LEAD-ACID BATTERIES AND POSITIVE PLATE AND ALLOYS THEREFOR

Abstract: Lead-acid cells and batteries and positive plates for such cells and batteries are provided. The positive plate comprises a grid supporting structure having a layer of active material pasted thereto, the grid supporting structure comprising a lead-based alloy consisting essentially of lead, from about 0.02% to about 0.06% calcium, from about 0.2% to about 3.0% tin, and from 0.01% to about 0.02% silver. A positive plate in accordance with the invention has excellent mechanical properties, is satisfactory for use in lead-acid cells and batteries, and imparts enhanced electrical performance to cells and batteries using such positive plates.

Description: LEAD-ACID BATTERIES AND POSITIVE PLATE AND ALLOYS THEREFOR TECHNICAL FIELD OF THE INVENTION The present invention relates to lead-acid cells and batteries, and, more particularly, to calcium-tin-silver lead-based alloys used for the positive grid alloys in such cells. 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. The principal criteria for providing satisfactory positive grids for starting, lighting, and ignition ("SLI" automotive lead-acid batteries) are stringent and are varied. In general, and by way of a summary, suitable alloys must be capable of being cast into satisfactory grids and must impart adequate mechanical properties to the grid. Still further, the alloys must impart satisfactory electrical performance to the battery in the intended application. Satisfactory alloys thus must impart the desired corrosion resistance, and avoid positive active material softening that will result in a loss of capacity. More particularly, and considering each of the criteria previously summarized, suitable alloys in the first instance must be capable of being cast into grids by the desired technique, i.e., the cast grids must be low in defects as is known (e.g., relative freedom from voids, tears, microcracks and the like). Such casting techniques range from conventional gravity casting ("book molds" or the like) to continuous processes using expanded metal techniques and to a variety of processes using alloy strips from which the grids are made, e.g., by stamping or the like. The resulting cast grids need to be strong enough to endure processing into plates and assembly into batteries in conventionally used equipment. Even further, suitable grids must maintain satisfactory mechanical properties throughout the expected service life. Any substantial loss in the desired mechanical properties during service life can adversely impact upon the battery performance as will be more fully discussed hereinafter. Considering now the electrochemical performance required, the grid alloy for the positive plates must yield a battery having adequate corrosion resistance. Yet, the use of a continuous direct casting process, or other processes using grid alloy strips, desirable from the standpoint of economics, ostensibly can compromise corrosion resistance. Continuous processes thus orient the grains in the grids, thereby making the intergranular path shorter and more susceptible to corrosion attack and to early failures. Casting a thick strip and then cold rolling or the like to the grid thickness desired even further exacerbates the problem. Positive grid corrosion thus can be a primary mode of failure of SLI lead-acid batteries. When positive grid corrosion occurs, this lowers the electrical conductivity of the battery itself. Battery failure occurs when the corrosion-induced decrease in the conductivity of the grid causes the discharge voltage to drop below a value acceptable for a particular application. A second failure mechanism, also associated with grid corrosion, involves failure due to "grid growth." During the service life of a lead-acid battery, the positive grid corrodes; and the corrosion products form on the surface of the grid. In most cases, the corrosion products form at the grain boundaries and grid surface of the lead-acid where the corrosion process has penetrated the interior of the "wires" of the grid. These corrosion products are generally much harder than the lead alloy forming the grid and are less dense. Due to the stresses created by these conditions, the grid alloy moves or grows to accommodate the bulky corrosion products. This physical displacement of the grid causes an increase in the length and/or width of the grid. The increase in size of the grid may be nonuniform. A corrosion-induced change in the dimension of the grid is generally called "grid growth" (or sometimes "creep"). When grid growth occurs, the movement and expansion of the grid begins to break the electrical contact between the positive active material and the grid itself. This movement and expansion prevents the passage of electricity from some reaction sites to the grid and thereby lowers the electrical discharge capacity of the cell. As this grid growth continues, more of the positive active material becomes electrically isolated from the grid and the discharge capacity of the cell decays below that required for the particular application. The mechanical properties of the alloy thus are important to avoid undue creep during service life. As is now appreciated, what has occurred in the last several 