WO2000035036A1

WO2000035036A1 - Lead-acid cells, batteries and battery grids

Info

Publication number: WO2000035036A1
Application number: PCT/US1999/027972
Authority: WO
Inventors: Thomas F. Uhlemann; John H. Miller
Original assignee: Gnb Technologies, Inc.
Priority date: 1998-12-09
Filing date: 1999-11-24
Publication date: 2000-06-15
Also published as: AU2030900A; WO2000035036A9

Abstract

A lead-acid battery utilizes positive grids made by fabrication techniques other than gravity casting that are treated so as to provide a modified surface, preferably having an Rq value (i.e., root mean square deviation of surface height and depth) of at least 50 greater than that of the untreated surface. Said surface modification may be carried out by media blasting (using glass beads or black diamond grit), steam curing techniques, chemical etching, passing the grid through knurled rolls, or using a water jet. The surface treatment zone is situated in unit (49) in the figure.

Description

LEAD-ACID CELLS, BATTERIES AND BATTERY GRIDS

FIELD OF THE INVENTION

This invention relates to lead-acid batteries and, more particularly, to grids and plates used in making such batteries and to the method of making such grids and plates.

BACKGROUND OF THE INVENTION

Over the last 20 or so years, there has been substantial interest in automotive-type, lead-acid batteries which require, once in service, little, or more desirably, no further maintenance throughout the expected life of the battery. This type of battery is usually termed a "low maintenance" or "maintenance-free battery." The terminology maintenance-free battery will be used herein to include low maintenance batteries as well. This type of battery was first commercially introduced in about 1972 and is currently in widespread use.

It has been well recognized over the years that lead-acid batteries are perishable products. Eventually, such batteries in service will fail through one or more of several failure modes. Among these failure modes are failure due to positive grid corrosion and excessive water loss. The thrust of maintenance-free batteries has been to provide a battery that would forestall the failure during service for a period of time considered commensurate with the expected service life of the battery, e.g., three to five years or so.

To achieve this objective, the positive grids used initially for maintenance- free batteries typically had thicknesses of about 60 to about 70 mils or so. The batteries were likewise configured to provide an excess of the electrolyte over that needed to provide the rated capacity of the battery. In that fashion, by filling the electrolyte to a level above that of the top of the battery plates, maintenance-free batteries contained, in effect, a reservoir of electrolyte available to compensate for the water loss, during the service life of the battery. In other words, while the use of appropriate grid alloys will reduce water loss during the service life of the battery, there will always be some water loss in service. Over the past several years, the manufacture of such automotive lead-acid batteries, typically termed SLI automotive batteries (principally used for the starting, lighting and ignition requirements of an automobile), has become substantially more complex. Battery grids have typically been made by gravity casting (e.g., the molten alloy is fed into what is termed a book mold and is then allowed to solidify, the book mold providing either one or two side-by-side grids). Production equipment using an alternate method to fabricate grids is now commercially available by which battery grids can be continuously formed by expanded metal fabrication techniques. For example, a rolled or wrought alloy strip or a cast strip is slit and expanded using reciprocating dies or the like and then cut into the desired width and height dimensions to form the grid with a lug. In addition, various other processes are available in which grids are made by stamping or other techniques (such process include the Wirtz ConCast™ and the Conroll™ processes). One complicating factor in attempting to provide satisfactory service life is the seemingly ever-increasing power and energy requirements demanded in current SLI automotive batteries used in modern automobiles. Many factors have contributed to the need and or desire for such higher power and energy for such batteries. One major measure of power currently in common usage is the rated number of cold cranking amps. The number of cold cranking amps is considered in the industry as some indication of the relative power of the battery to start an automobile in cold temperature conditions.

