GB2084790A - Lead-acid batteries - Google Patents

Abstract

A maintenance-free, sealed lead-acid battery has the electrolyte absorbed in a porous separator. The a battery case has a relief valve.

Description

SPECIFICATION Sealed, maintenance-free, lead-acid batteries for float applications This invention relates to lead-acid batteries, and, more particularly, to sealed, maintenance-free, lead-acid batteries capable of use for float applications such as automotive starting, lighting and ignition applications.

For many applications, the trend in lead-acid technology is to provide batteries which are maintenance-free, i.e.,-a type of battery which may be operated without adding water to the electrolyte during its recommended life. The life of such batteries is limited by the water loss due to gas evolution; and, therefore, excess electrolyte must be used to compensate for the water loss which occurs so as to provide a satisfactory life.

Typically, such batteries have minimized the loss of water by using grid alloys having high hydrogen overpotential. Rigid, self-supporting, and sometimes structurally reinforced grids may be employed, made from a variety of either antimony-free, or low-antimony, lead alloys.

Examples of grid alloy systems used include calcium-lead, calcium-tin-lead, cadmium-antimonylead, selenium-antimony-tin-lead with various optional alloying ingredients such as silver and arsenic as well as combinations of these alloys.

Further, lead-acid batteries in which the electrolyte is immobilized in a gel form are known.

Such batteries can provide not only maintenance-free but also spill-free characteristics, viz.-the battery may be used in any attitude without electrolyte leakage. However, the cracks which develop in the gel during charge, while essential for oxygen transport, result in conditions which can adversely affect the desired performance. Also, such batteries invariably are characterized by higher internal resistance and, hence, cannot be used in applications where high ratio discharge capability is a requirement, such as, for example, an automotive starting, lighting and ignition battery.

To provide a sealed design yet avoid the potential problems with gelled electrolytes, sealed systems have been utilized in which electrolyte is immobilized and absorbed in special separators. The separators are not fully saturated, and the gases evolved during overcharge or at other times can diffuse rapidly from one electrode to the other. Thus, under the right conditions, the oxygen that is evolved at the positive electrode can diffuse to the negative electrode where it will rapidly react with active lead. Effectively, this reaction partially discharges the negative electrode, preventing the negative electrode from reaching its fully-charged state so as to minimize the evolution of hydrogen. This sequence results in what has been termed an "oxygen cycle".While the oxygen recombination rate is greater than the rate of oxygen being produced at the positive electrode, there should be minimal water loss and pressure build-up.

At the present time, sealed lead-acid systems of this type have been available commercially in only small ampere-hour capacity sizes. Usage has thus been generally confined to standby applications such as emergency lighting, alarm systems and limited cycle life portable equipment such as television, lanterns and garden tools.

U.S. 3,862,861 to McClelland and Devitt is an example of a cell configuration recombining oxygen using relatively #high internal pressures. The cell is thus said to enhance the rate of recombination by operating under increased pressure. For this reason, a vent relief is used which should be biased to vent at as high a pressure as possible. A Bunsen-type relief valve capable of retaining at least 10 to 15 pounds of internal pressure is disclosed. This type of cell can be used in float applications and in deep cycle applications in which limited life is acceptable.

A cell having a prismatic container is described in Progress in Batteries 8 Solar Cells, Vol. 2, 1979, pp. 167-170, and is a type which apparently operates at relatively low internal pressures. This type is used primarily for float applications.

A further type of battery is reported in the International Symposium on Batteries, October 21-23, 1958. As described, in general terms, the batteries are made by building a stack of plates and flat, softish separators, the stack, after compression, being inserted into a suitable container. In this stack or block of plates and separators, the active material of the plates if fully supported by the separators. The block, after compression, must have a high enough porosity to allow it to absorb and hold all the electrolyte required for efficient and economic functioning.

The separators are made from diatomaceous earth. Batteries suitable for automotive starting, lighting and ignition applications are disclosed, the advantages described being resistance to vibration, unspillability, and increased high rate performance at normal and low temperatures.

While the report describes the wide introduction onto the market as having been limited due to commercial considerations rather than to performance or quality, it is believed that the performance characteristics do not meet present requirements for such applications. Regardless, the type of battery described has not achieved widespread usage and has not been commercially successful.

Still further, it has been suggested that a sealed, lead-acid system might be scaled up to larger ampere-hour capacity sizes than are being used commercially at the present time (Engineering, October, 1978, The Age of the Sealed Battery, pp. 1020-22). It was, indeed, stated that the characteristics of the sealed battery system would be particularly advantageous for an automotive SLI (viz.-starting, lighting and ignition) application.

In addition to this application, there are a number of what may be termed "float-pulse" applications (viz.-a system capable of providing relatively high peak power at fairly high discharge rates for brief periods of time) where characteristics attributed to a sealed system would seem to make such a system desirable for use. Batteries for motorcycles, starting outboard motors and standby power for computers are examples which fall into the category.