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 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. A breakthrough was achieved in utilizing the positive grid alloys disclosed in U.S. Patent 5,298,350 to Rao. Utilizing such positive grid alloys provided batteries that exhibited substantial improvements in service life and have effectively eliminated premature positive grid corrosion at elevated temperatures as being the primary mode of failure. The subject Rao patent has spurred considerable interest in the type of positive grid alloys utilized, i.e., calcium-tin-silver lead-based alloys. Thus, substantial effort has been made to investigate this type of alloy through testing of various properties with varying levels of the alloying constituents. Yet, despite this effort, what has not been achieved are positive grid alloys • of this type that possess, in lead- acid batteries, excellent high temperature corrosion resistance while having enhanced electrical performance. Thus, while the electrical performance resulting from the use of this type of positive grid alloys is certainly considered acceptable, it would be highly desirable to achieve improved electrical performance. Further, the alloys must maintain adequate contact for electrical conductance throughout the desired service life. Otherwise, the cell will experience what has been termed as "premature capacity loss" ("PCL"). PCL can also occur through loss of contact due to cracking of the corrosion layer or from a nonconductive film generated in the corrosion layer. Because of the complexity and the substantial potential adverse effects, this is a difficult criteria to achieve in combination with the other necessary criteria. Thus, despite all of the prior effort in this field, there exists a need to provide positive grid alloys that combine superior high temperature corrosion resistance with improved electrical performance. Accordingly, it is an object of the present invention to provide a lead-based alloy for a positive plate for a lead-acid battery that possesses excellent high temperature corrosion resistance while providing improved electrical performance. It is an additional object of the invention to provide alloys cast into grids by conventionally used techniques and having satisfactory mechanical properties to allow use in conventional lead-acid processes and assembly. Another object of this invention is to provide a positive grid alloy that achieves the desired corrosion resistance and electrical performance characteristics while satisfying the diverse criteria for SLI lead-acid positive grids. Other objects and advantages of the present invention can be seen from the following description of the invention. BRIEF SUMMARY OF THE INVENTION In general, the present invention is predicated on the discovery that the corrosion layer which develops during service on the positive grids made using lead- based calcium-tin-silver alloys can be desirably modified so as to achieve a highly desirable combination of properties. More particularly, by carefully adjusting together the respective level of the alloying ingredients, alloys can be provided that retain excellent high temperature corrosion resistance, yet impart enhanced electrical performance and have satisfactory mechanical properties. Thus, it has been found that lead-based alloys having from about 0.02% to about 0.06% calcium, preferably 0.025% to 0.045%, from about 0.2% to about 3.0% tin, preferably about 1.0% to 3.0%, more preferably about 1.5% to about 3.0%>, and from about 0.01%) to about 0.02% silver, the percentages being based upon the total weight of the alloy, possess these desirable characteristics. Optionally, the alloys of this invention can include from about 0.003%) to 0.04%> by weight aluminum. What is particularly surprising is that despite the widespread use and investigation of lead-based calcium-tin-silver alloys, it has not been appreciated that a coordination of the alloying ingredient levels can retain the desired characteristics of these alloys, yet achieve enhanced electrical performance. 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 a battery grid made utilizing an alloy composition in accordance with the present invention; FIG. 3 is a three-dimensional bar graph showing the Yield Strength of various alloy compositions; FIG. 4 is a bar graph showing the initial discharge capabilities achieved using positive grids having varying alloy compositions; FIG. 5 is a bar graph showing the residual reserve capacities achieved as the positive grid alloy composition is varied; FIG. 6 is a bar graph showing the nominal corrosion layer of various grids of varying alloy content; FIG. 7 is a diagrammatic view showing the apparatus used to evaluate the effects of alloy composition on gassing at the positive electrode; FIG. 8 is a bar graph showing the effect on oxygen overvoltage of the alloy composition; FIG. 9 is a graph of current versus time and showing the float behavior at elevated temperatures for two alloys; FIG. 10 is a bar graph showing the growth of the width of the positive grids under high temperature float conditions as the grid alloy composition is varied; FIG. 11 is a bar graph similar to FIG. 10, except showing the growth of the length of the positive grids as the grid