Yet another complicating factor is the "under-the-hood" space requirements. Automobile manufacturers have significantly decreased the overall space available for batteries in the engine compartment. Typically, this has required that battery manufacturers provide a lower profile battery, viz., a battery having less overall height than previously required so as to meet current aerodynamic styling needs in automobiles. Such lower profile batteries will have less acid above the plates. These complicating factors (i.e., a need for increased power and energy with less available space for the battery) have required battery manufacturers to alter the battery internal design configurations to provide the needed power in a lower profile battery container. These internal alterations have typically involved increasing the number of plates used in each cell by employing battery grids with reduced thickness. For example, the number of plates in a BCI Group 24 battery has increased from about 13 to about 19 or so over the last few years while the thickness of the positive grids has decreased from about 65 to 75 mils or so down to about 45 mils and even less in some cases. The reduction in the thickness of the positive grids together with an increase in the number of plates has allowed battery manufacturers to provide Group 24 batteries having rated power output capabilities of 875 cold cranking amps or so. Battery manufacturers currently offer batteries in other BCI sizes having rated power output capabilities of up to 1000 cold cranking amps and even more.

Another aspect that has occurred in recent years is the substantial increase in the under-the-hood temperature to which the battery is exposed in automobile service. Obviously, the under-the-hood temperature is particularly high in the warmer climates. One automobile manufacturer has perceived that, in the past three years or so, the temperature to which an SLI battery is exposed under-the- hood in such warmer climates has risen from about 125°F to about 165°F-190°F in new automobiles. The specific temperature increase which is involved is not particularly important. What is important is that such under-the-hood temperatures have in fact increased. The impact of the under-the-hood vehicle service temperature increases on the failure modes has been to substantially increase the occurrence of premature battery failures. The incidence of premature battery failures due to excessive positive grid corrosion has been significant.

One attempt to deal with the acute problem of relatively high under-the- hood temperatures by one battery manufacturer has been to provide a battery designed for such high temperature conditions. This battery goes back to the use of thicker positive grids (about 70 mils or more) while using a smaller number of plates (back down to about 10 per cell). In addition, the head space in each cell is filled with hollow plastic microspheres. The use of such microspheres is perhaps to serve as a vapor barrier to the electrolyte for minimizing evaporative loss of water in the electrolyte or perhaps for limiting heat transfer or the like.

These overall battery requirements drastically increased the need for a positive grid that will impart, in the resulting battery, enhanced resistance to positive grid corrosion, particularly at elevated temperatures. A breakthrough was achieved by utilizing the alloys disclosed in U.S. 5,298,350 to Rao. The resulting batteries exhibited substantial improvements in service life and have effectively eliminated premature positive grid corrosion at such elevated temperatures as being the primary mode of failure. As previously noted, in addition to forming battery grids by gravity casting, equipment is now commercially available by which battery grids can be continuously cast on a rotary drum grid caster. Additionally, battery grids can also be continuously formed, or semi-continuously formed, by expanded metal fabrication techniques. Historically, the most widely used technique for making SLI battery grids has been conventional book mold gravity casting techniques. However, it has long been recognized that this batch technique can cause several production problems. In the first place, gravity casting techniques are subject to various problems which result in scrap as well as lack of product consistency and the like. These problems include operator errors; wide variation in grid wire thickness and hence overall weight due to mold coating variations and irregularities; substantial material handling in production and difficulty in automating such processes and the accompanying inconsistencies due to human error and the like.

Feeding of these individual grid panels made by gravity casting technique into the pasting machine during high speed production conditions can also result in frequent grid jam ups and with resultant scrap. Further, such jam ups result in production stoppage, lost production, clean-up.of jams and variation in paste machine set-up and attendant active material paste weight and thickness variations. Further, as is known, grids pasted with active material are typically stacked for paste curing prior to assembly of the battery. It is therefore necessary to remove a small quantity of paste surface moisture from the active material paste prior to stacking so that adjacent stacked, pasted plates will not stick together during curing and post-curing, before they are dried. As a practical matter, however, the tendency in commercial production is to surface dry more than is required so as to ensure that any possible sticking problems are eliminated. This further exacerbates the problem of providing product consistency.