Applications requiring a relatively pure form of direct current also have similar requirements.

Batteries for such applications require at least 15 ampere-hour capacities, or even 25 amperehours and substantially more.

Yet, for whatever reason, sealed systems of these capacity sizes have not become commercially available to any extent. It may well be that the necessary performance characteristics have simply not been capable of being provided. One may thus speculate that it has proven difficult at best to achieve the capacity and life needed for such applications. Also, it may be felt that a stronger, more rigid battery container is required to withstand the high internal pressures believed necessary to provide satisfactory oxygen recombination efficiency and that this would be particularly acute in batteries of relatively large capacity sizes. The use of such stronger battery containers would. of course, result in an increased weight for the battery.This would be particularly detrimental in the automotive area since the present trend is to provide battery systems in which the weight can be decreased as much as possible.

It is accordingly a principal object of the present invention to provide a sealed maintenancefree, lead-acid battery which is capable of providing performance characteristics satisfactory for use in float-pulse applications.

Another and more specific object of this invention provides a sealed, lead-acid battery capable of performance which equals or exceeds the characteristics required of batteries for automotive starting, lighting and ignition applications.

A further object lies in the provision of a sealed, lead-acid battery characterized by improved volumetric and gravimetric energy density.

A still further object of the present invention is to provide a sealed, lead-acid battery which can be designed in sizes ranging from small to extremely high capacities.

Yet another object lies in the provision of a sealed, lead-acid battery capable of being manufactured with standard equipment used for conventional float-pulse battery production.

A A still further object of the present invention is to provide a sealed, lead-acid battery capable of using the standard, thinwall plastic containers often employed for float-pulse batteries.

Another object of this invention provides a sealed lead-acid battery capable of operation at extremely low internal pressure yet providing maintenance-free characteristics over an extended life.

Other objects and advantages of the present invention will be seen from the following description and the drawings, in which: Figure 1 is a perspective view of a battery made in accordance with the present invention, partially cut-away to show the internal configuration; Figure 2 is an exploded, sectioned side elevation view and illustrating the arrangement of the plates and separators of the battery of the present invention; Figure 3 is a cross-sectional view taken generally along line 3-3 of Fig. 1, and further showing the internal configuration of the battery; Figure 4 is a graph of voltage versus time and showing the improved cold cranking behavior with a sealed lead-acid battery made in accordance with the present invention in comparison to a a commercial maintenance-free battery; and Figure 5 is a graph illustrating the performance of a sealed, lead-acid battery of the present invention in an industry cycle life test.

While the present invention is susceptible to various modifications and alternative forms, there is shown in the drawings and will herein be described in detail, the preferred embodiments. It is to be understood, however that it is not intended to limit the invention to the specific forms disclosed. On the contrary, it is intended to cover all modifications and alternative forms falling within the spirit and scope of the present invention as expressed in the appended claims. Thus, while the present invention will be principally described in conjunction with an automotive starting, lighting and ignition application, it should be appreciated that the present invention may be utilized for any other float-pulse application. Further, the invention is, of course, equally applicable to either a battery or to a single cell. Also, while the present invention will be described in connection with batteries of larger capacity sizes, it should be appreciated that it is likewise useful in providing small capacity sizes as well. Likewise, while all of the advantages of this invention will not be obtained, a container more rigid than that required by the internal pressures developed in service can be employed, if desired.

In general, the present invention is predicated on the discovery that a sealed, maintenancefree, lead-acid battery suitable for various float applications such as, for example, an automotive starting, lighting and ignition application, can be provided by immobilizing the electrolyte in highly absorbent separators while operating at extremely low internal pressures, allowing use of conventional, thinwall containers. The battery of the present invention will provide superior peak power at relatively high discharage rates in comparison to conventionally used flooded electrolyte-type batteries as well as satisfying the other performance characteristics required.

Turning now to the drawings, Figs. 1 and 2 show the battery 10 in accordance with the present invention. The battery 10 has a container 12, separated into individual cells by internal partitions 14. In each cell, a plurality of positive electrodes 16 and negative electrodes 18 are separated by absorbent separators 20.

The electrical connections necessary can be made by any of the several techniques which are known in the art. The particular technique employed does not form a part of the present invention. As shown, conductive straps 22 join the electrodes together, and the intercell connections are shown in Fig. 3. The straps 22 of the end cells are connected to external terminals 24 by conventional means.

As seen in Figs. 1 and 3, release venting is provided through manifolds 26 to low pressure, self-resealing relief valves, such as, for example, bunsen valves 28. While venting through a manifold in illustrated, individual cells could each be provided with a relief valve if desired. On the other hand, a single manifold for the six cells shown could be used or more than two manifolds can be utilized.