alloy composition varied; and FIG. 12 is a bar graph illustrating how the positive grid corrosion varies, on formation and on float at an elevated temperature, as the grid alloy composition is varied. DESCRIPTION OF THE PREFERRED EMBODIMENTS Although each of the alloying ingredients utilized in the alloys of the present invention contributes to the overall performance of the alloy, it is difficult to separate the benefits provided individually by the alloying ingredients, because of the synergy achieved when these ingredients are used collectively in the amounts specified herein. Thus, there is a careful balance which results when the proper alloying ingredient level is maintained. Upsetting that balance can affect many of the desired characteristics. However, to provide an understanding of the considerations involved in selecting the various amounts of alloying ingredients, the function of each of these ingredients will be separately discussed. With respect to calcium as an alloy constituent, calcium should be present in an amount sufficient to impart to the cast grids of this invention the desired casting characteristics and mechanical properties. To achieve such characteristics, it has been found that the calcium content should be at least about 0.02% by weight of the total alloy. However, the calcium level must be carefully controlled to avoid excessive amounts which would provide an alloy composition that has an unduly high tendency to recrystallize after solidification, significantly altering the structure from that of the as-cast structure. More particularly, when the calcium content is excessive, there is a propensity for recrystallization after solidification, creating a grid structure that is unduly susceptible to premature failure due to highly erratic intergranular corrosion. The corrosion thus occurs through intergranular corrosion, and recrystallized alloys tend to have smaller grains which in turn are more susceptible to intergranular corrosion due to higher calcium-based intermetallics in the new recrystallized grain boundaries. Accordingly, to impart adequate mechanical properties while avoiding calcium levels increasing the propensity for recrystallization, it has thus been founc suitable to mention the calcium in the alloys of this invention in the range of from about 0.02%) to 0.06% by weight of the total alloy. More preferably, the calcium content is from about 0.025% to about 0.045% or even 0.05%. These more preferred calcium contents are particularly desirable so as to minimize the propensity for reciystallization of the resulting alloys, particularly given the relative amounts of the other alloying constituents utilizing in accordance with the present invention. As to the silver constituent, this cooperates with the other alloying ingredients to provide the resulting alloy with the requisite casting and mechanical property characteristics. More particularly, silver present at an appropriate level, imparts highly desirable mechanical properties to the resulting alloy that could not otherwise be provided using the other alloying ingredients. Thus, it has been found that the inclusion of silver in an amount of at least about 0.01% by weight of the total alloy will provide the desired casting and mechanical properties. An important aspect of the inclusion of silver is that the resulting alloys can be heat treated to even further enliance the mechanical properties of grids made using these alloys. Such heat-treating enhancements are not obtained in calcium-tin lead-based alloys not containing appropriate levels of silver. Even further, an appropriate level of silver tends to stabilize such alloys against over aging. Thus, in the absence of adequate silver levels, calcium-tin lead- based alloys tend to lose their desirable mechanical properties upon aging. Such a substantial loss in such mechanical properties cannot be tolerated for positive grid alloys for many applications. Moreover, pursuant to the present invention, it has been found that selecting a silver level below that typically employed in commercial alloys of this type achieves enhanced electrical performance while still retaining the other desired positive grid characteristics as discussed herein, so long as the levels of calcium and tin are obtained at suitable selected levels. Accordingly, the silver content should be no more than about 0.02%> by weight of the total alloy. The preferred composition includes silver in a range of about 0.015% to 0.02%. Additional benefits obtained from lessening the amount of silver are that recycling problems can be likewise lessened. Thus, most economically, lead-acid alloys are made using secondary lead. Silver tends to build up in secondary lead sources inasmuch as silver removal is generally not cost effective. Accordingly, lowering the silver content minimizes such silver build up issues. Also, silver in some cell designs is considered a contaminant for some applications, especially in oxides used to make battery pastes, due to gassing and cell dry-out concerns. As to the tin constituent, the issue is even more complex. Thus, while the tin level will certainly affect the characteristics as the grid is being cast and the mechanical properties of the cast grid, the tin level will also impact upon