Still frirther, a related problem is the development of what are often termed "checking cracks" or shrinkage cracks in the cured or dried active material paste on the plates, particularly adjacent to the grid wire surface. Such checking cracks can result from either excessive drying or from drying (i.e., moisture removal) too quickly. Such checking cracks not only decrease the expected service life but also the low and high rate discharge performance of batteries using plates having checking cracks because of poor paste adhesion to the underlying grid surface.

Another problem of substantial significance stems from the environmental issues involved in pasting, curing and assembly of batteries using gravity cast SLI battery grids. Lead dust is a major problem, stemming from loss of powdery active material from cured and dry paste during processing and handling while assembling batteries. Mechanical handling loosens powdery active material since there are no surface barriers. The resulting lead dust must be dealt with in an environmentally satisfactory manner, and production staff have to wear respirators while carrying out pasting and battery assembly operations. Indeed, a great many production safeguards need to be provided to handle powdery lead oxide dust.

Potentially, the use of any continuous process like continuous grid casting or other continuous or semi-continuous expanded metal fabrication techniques to make battery grids is capable of mimrnizing, if not eliminating, one or more of the problems associated with gravity casting techniques. There has accordingly been substantial interest and effort directed to the use of such techniques over the years. This effort has resulted in what is believed to be rather widespread use of various continuous, expanded metal fabrication processes for making SLI negative battery grids. The same benefits would result when using continuous process for making grids and plates for SLI positive battery grids. However, one major issue present with positive grids and plates, not an issue with negative battery grids and plates, is, as has been previously discussed herein, the corrosion of the positive battery grid and failure of SLI batteries because of such corrosion. U.S. 5,434,025 to Rao et al. provide a singular success as regards the use of selected calcium-tin-silver lead-based alloys used to make positive grids by a continuous process. The corrosion resistance and performance characteristics of the resulting batteries has heretofore been considered quite acceptable. Indeed, a variety of other expanded metal fabrication techniques using alloy strips have been tried in an effort to mimic the success of batteries made pursuant to the '025 Rao et al. patent. However, more recently, reflecting the increasing under-the-hood temperatures to which automotive batteries are subjected in service, the industry has modified the conventional SAE J240 life test. Previously, this test was performed at 40°C, but has now been modified to utilize a temperature of 75°C.

The performance of batteries made using grids produced by expanded metal fabrication processes are less than desired when utilizing this relatively newly- adopted, modified life test. Indeed, some skepticism has been expressed as to whether expanded metal or stamping techniques are suitable for the production of satisfactory positive grids and plates.

The issue is not one of positive grid corrosion perse. Rather, the problem has been characterized as undue positive active material softening. Because of the superior characteristics imparted to the resulting batteries when using the alloys in the patents hereinbefore identified, premature failure due to positive grid corrosion has been essentially eliminated. However, the use of this family of alloys, as well as other alloys used for SLI, maintenance-free batteries, present a variety of factors which, individually or collectively in sorne fashion, result in undue positive active material softening, thereby causing the resulting batteries made using positive plates with expanded metal fabrication techniques to have less than desired service life and performance characteristics. The phenomenon surrounding such positive active material softening is an issue which could impair the effectiveness of using these highly desired, expanded metal fabrication techniques for making positive grids and plates.

While the eventual key requirement for SLI batteries is field durability (i.e., the service life), it is likewise key that such batteries be capable of enduring elevated temperature life tests. More particularly, as is known, and as was previously alluded to, the industry utilizes a 75 °C SAE J240 cycle test as a benchmark, or indication, of field durability. Accordingly, the number of such cycles that SLI batteries can survive is important. Indeed, some commercial battery purchasers are setting standards for acceptability based on the ability to service a rninimum number of J240 cycles.

What has occurred is major disparity between the number of J240 cycles that can be achieved by SLI batteries using gravity cast, positive grids and the number achieved with batteries having positive grids made by expanded metal, stamping or the like. Thus, the performance of gravity cast grids is twice that (or even more) achieved using grids made with other techniques. This disparity seems to result regardless of the type of positive grid alloy used and irrespective of which process is used to replace gravity casting and whether the strip used is directly cast to the thickness desired or is cast thicker and then rolled to the grid thickness. The reason (or reasons) for this disparity are not known. Yet a solution is certainly needed to avoid the battery industry having to retreat to using only gravity casting techniques for making positive grids with its other significant disadvantages while foregoing other substantial performance and economic advantages made capable by using other grid fabrication techniques. The need is of such major importance that it cannot be easily overstated.

