 thereof as a result of the elevated pressures and temperatures associated
 with operation of a lead-acid battery.
 In accordance with the invention, end walls 30 of the container 14 are of
 an integrally molded structure, having a base portion 32 from which
 extends a series of ribs 34 which likewise restrain the end walls 30 of
 the container 14 operation. According to an important feature of the
 invention, the ribs 34 are disposed at other than a normal angle to the
 horizontal bottom 28 of the battery container 14, that is, diagonally. In
 this way, the ribs 34 present a honeycomb type configuration having a
 series of approximately diamond-shaped relatively flat spots 38 at which
 the ribs 34 meet the base portion 32. It has been determined that ribs 34
 disposed at approximately +/-45.degree. to the horizontal bottom surface
 of the container 14, as shown in the figures, provides the best opposition
 to the deflection forces asserted against the container end walls 30. It
 will be appreciated, however, that the ribs may be disposed at slightly
 greater or less than +/-45.degree. and still provide improved, though
 slightly less effective, deflection resistance.
 In order to effectively minimize deflection in the end walls 30 of the
 container 14 during use, a sufficient number of ribs must be incorporated.
 The number of ribs 34 incorporated depends upon the spacing, as well as
 the height and width of the container end wall 30. Preferably, the crests
 40 of the ribs 34 are on the order of no more than 1.25 inches apart. In
 the currently preferred design, six ribs 34 are disposed at each
 +45.degree. and -45.degree., such that the crests 40 of the ribs 34 are
 less than one inch apart. It will be appreciated by those skilled in the
 art, however, that the spacing and number of ribs 34 may vary, so long as
 the desired strength is obtained.
 In order to minimize differences in shrinkage of various portions of the
 container 30, the container side walls 26 and the base portion 32 of the
 end walls 30 are of substantially similar thickness, and are not
 disparately thicker than the partitions 22. In this way, the container
 walls 26, 30 retain substantially the same relative geometry during
 molding to ultimately produce an appealing molded part of the desired
 dimensions. It will be appreciated by those skilled in the art that the
 ribs 34 thus will have minimal affect on the shrinkage of the end walls
 30, while increasing the over resistance of the end wall 30 to deflection
 due to internal forces.
 In the preferred embodiment of the container design, the container side
 walls 26 have a thickness on the order of 0.150 inch, while the partitions
 22 have a thickness of 0.065 inch at the top edge and a thickness of 0.130
 to 0.140 inch at the bottom. The base portion 32 of the container end
 walls has a thickness on the order of 0.210 inch. The ribs 34 are on the
 order of 0.175 inch. In a group 27 battery of this design, the deflection
 of the end walls 30 at 2 p.s.i. it was approximately 0.014 inch; at 4
 p.s.i. it was approximately 0.028 inch; and at 6 p.s.i. it was
 approximately 0.045 inch. Accordingly, a container constructed according
 to teachings of the invention yielded considerably less deflection than
 the standard container design having vertical ribs (set forth in the
 Background of the Invention section).
 According to another important feature of the invention, in order to
 facilitate molding and minimize costs associated with production of the
 container 14, each of the corners of the rib structure are rounded. As may
 be seen in FIG. 5, the ribs 34 are preferably rounded along their crests
 40, as well as along the intersection or edges 42 at which the sides 41
 meet the base portion 32. Additionally, the corners of the diamond shaped
 flat spots 38 are similarly rounded at the corners, and round up into the
 intersection 44 of the ribs 34, as best seen in FIG. 4. In the preferred
 embodiment, the corners of the diamond-shaped flat spots 38 (i.e., at the
 intersections 44 of the ribs 34) have a radius w on the order of 0.090
 inch; the crests 40 of the ribs 34 have a radius x on the order of 0.090
 inch; the inner edge 42 at the base portion has a radius y of
 approximately 0.060 inches; and the sides of the crests 40 are at a draft
 angle z on the order of 9.degree. from vertical. It will be appreciated by
 those skilled in the art that the measurements identified for the
 preferred embodiment are, of course, only exemplary figures. These numbers
 may vary while achieving similar desired results within the purview of the
 invention. For example, the crests 40 of the ribs 34 may typically have a
 radius x within a range of 0.060 and 0.120 inch.
 It will further be appreciated that that these rounded features of the
 container 14 provide a part that may be more easily demolded than would be
 possible with sharper edges. Additionally, the rounded edges minimize
 water retention during processing. As a result, the inventive design
 reduces plant labor costs associated with manufacturing the container 14.
 It will be further appreciated that the rounded edges of the part provide
 for optimal plastic flow within the mold. As a result, the molded part
 exhibits superior density characteristics. More specifically, the molded
 part has a lower porosity than a part molded with edges having a square
 characteristic. Further, the improved flow characteristics within the mold
 result in quicker molding time, further minimizing manufacturing costs.
 It will be appreciated that the ribs 34 may result in additional effective
 thickness of the end wall 30, even though the end wall 30 is not a
 uniformly thick structure. In order to maintain the original footprint of
 the battery 10, the ribs 34 along the edge of the end wall 30 adjacent the
 bottom 28 of the container 14 are angled toward the base portion 32, as
 may best be seen in FIG. 1.
 As may be seen in FIGS. 1-3, the intersections 44a of the ribs 34 located
 toward the side walls 26 of the container 14 are disposed slightly inward
 from the outside edge of the end wall 30. Preferably, the height of these
 intersections 44a extends outward to the edge of the end wall. In this
 way, additional strength is provided to the end wall 30 along the critical
 joint with the side walls 26.
 As shown in FIGS. 1 and 2, the attachment point 48 for the handle is
 disposed along the upper portion of the end wall 30. It will be
 appreciated, however, that any appropriate handle design and attachment
 location may be utilized.
 Any thermoplastic material and filler may be utilized which possesses the
 desired characteristics for molding battery containers pursuant to this
 invention. As is well known, the currently used materials for SLI
 lead-acid batteries comprise an ethylene-propylene impact-modified
 copolymer in which polypropylene is a major constituent.
 In summary, the invention provides a container design that provides
 enhanced resistance to deformation of the end walls due to internal
 pressures and internal and external temperatures developed during use. The
 rib structure increases the effective thickness and strength of the end
 wall without uniformly increasing the thickness of the end wall.
 Accordingly, the container may be fabricated minimizing molding
 difficulties related to differences in shrinkage rates. Additionally,
 rounded edges of substantially all locations where the ribs meet the base
 portion provide good material flow characteristics, an easily demolded
 part, and minimal retention of water during processing, thus providing an
 easily and economically fabricated part.