the issues of corrosion, cycling, and capacity loss characteristics. These diverse criteria are not fully understood; and, despite the prior work in this field, the impact of the tin level on the characteristics of lead-acid batteries, it is believed, has not been appreciated to any great extent. However, in accordance with the present invention, it has been found that the inclusion of tin in the range of from about 0.2% to about 3.0% by weight of the total alloy will impart the desired characteristics to the alloys, grids made using such alloys, and to batteries using such alloys for the positive grids, when such alloys possess appropriate calcium and silver levels. More particularly, it is preferred to maintain the tin in the range of from about 1.0% to about 3.0%, by weight of the alloy, even more preferably, 1.5% to 3.0%. The tin level employed may well be dictated by economic considerations, making levels of 0.5% to 1.0% tin more desirable. This is particularly true where the acceptable service life lies in the 2 to 5 year range. Use of the higher tin levels enhances service life and are desired for those applications where a longer service life is desired. However, casting grids becomes more difficult as the tin level rises to the 2.5% to 3.0% level. Thus, in the preferred embodiment, the alloy consists essentially of lead, calcium, tin, and silver. If desired, however, the alloy may include an amount of aluminum effective to prevent drossing of calcium from the alloy. Aluminum may be present in an amount ranging from about 0.003% to about 0.04%. Preferably, ingredients other than those previously described are excluded from the alloy, or are present only in trace amounts, such as amounts typically present in commercially available metals. Of course, other ingredients may be added to the alloy if desired, provided the beneficial properties of the alloy are not disturbed by the addition of such ingredients. The alloy preferably is prepared by blending the ingredients at temperatures of about 800°F to about 950°F (426°C to about 510°C) until a homogeneous mixture is achieved, and allowing the ingredients to cool. The particular manner in which the alloys of this invention are prepared does not form a part of the present invention. Any desired technique can be used, and appropriate techniques are known. The alloys described herein may be cast into grids by any of the known techniques used for lead-acid grids. Thus, conventional gravity casting techniques are known in the art and may be used. Other known techniques for casting lead-acid grids include employing wrought, expanded metal techniques, or utilizing a strip from which the grids are made by stamping or the like. Such techniques may likewise be used, as is desired for the particular application. With regard to the grid casting parameters, it is preferred to at least minimize, if not eliminate, the generation of temperature gradients. To this end, in contrast to the casting of other calcium lead-based alloys, it is preferred to employ cooler lead temperatures and higher mold temperatures, while providing more insulation (e.g., obtained by conventional corking) in the upper frame and gate area to prevent premature cooling of the lead and associated temperature gradients during solidification. Accordingly, preferred lead/ladle temperatures range from about 770°F to 800°F with mold temperatures of about 350° to 575°F, more preferably about 475°F to about 575°F. Still further, process stability is important so that the calcium content selected is maintained during the grid manufacturing process. It is thus important to avoid contamination, particularly when aluminum is utilized. The present invention is equally effective for cold-rolled alloy strips or any other method of providing strips for a continuous cast process or any other grid- making process. 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. In any event, as compared with gravity cast grids, the alloy weight per grid can be significantly less in the method of the present invention while achieving satisfactory performance in service. A significant savings in raw material costs can thus be achieved. 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 and then slitting the moving alloy strip. 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 40 to 120 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. As has been previously noted, the calcium-tin-silver lead-based alloys used in the present invention can be heat-treated to provide enhanced mechanical properties. Any heat-treating techniques may be used. As one illustrative example, it has been found suitable to heat-treat the resulting grids for about 3 hours or so at a temperatu of 212°F (100°C). Such heat-treating can increase the yield strength from levels of about 3,500-4,000 psi or so up to yield strengths in excess of about 6,000 psi or so. The particular grid configuration and that of the lead-acid cells or batteries in which such positive grids are used can be varied as desired. Many configurations are known and may be used. As one illustrative example, FIGS. 1 and 2 show a maintenance-free battery utilizing the positive grids having 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 from any desired antimony-free alloy. The