It is accordingly an object of the present invention to provide a maintenance-free, lead-acid battery using positive grids produced from grid manufacturing methods other than gravity casting techniques that are capable of satisfactory service life when operated in relatively high temperature environments. Yet another object provides a positive grid using commercially viable, expanded metal fabrication techniques for such maintenance-free batteries that will impart performance characteristics comparable to batteries using gravity cast positive grids. Another object provides a continuous method for making lead-acid battery positive plates also characterized by superior high temperature positive grid corrosion resistance.

Other objects and advantages of the present invention will become apparent as the following description proceeds, taken in conjunction with the accompanying drawings.

SUMMARY OF THE INVENTION

In accordance with the present invention, positive grids are made by techniques other than gravity casting which have modified grid surfaces. Such modified grid surfaces can be obtained by mechanical and/or chemical treatment that suitably alters the surface characteristics relative to that of the untreated, unmodified grid surface.

In the preferred embodiment, the treatment is carried out such that the surface modification, as viewed with SEM equipment, is generally consistent throughout. Stated differently, the treatment should result in uniform modification of the surface.

Further, while there are several techniques known for measuring surface modification, it has been found that the most meaningful parameter to measure the modified surface is Rq, as will be discussed more fully herein. The modification treatment should provide an Rq value of at least about 50 greater than that of the untreated grid, more preferably, at least about 75.

In this fashion, it is believed that batteries made using positive grids made by methods other than gravity casting will be provided that have desirable performance characteristics. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof will hereinafter be described in detail. It should be understood, however, that it is not intended to limit the invention to the particular form disclosed, but, on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as expressed in the appended claims. Thus, while the lead-acid cells and batteries herein have been described in conjunction with SLI automotive applications, it should be appreciated that such cells and batteries may be used for any other application, including for marine and other vehicle use. Further, the present invention may be used with sealed lead-acid cells, sometimes termed VRLA cells and batteries (valve-regulated lead-acid).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a maintenance- free battery of the present invention;

FIG. 2 is a cross-sectional view taken generally along the line 2-2 of FIGURE 1 and showing the battery plate and grid;

FIG. 3 is a schematic view of a preferred continuous method for making lead-acid positive battery grids and plates according the present invention; and FIG. 4 is a side elevation view of a grid made using the method shown in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 show a maintenance-free battery utilizing the continuously cast positive grids having roughened surfaces of the present invention. Thus, a maintenance- free battery 10 is shown which includes a container 12, a pair of side terminal posts 14 and a cover 16 sealed to the container by any conventional means. The container is divided into a plurality of cells, a portion of one cell being shown in FIG.2; and a battery element is disposed in each of these cells. The battery element comprises a plurality of electrodes and separators, one of the positive grids being shown generally at 18. The^'negative grids are of identical or similar construction but are formed with any desired antimony-free alloy. The electrode has a layer of active material 20 pasted thereto and has an integral lug 22. A strap 24 joins the lugs 22 of the respective positive and negative grids together.

Intercell connectors are shown generally at 26 and include a "tombstone" 28 which forms a part of the strap 24. The strap 24 may be fused to the grid lugs 22 in assembling the components into an element as is known. The terminals 14 are similarly electrically connected through separate straps 24 to the grid 18 during assembly, the base of the terπiinal forming a part of the strap 24. Suitable manifold venting systems for allowing evolved gases to escape in flooded electrolyte SLI batteries are shown at 34. Many satisfactory venting systems are well known. In addition, it is believed that all the present maintenance-free batteries manufactured in the United States will typically utilize flame retardant explosion-proof vent designs.

The particular design configurations of the battery may be varied as desired for the intended application. The positive grids described herein may be advantageously utilized in any type and size of lead-acid automotive battery. For example, the battery grids of the present invention may be advantageously used in dual terminal batteries such as those shown in U.S. Patent 4,645,725. Similarly, while a battery having side terminals has been exemplified, the battery of this invention could comprise a top terminal battery. The thickness of the positive grids can vary as is desired for a particular service life and a particular desired rated capacity. However, with any given thickness positive grid, the batteries utilizing the grids of the present invention will impart enhanced positive grid corrosion resistance to the battery in comparison to conventional maintenance-free batteries having positive grids formed from previously used continuously cast methods. In general, the grid thickness in the batteries of this invention can desirably vary from about 30 to about 75 mils for most applications. These grid thicknesses should be considered merely exemplary.

In general, cast positive grids will be made utilizing either a direct cast strip or an alloy strip that is cold-rolled. The grid is then made by a variety of known expanded metal fabrication techniques. As is known, direct cast strips are cast to the desired thickness whereas cold-rolled strips are cast and then rolled by various means to provide a strip of the desired thickness. The grids produced by both direct cast and cold-rolled strips exhibit strong orientation of grain boundaries, which is typically associated with variable corrosion resistance. The performance achieved with direct cast strips is considered far superior to that achieved with other cast and rolled strips due in large part to the microstructural instability caused by residual stresses and recrystallized zones present in the cold rolled strip. For this reason, the most preferred embodiment of the present invention utilizes directly cast strips. Accordingly, the positive grids treated to provide the modified, enhanced surface according to this invention make it commercially feasible to utilize expanded metal fabrication techniques to make the positive plates while achieving high temperature positive grid corrosion resistance and other characteristics required. The potential commercial implications are very significant. In addition to the environmental considerations, the economic benefits, it is believed, should amount to at least ten cents per battery, and, when all other beneficial aspects are considered, should be several times that amount.

Although the present invention will be described herein in the context of a directly cast alloy strip, this discussion is in no way intended to limited the scope of the invention to directly cast strips. In fact, as previously noted, the present invention is equally effective for any grid fabrication process other than gravity casting, i.e., using cold-rolled alloy strips or any of the other known methods for fabricating grids.

As noted, the most preferred method of the present invention thus involves, initially, providing an alloy strip directly cast to the desired thickness. The thickness of the alloy strip can be varied as is necessary to satisfy the service life and other requirements of the particular application. In general, for present SLI lead-acid battery applications, the strip thickness can vary from about 0.020 inches to about 0.060 inches. As used herein, the terminology "directly cast" refers to a continuous strip that is cast directly from molten lead alloy into the thickness desired for making the positive grids. The casting process thus does not include any cold rolling or other reduction in the thickness of the strip from the cast thickness to the thickness desired for making the positive grid. Equipment for making a suitable directly cast alloy continuous strip from molten lead alloy is commercially available (Cominco Ltd., Toronto, Canada). U.S. 5,462,109 to Vincze et al. discloses a method for making a directly cast strip.

This directly cast strip can then be converted by known expanded metal fabrication techniques to achieve a continuous source of an expanded lead-alloy grid mesh strip suitable for conversion into positive lead-acid battery plates. In general, as is known, these operations involve first expanding into grids and pasting with positive or negative paste and then slitting the moving alloy pasted grid mesh strip to provide, after expansion and other processing, as will be described herein, the desired plate size and lug configurations.

As is known in conjunction with making negative grids, slits are generally made in the longitudinal direction of travel, leaving the transverse edges free from slits. For SLI positive plates, the continuously cast strip may be, for example, from about 3 inches to about 4-5 inches wide, preferably about 4 inches wide. In this fashion, the strip can be slit and expanded at speeds of up to about 60 to 150 feet per minute or so to make transversely positioned, side-by-side grids with the lugs being located toward the center of the expanded strip.