electrode illustrated includes a supporting grid structure 20 having an integral lug 22 and a layer of active material pasted thereto; and a strap 24 joining 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 supporting grid structure 20 during assembly, the base of the terminal 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 i 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 electrical performance characteristics 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. As is known, there are many different configurations for the grid. For some applications and to complement process control and minimize cracking, tears, voids and the like, it may be desirable to utilize the optimized internal positive grid wire geometry as disclosed in the co-pending Rao application, Serial No. 08/925,543, filed September 8, 1997, assigned to the assignee of the present invention. As is thus discussed at pages 7 and 14-15 and illustrated in FIGS. 6 and 7 therein, which disclosures are herein incorporated by reference, positive grid internal configurations which are generally cylindrical or elliptical in cross-section facilitate uniform solidification during grid casting and should assist in minimizing, if not eliminating, casting defects. As previously noted, the present invention provides a modulated corrosion layer to be developed that achieves enhanced electrical performance. In general, this provides cells and batteries that are characterized by higher or more optimal formation efficiencies. This translates to allowing less severe and shorter formation regimes, enhanced initial electrical performance, and improved characteristics while on stand prior to being placed into service, as well as during service. The level of improvement will vary, of course, but the enhanced formation efficiency should be highly beneficial in many systems throughout the life of the battery. Thus, as an indication of the expected performance of an SLI battery, the enhanced corrosion layer developed during formation can result in increased residual Reserve Capacity that is 5% more than identical batteries using positive grids with higher silver contents (e.g., 250 or 350 ppm), even 8%> or 10%, perhaps up to 15%> or so. The level of improvement in VRLA cells and batteries, in terms of initial discharge capacities, likewise can be enhanced at least 5%. Further improvements up to 10% or even 15% or so can be achieved. The importance of this enhanced initial electrical performance is significant. This is the stage where testing is often carried out to insure that the batteries will achieve the desired performance. Accordingly, less than the necessary performance level could well result in such batteries being inaccurately determined to be unsatisfactory whereas testing after some service would have shown satisfactory performance. The enhanced formation efficiency achieved by the present invention is likewise beneficial when considering stand (or storage) issues. Long storage of batteries before service puts a premium on the formation efficiency, as less formation-efficient batteries will tend to corrode faster, ultimately causing problems when being placed in service or thereafter. Enhanced formation efficiency should likewise translate to more uniform performance in service. In other words, all other parameters being the same, batteries according to the present invention should have less variations in performance, battery-to-battery. The following Examples further illustrate the present invention but, of course, should not be construed as in any way limiting its scope. The mechanical properties set forth in these Examples were determined by the following procedures: Ultimate Tensile Strength (UTS) Yield Strength (Yield) (0.2% offset) Strain (elongation) Toughness These properties were tested in accordance with ASTM Test No. D638. The compositions of the alloys set forth in these Examples were deteπnined on the cast alloys. Example 1 This Example illustrates casting lead-based alloys while varying the silver and tin contents while maintaining a constant calcium content. Bars (0.5" x 0.25" x 4.0") were gravity cast using alloy blend at 850°F while maintaining the mold at about 350°F (176°C). Table 1 sets forth the respective alloy compositions which were cast: Table 1 Moving Constituent (Wt.% Alloy Ca Ag Sn Lead Alloy A 0.040 0.0165 3.0 Balance Alloy B 0.039 0.03363.0 Balance Alloy C 0.038 0.045 3.0 Balance Alloy D 0.040 0.045 2.0 Balance Example 2 This Example illustrates the mechanical properties of the cast bars obtained using Alloys A-D of Example 1. The mechanical properties of such alloys were tested, and the results are set forth in Table 2: Table 2 Mechanical Properties Allov UTS (psi) Yield (psi) Strain (%) Toughness (in#/in 3 ) A 1 5025 3734 16.7 664 B 2 4761 3534 15.0 540 C 3 4596 3313 14.4 489 D 4 4012 3120 13.2 373 1 0.04% Ca, 0.0165% Ag, 3.0% Sn 2 0.039% Ca, 0.0366% Ag, 3.0% Sn 3 0.038% Ca, 0.045% Ag, 3.0% Sn 4 0.040% Ca, 0.045% Ag, 2.0% Sn Example 3 This Example illustrates the effects of aging and heat-treating the alloys described in Example 1. The alloys of Example 1 were allowed to stand at ambient temperature for three days. The mechanical properties evaluated in Example 2 were again evaluated for alloy aging. To evaluate