FIG. 3 schematically depicts the various steps, and equipment utilized, in the preferred embodiment of making the positive battery plates of the present ^' invention. The equipment utilized comprises a commercially available continuous expanded battery plate production line (Cominco Ltd., Toronto, Canada). U.S. 4,315,356 to Laurie et al. also illustrates, in general, the method and apparatus for forming the expanded mesh strip. In utilizing this line, the strip is in the form of coils, each coil weighing about 1500 pounds. Strip 40 from a coil 42 stacked in the horizontal position is continuously fed into the grid expander line. Successive coils can be processed without re-threading by using a strip welder 44 hich bonds the end of one coil to the beginning of the next coil. Suitable strip welders can achieve the desired bond with cold pressure. As may be appreciated from the foregoing, the grids and plates formed from the strip ends that are bonded together may well have less than optimum high temperature positive grid corrosion resistance. If desired, such grids could be separated out and not used.

In the grid expander section, the strip 40 is converted into a grid mesh of the desired size and pattern. In general, the rotary expansion, shown generally at

46, involves an expander tooling module having an assembly of circular cutters mounted onto three shafts which cut and perform the strip 40 into an expandable pattern. Center and outside guide protrusions are also cut into the strip which allows engagement by three sets of silent chains in the expansion section. The outside silent chains diverge, causing the mesh to expand away from the center, forming a diamond pattern. As the mesh is expanded, the outside edges elongate more than the center. A stretcher pulls the center portion forward to match the outside edge.

Grid mesh flatteners and coining rollers may be employed to roll the grid expanded mesh to the desired thickness (i.e., flattening out any high spots). Edge trimmers may be used to remove the outside edges of the mesh so as to provide dimensional uniformity and eliminating any ragged or protruding portions.

A tab blanker 48 forms the lug and top frame bar configuration of the plate by punching a slug pattern from the center solid strip. The mesh strip is thus guided through a rotary male/female die assembly which cuts the slugs and ejects them as salvage. A center guide protrusion then is flattened as the grid mesh exits the die set.

Pursuant to the present invention, the grid mesh is then exposed to suitable treatment to modify the surface to provide the desired surface characteristics. As shown, the grid mesh continues through a surface treatment zone 49 whereby a mechanical and/or chemical surface treatment technique is performed. For example, the surface treatment zone 49 may comprise physically blasting the grid with an abrasive media, such as glass beads or Black Diamond grit, just to name a few. Additionally, the treatment zone may comprise subjecting the grid to a steam cure of a desired chemical composition. Still further, the surface treatment may be carried out using a chemical etching bath. Knurled rolling of the formed strip could also be employed, although this technique is not preferred as the interior of the grid wires will not be modified. A still further treatment involves the use of a water jet, contacting the grid with a stream of water under pressure (e.g., 36,000 psi). Use of this technique offers the advantages of eliminating any trace of lubricant used in grid fabrication and not involving the introduction of any foreign substance to the grid.

It is desirable, as seen from an SEM analysis, that the surface modification is relatively uniform. The Rq of the treated grid should have a value of at least 50, more preferably at least 75, greater than that of the untreated grid. Surface modification parameters, such as R_a - the average height of depth deviation from the graphical centerline of the surface profile, and R^ - the total range of surface roughness from the highest peak to the lowest valley on the surface, may be used to characterize the modified grid surface. However, it has been found that these surface measurement parameters may not provide uniform, discriminating results in accordance with the present invention.

Rather, pursuant to this invention, it has been found that the Rq parameter (i.e., the root mean square deviation of surface height and depth, as previously noted) is sufficiently discriminating to characterize the modified grid surfaces in accordance with this invention. Suitable equipment for carrying out Rq measurements (and the other surface measurements as well) is commercially available.

The grid mesh strip is subsequently moved onto conveyor belt 50 with bottom absorbent paper layer 52 provided from roll 54 being positioned between strip 40 and the surface of the conveyor belt 56. Positive active material paste from paste hopper 58 is applied to the desired areas of strip 40 in the pasting zone shown generally at 60. Suitable paste-applying apparatus for expanded mesh is known and may be used. As an illustrative example, a suitable paste-applying apparatus is Auto Mac 170 Paster (MAC Engineering, Benton Harbor, Michigan). After exiting from the pasting zone, in an optional step which may otherwise be eliminated depending upon processing desires, a top absorbent layer of paper is positioned on the upper pasted surface of the pasted plates so as to shroud the pasted plates, the pasted plates being thus sandwiched between the top and bottom absorbent layers. In this fashion, any environmental concerns due to lead dust or the like getting into the air should be minimized or generally eliminated because the active material is virtually encapsulated between the paper layers.

Still further, the top absorbent layer of paper functions to simplify any surface drying of the paste required which enhances the consistency of the electrical performance and service life that will be achieved since active material checking and shrinkage cracks next to the grid wires is minimized. Also, when separated into individual plates and stacked, the absorbent paper layer shroud minimizes any sticking problems between adjacent plates in the stack. The paper layer also helps in keeping the plate divider knives clean and sharp.

As regards the absorbent layers, a wide variety of materials can be used. The principal requirements are wet strength, tensile strength, and electrochemical cleanliness. As illustrative examples, it has been found suitable to use 8 pound basis weight battery grade tissue paper from Zellerbach (Cincinnati, Ohio).

As shown in FIG. 3, a top absorbent paper layer 62, unwound from roll 64, is fed onto the upper surface 56 of the pasted strip 40. The resulting pasted plate sandwich can then be further processed as desired. Typically, such further processing includes, as in the illustrative preferred embodiment, plate parting (or dividing) and flash drying followed by paste curing, as shown in FIG. 3 at 66 and 68, respectively. These steps can be carried out in any desired order. However, it is preferred to first carry out the plate parting step because the paper present on either side of the pasted grid mesh prevents the cutters used for plate parting from removing too much paste; and, also, the active material is soft and less susceptible to cutter damage prior to curing.

Plate parting or dividing employs a rotary cutting die which alternately cuts the pasted grid mesh into left and right plates (viewed from the top). The mesh is suitably guided through this step by using an index ring which engages the center lug cut-outs. The divided individual plates go through a rapidly moving conveyor where pasted plates are heated to remove a small amount of surface moisture. Typically, 15-20% of the total moisture from the plates is removed in this step.

The flash-dried plates are stacked in plate trays for further paste curing.

Curing can be carried out by any of the many known techniques. In the preferred practice of this invention, curing of positive pasted plates is carried out by using conditions that favor conversion of tribasic to tetrabasic lead sulfate.

Such conditions include temperatures of 175°F up to 210°F at relative humidities of 95 to l00%.

Further optional processing steps that could be carried out, if desired, include forced drying of such cured plates at temperatures up to 175°F and low relative humidity to reduce the free lead content to below 3% and reduce moisture to below 3% level. The negative pasted plates, after flash drying, are usually cured at room ambient temperature for up to 72 hours or can be cured at 110°-

148°F and 95% humidity for 24 to 48 hours.

FIG. 4 illustrates a preferred embodiment of a grid made by expanding metal techniques using a directly cast strip and made in accordance with the present invention. Grid 70 includes a lug 72, a top bar 74 and a bottom bar 76.

The mesh design is generally in the form of diamond shapes as indicated at 78.

The grid has no side frame bars.

As is often employed with positive plates, the positive plates of this invention may be enveloped with any desired separator. Care should be taken in such process since the grids made by the expanded technique and plates do not include side bars, and the exposed mesh sides or edges thus present a potential problem as regards puncturing the separator if appropriate care is not taken in the enveloping process. For this reason, it is preferred that the negative plates be enveloped. Susceptibility to separator puncture and tear and eventual oxidation of separator and separator failure is much greater at the positive side. This can be greatly minimized by enveloping negative plates.

The method of the present invention should be capable of making up to about 400 plates/minute or so while achieving significant improved performance in many respects in comparison to what is achieved using gravity cast grids. The paste weight, density and thickness are thus more readily controlled, as is the paste adhesion during post-curing so as to minimize checking cracks in the paste. This latter aspect enhances the low and high rate discharge performance as well as the expected service life.

The following Examples are illustrative, but not in limitation, of the present invention. Unless otherwise indicated, the percentages set forth are based upon the total weight of the alloy, as added.