the effects of heat-treating, the alloys were heat-treated in an oven at 200°F (93°C) for one hour and at 200°F for three hours. Table 3 UTS Yield Strain Toughnes (psi) (psi) (%) (in#/in 3 Allov A 3-day age 5527 4330 14.1 661 1-hr.200°F 7576 6216 11.5 644 3-hr.200°F 7531 6512 12.5 700 AllovB 3-day age 5096 3816 15.5 607 1-hr.200°F 7236 6040 12.6 681 3-hr.200°F 7672 6449 9.0 523 Allov C 3-day age 4069 2926 8.4 260 1-hr.200°F 6960 6105 7.6 346 3-hr.200°F 6843 6109 5.9 247 Allov D 3-day age ~ ~ ~ « 1-hr.200°F 6329 5510 10.1 469 3-hr.200°F 6988 6269 9.2 480 As is thus shown, heat treating serves to significantly enhance the mechanical properties of these alloys. Example 4 The following Example illustrates the effects of the silver level on the properties of the alloys. An alloy, Alloy E, having the following composition was prepared: Table 4 Ingredient Amount (wt.%) Tin 2.0 Silver 0.006 Calcium 0.040 Lead Balance Thus, Alloy E was comparable to Alloy (D (i.e., 0.049% Ca, 0.045% Ag, and 2.0% Sn), except that the silver concentration was reduced to 0.006%. Alloy E was subjected to testing as previously described, and the following results were obtained: Table 5 UTS Yield Strain Toughness (psi) { si (%) (in#/in 3 ) Allov E As cast 2880 1578 28.6 663 l-hr. 200°F 3160 2018 22.6 567 3-hr. 200°F 3756 2811 17.7 534 As can be seen, the mechanical properties of Alloy E were substantially lower than those of Alloy D. Example 5 This Example illustrates the evaluation of the mechanical properties of varying compositions. ASTM test bars (described in Example 1) were heat-treated at 100°C for 3 hours. The Yield Strengths are determined at the tin contents and graphed in a three- dimensional plot as shown in FIG. 3 (the average of 5 samples). The results show that alloys having unduly low levels of calcium develop unsatisfactory mechanical properties, regardless of the tin content. However, once an appropriate level of silver is included, i.e., 0.015 wt.%, the grids having acceptable yield strengths are obtained, the strength levels increasing as the tin content is increased from 0.5 wt.% to 3.0 wt.%. Increasing the level of silver in the alloys used from 0.015 wt.% to 0.030 wt.%, and even to 0.045 wt.%, do not result in any meaningful increase in mechanical performance. By reference to Example 4, it can be seen that a silver content of 0.006% by weight is inadequate to provide satisfactory mechanical properties. Example 6 This Example illustrates the effect on the initial discharge capacities of valve- regulated lead-acid (i.e., sealed) cells using positive grids of varying alloy compositions. Six cell strings (i.e., 12 Volts), each cell having a rated capacity of 160 Ampere Hours, were assembled. Each cell included 5 positives (grid weight - 404 grams and 580 grams of positive active material 580 grams) and 6 negatives (grid weight - 254 grams and 530 grams of active material). The discharge capacities were then determined at a C/5 rate (i.e., 32 Amp discharge to 1.0 Volt per cell). The results are set forth in FIG. 4. As can be seen from that graph, a 10% increase in initial capacity was obtained with the cells having positive grids made from alloys not containing silver. Similarly, at the level of silver utilized, increasing the tin content from 0.5 weight percent to 3.0 weight percent did not improve the electrical performance. This data thus shows that there would be a silver level (below 250 ppm) at which satisfactory mechanical properties can be obtained while still achieving improved electrical performance. Example 7 This Example shows the effect upon the Residual Capacity of batteries made using positive grids of varying alloy configurations. BCI Group 25 batteries having 11 plates (6 positive, 5 negative) were used. Formation was carried out using the following charge regime: 22 Amps x 4.5 hrs., followed by 8 Amps x 16 hrs. The total Ampere Hour input was 227. The Residual Capacity, RC in minutes, was then determined for each battery. The test sequence involved discharge at a 25 Amp rate until the voltage of the battery decreased to 7.2 Volts. A graph of the results is shown in FIG. 5 (the results being an average of 15 batteries). As can be seen, the batteries utilizing grids made from alloys with either no silver or 0.015 wt.% silver have nearly identical capacities. Further, and importantly, such batteries exhibit about an 11% improvement relative to batteries which have positive grids made with alloys having 0.035 wt.% silver. Further, the results here supplement those in Example 6. Thus, the improvement in Residual Capacity confirms that there is indeed a silver level at which enhanced electrical performance can be achieved (relative to higher silver content alloys) and that the reduced level will provide alloys with the necessary mechanical properties. Example 8 This Example examines the corrosion layer build up of various Ca-tin lead- based alloys, with and without added silver, at various stages: dry unformed ("DUF"), after formation ("Formed"), and after a BCI sequence (Residual RC1, i.e., first RC after formation, followed by CCAl (i.e., cold cranking), RC2, CCA-2, RC-3 and than a 20 hour capacity) ("BCI"). The batteries used were those described in Example 6. The thickness of the corrosion layers of three