EXAMPLE 1

A number of positive grid samples (made with a calcium-tin-silver lead- based alloy) were analyzed using surface profilometry and a scanning electron microscope (SEM) to determine their surface character. The samples analyzed included an as-cast book mold grid, a book mold grid after steam cure, an as-cast grid made using the method shown in FIG. 3 (but not treated as shown at 49), and grids made using the method of FIG. 3 that were treated by sandblasting with glass beads or with Black Diamond grit material.

The results of the analysis are set forth in Table 1 below (FIG. 4 grid referring to the grids made by the method of FIG. 3):

TABLE 1 Sample R_a (μin.) Rq (μin.) R_max (μin.)

FIG. 4 Grid - As-Cast 145 183 1940

FIG. 4 Grid - Media Blasted 200 250 3410 (Glass Bead)

FIG. 4 Grid - Media Blasted 281 304 3570 (Black Diamond Grit)

Book Mold As-Cast 305 381 2110

Book Mold Steam Cured 238 381 1890

As can be seen, among the surface characterization parameters, it is the Rq parameter that is more discriminating. Also, the surface treatments used have suitably modified the root mean square deviation parameter pursuant to this invention. The as-cast FIG. 4 grid sample had relatively flat regions punctuated with a network of exposed grain boundaries and possessed the lowest surface roughness of any of the samples. The FIG. 4 grid samples subjected to media blasting with glass bead smeared and mottled the grain boundaries resulting in a faceted surface. Media blasting the FIG. 4 grid with Black Diamond grit similarly mottled the grid surface and obliterated grid boundaries but produced a rougher surface than did the glass beads.

The as-cast, glass bead and Black Diamond grit FIG. 4 grid samples were then subjected to 2 hours of steam at 95°C in a steam chamber to determine if steaming would impart additional surface roughness. SEM analysis showed that the steaming had produced increased surface roughness in all three samples.

The as-cast book mold sample appeared wavy with check marks from corking or mold imprints. The steam cured book mold sample had poorer definition of the check marks and other features. Thus, as has been seen, the batteries of the present invention utilize positive grids having desirably modified surface yielding altered surface characteristics. These modified surface grids should equate to better service performance of the batteries of this invention as the service life of the battery continues due to the greater degradation experienced by batteries not employing grids treated as in this invention. It is believed that the present invention can minimize the vexing and substantial issues plaguing batteries made with grid fabrication techniques other than gravity casting that itself has undesirable characteristics. As used in the following claims, the terminology "fabricated strip grid" refers to grids made by any fabrication technique other than by gravity casting.

Claims

WE CLAIM:

1. A lead-acid battery comprising a container having a plurality of cells, a battery element disposed in each of said cells, each battery element comprising a plurality of positive and negative plates and separators, each cell being electrically connected together, at least said positive plates comprising an electrically conductive grid, having positive active material pasted thereto, and at least one of said grids comprising fabricated strip grid having a surface modified to an extent sufficient to enhance the battery performance.

2. The lead-acid battery of claim 1 wherein said grid has an Rq value of at least 50 greater than that of the unmodified grid.

3. The lead-acid battery of claim 2 wherein the Rq value is at least 75 greater than that of the unmodified grid.

4. The lead-acid battery of claim 2 wherein said grid is a directly cast grid.

5. The lead-acid battery of claim 2 wherein said electrically conductive grid is a cold-rolled grid.

6. The lead-acid battery of claim 2 wherein said surface modification is carried out by is media blasting.

7. The lead-acid battery of claim 6 wherein said media blasting utilizes glass beads.

8. The lead-acid battery of claim 6 wherein said media blasting utilizes Black Diamond grit.

9. The lead-acid battery of claim 2 wherein said surface modification d out using steam curing techniques.

10. The lead-acid battery of claim 2 wherein said surface modification ed out using chemical etching.

11. The lead-acid battery of claim 2 wherein said surface modification ed out using knurled rolls.

12. The lead-acid battery of claim 2 wherein said surface modification ed out using a water jet.