or four batteries were determined via SEM (scanning electron microscopy), and the ranges of the actual measurements are shown in FIG. 6, the averages for each of the four alloy compositions being shown in bar graph form. The DUF and BCI results are considered particularly informative. Thus, it is believed that lowering the silver level enhances the corrosion layer development. The more rapid corrosion layer development, resulting from lowering the silver content, may explain the enhancement in electrical performance achieved by alloys of this type having such lowered silver contents. Example 9 This Example illustrates the testing of calcium-tin-silver lead-based alloys to determine the effect of alloy composition on the oxygen overvoltage at the positive electrode. The test set-up is shown in FIG 7. Each of the alloys used was cast into a wire and potted in an epoxy resin, polished to a level of 0.3 microns. The polished surface area was 0.164 cm 2 . In the schematic, as is shown in FIG. 7, alloy wire tested is shown generally at 50, immersed in 1.310 specific gravity sulfuric acid, shown at 52, positioned in a small reaction vessel 54. A reference electrode (mercury-mercurous sulfate) 56 was immersed in the sulfuric acid solution adjacent the counter-electrode 50 as is shown. The wire was anodized at 5 mA/cm 2 for 45 minutes. Then, the voltage on a reference scale from 1.6 V to 1.2 V was swept, and the oxygen gassing current during the sweep was recorded. The results are shown in FIG. 8 for the test conducted at 78°F (25°C). As can be seen, increases in tin content reduced the extent of gassing as the tin content was increased from 1.5% by weight of the alloy up to 2.5% tin. Further increases in the tin level then begin to increase gassing. The performance of such alloys with appropriate tin levels demonstrates that gassing at the positive electrode should not be unduly excessive. Accordingly, since such alloys do not poison the negative electrodes, as do virtually all antimony- containing alloys, the alloys of the present invention should be capable of being used without a tendency for gassing and thermal runaway. Thus, as has been seen, the alloys of the present invention satisfy the diverse criteria needed for VRLA motive power and stationary applications. The casting characteristics are satisfactory. The mechanical properties are excellent, and, importantly, are not unduly susceptible to loss of such desired properties upon aging. Likewise, positive grids made from such alloys impart adequate electrical performance to the VRLA cells for use in the desired application. In addition, as should be appreciated, these results are likewise applicable to SLI batteries. Thus, from the standpoint of gassing characteristics, improved performance is achieved at tin contents in the range of 2.0% to 2.5% by weight. Example 10 This Example compares the performance of cells using the positive grid alloys of the present invention with that of other positive grid alloys, as well as comparing the grid growth characteristics and the grid microstructures. The cells tested were assembled using positive grids made of alloys of varying compositions as described hereinafter. In general, the cells tested can be characterized as follows: 200 Ampere-Hour VRLA cells having 5 positive and 6 negative plates (calcium-lead alloy) with a glass separator and a flame retardant polypropylene container and set to operate at about the 97-98% saturation level. The float behavior of the cells was determined by floating six cell (12 volt) strings at 2.23 volts per cell in an air oven at 60°C and 65°C after about 115 days. FIG. 9 is a graph of the current versus days and compares the float behavior of the cell strings using positive grid Alloy 1 with the cell strings using a commercially used cadmium-antimony-lead positive grid alloy ("Prior Art"). The float behavior of each is considered acceptable. Additional cell strings using various positive grid alloys were evaluated for grid growth and corrosion. The various alloys used are described as follows: Positive Grid Alloy Identification Nominal Composition Ca Sn Ag_ Alloy F 0.04 1.5 - Alloy G 0.04 ' 1.5 0.025 Alloy H 0.04 2.0 - Alloy I 0.04 2.0 0.025 Alloy J 0.04 3.0 - Alloy K 0.04 3.0 0.025 Alloy L 0.04 4.0 - Prior Art Cadmium-Antimony-Lead FIGS. 10 and 11 graphically show the grid growth (FIG. 10 being the growth in the width of the grid and FIG. 11 being in the height) after being floated at about 2.23 volts per cell in an air oven held at 60°C for 12 weeks. As can be seen, the grid growth characteristics of the positive grids in cells having silver-containing positive grids were superior to those where the positive grids had the same tin content but no silver, i.e., G versus F, I versus H, and K versus J. In addition, the positive grid alloys with silver and tin contents in the range of 2-3% appear preferable. FIG. 12 shows the grid corrosion characteristics of positive grids made from the various alloys identified after formation and after being floated for 12 weeks under the conditions previously identified regarding the grid growth tests. Again, the positive effect of including silver in the positive grid alloys can be seen. The microstructure of positive grids using various alloys was also examined. Severe intergranular corrosion occurred under the test condition in the positive grids made with a prior art alloy. In contrast, the primary corrosion which occurred in the positive grids made from Alloy I and from Alloy K, was uniform; and no intergranular corrosion was noted. A primary defect in all the grids was cracking with some voids and tears occurring when Alloys I and K were used. It is believed that such defects can be satisfactorily controlled by process design as previously discussed herein. Thus, as has been seen, utilizing positive grids made from alloys having a very specific range of relatively low silver content, combined with a relatively low level of calcium and an elevated tin content, results in lead-acid cells and batteries that have increased overall electrical performance at high and low discharge rates. Such enhanced electrical performance, which is considered significant, thus concomitantly results in batteries having increased power, specific energy, volumetric energy density and gravimetric energy density at high and low discharge rates, while retaining the considerable benefits of alloys of this type. While not wishing to be held to any theory, it is believed that the enhanced electrical performance may be achieved due to the development, during service, of a corrosion layer having improved conductivity. It is accordingly believed that the modulation of the conOsion layer, which exists at the interface between the grid members and the active material of the positive electrodes, improves the conductivity of such a layer and thereby results in increased capacity. While particular embodiments of the invention have been shown, it will of course be understood that the invention is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings. Thus, while the present invention has been described in conjunction with SLI batteries, it should be appreciated that the alloys disclosed herein may be used in any other lead-acid cells or batteries including, for example, bipolar and the like.

Claims: WHAT IS CLAIMED IS : 1. A lead-acid battery comprising a container, at least one positive plate and a negative plate disposed within said container, a separator disposed within said container and separating said positive and negative plates, and an electrolyte, said positive plate comprising a grid supporting structure having a layer of active material pasted thereto, said grid supporting structure comprising a lead-based alloy consisting essentially of lead, from about 0.02% to about 0.06% calcium, from about 0.2% to about 3.0% tin, and from about 0.01%) to about 0.02% silver, the percentages being based upon the total weight of said lead-based alloy. 2. The battery of claim 1, wherein the calcium content of said lead- based alloy is in the range of about 0.25% to about 0.045%. 3. The battery of claim 1 , wherein the tin content of said lead-based alloy is in the range of about 0.5% to about 2.0%>. 4. The battery of claim 1, wherein the silver content of said lead-based alloy is in the range of about 0.015% to about 0.02%. 5. The battery of claim 1 , wherein the Residual Capacity of said battery is at least 5% greater than that of a battery having positive grids of a calcium-tin-silver lead-based alloy in which the silver content is about 0.035%o, based on the weight of the positive grid alloy. 6. The battery of claim 1 , wherein the Residual Capacity of said battery is at least 8% greater than that of a battery having positive grids of a calcium-tin-silver lead-based alloy in which the silver content is about 0.035%, based on the weight of the positive grid alloy. 7. The battery of claim 1 , wherein the Residual Capacity of said battery is at least 10% greater than that of a battery having positive grids of a calcium-tin-silver lead-based alloy in which the silver content is about 0.035%>, based on the weight of the positive grid alloy. 8. A positive plate for a lead- acid cell or battery comprising a grid supporting structure and positive active material pasted thereto, said grid supporting structure comprising a lead-based alloy consisting essentially of lead, from about 0.02%> to about 0.06% calcium, from about 0.2% to about 3.0% tin, and from about 0.01% to about 0.02% silver, the percentages being based upon the total weight of the lead-based alloy. 9. The positive plate of claim 8, wherein the calcium content of said lead-based alloy is in the range of about 0.025%) to about 0.045%, the tin content of said lead-based alloy is in the range of about 1.0% to about 3.0%, and the silver content of said lead-based alloy is in the range of about 0.015% to about 0.02%>. 10. A method of enhancing the electrical performance of SLI lead- acid batteries which comprises making positive plates having a grid supporting structure comprising a lead-based alloy consisting essentially of lead, from about 0.02%) to about 0.06%> calcium, from about 0.2% to about 3.0% tin, and from about 0.01% to about 0.02% silver, the percentages being based upon the total weight of said lead-based alloy, and assembling the positive plates into a lead-acid battery.

Inventors: LARSEN STEVEN R, LARSEN, STEVEN, R.
Applicants: EXIDE TECHNOLOGIES
Application year: 2001
Main classification: H01M
IPC codes: C22C11, H01M4, H01M10