The electrodes, 16 and 18, and separators 20 should be snugly fit within the cells, i.e.-the electrodes and separators should stay in the assembled condition when the container is inverted.

The electrodes can thus be sized to almost the interior dimensions of the cells. To eliminate the possibility of shorts, it is, however, desirable to size the separators used such that the edges extend slightly past all of the edges of the electrodes, as is shown in Fig. 2. One means of achieving this at the bottom of the electrodes is to fold the separators around the electrode with a U-fold as depicted in Fig. 2.

Highly efficient use of the internal container is obviously thus provided. However, if desired, spacing means, such as shims, could be employed if the cells employed are oversized for any reason.

Considering the present invention in greater detail, the grids used for the positive and negative grids can be any of the several known grid alloys used for conventional maintenancefree batteries. As one example, it is thus suitable to utilize a calcium-tin-alloy in which the calcium content is from about 0.06 to 0.20% and the tin is in the range of 0.1 to 0.5% (preferably 0.2-0.3), both percentages being based upon the total weight of the alloy. The alloys used should be capable of providing self-supporting grids. The grids may be formed by any of the known techniques, such as direct casting or expanded metal.

There are several difficulties associated with providing a satisfactory internal structure for a sealed system. One such difficulty arises from the need to provide an absorptive capacity for the system to insure that sufficient electrolyte will be retained to yield the desired capacity. This can be provided to some extent by using thicker separators than would be needed for a flooded-electrolyte system. However, this will generally not be a complete answer as the internal resistance will increase as the plate spacing is increased. There is thus a practical limit in the trade-off of performance characteristics which will result. Some benefit can be obtained by using somewhat higher specific gravity electrolyte than conventional flooded systems; but, here too, there is a practical upper limit due to factors such as the decreased conductivity which results.A further possibility is, of course to decrease the density of the active material pastes.

Since mossing of the negative active material and shedding of the positive active material should be prevented, or at least substantially minimized, by the snug fit of the electrodes within the absorbent separators utilized, lowering the active material paste density should conceptually be possible. However, here again, there are limitations because of processing problems which arise with such reduced densities when conventional pasting machines are employed, as in an automotive application.

A further difficulty arises from the shrinkage in the negative active material which occurs in batteries exposed to float conditions, as is known. This can be a primary mode of failure, significantly lessening the useful life of a battery. It is for this reason that battery manufacturers have conventionally used various expanders to maintain the desired porosity and active material surface area of the negative plates. It has been found that this difficulty is particularly acute in sealed systems for float applications.

Accordingly, to obtain all of the advantages of the present invention, it is preferred to utilize active material pastes which have increased absorptive capacity yet which can be readily processed with conventional equipment and provide a system with satisfactory life and performance characteristics. To this end, the paste densities for the positive active materials are lower than the paste densities used for conventional, flooded, lead-acid batteries using long life, leady-oxide materials, such as those obtained from a Barton pot. It has thus been found suitable to utilize plate densities in the range of from about 2.9 to 4.1 grams/cm.3 for the cured, unformed positive active material. A density range of 3.0 to 4.2 grams/cm.3 for the cured and formed positive active material is acceptable.

To provide the desired electrolyte retention properties, it is preferred to incorporate an electrolyte-retaining agent such as colloidal silica into the paste. Incorporation of colloidal silica in an amount of from about 0.05-1.0%, based upon the weight of the unformed positive active material, has been found suitable. Other electrolyte-retaining materials are known and may be similarly utilized as substitutes, in whole or in part, for the silica. Desirably, the electrolyteretaining agent should also function as a bulking agent, viz.-increase the consistency of the wet paste so as to improve handleability. Colloidal silica satisfies this function.

In accordance with a further and preferred aspect of the present invention, the characteristics of the negative active material is enhanced not only to increase the absorptive capacity but also to compensate for the effects of shallow cycling to provide satisfactory life and performance characteritics. To this end, the negative active paste preferably includes a combination of conventionally used paste additives above the levels typically used. As one component, it is preferred to utilize a conventional organic expander at a level of from about 0.3 to about 1.0%, based upon the weight of the dry, unformed paste. Various lignosulfonates such as, for example, sodium lignosulfonate are known and are suitable.In addition, it is preferred to utilize an additive serving to minimize the physical consolidation of the lead sulfate crystals formed during cycling into larger crystals and eventually into an essentially solid layer with minimal porosity, resulting in failure of the battery. Cellulose floc serves this purpose, and a level of about 0.3 to about 1.0% by weight may be used.

It is not believed that the maximum benefits will be achieved by utilizing only one type of expander as the functions are believed different. In some fashion, the organic expander affects the morphology of the lead sulfate structure to retard loss of porosity. The additive which retards consolidation is believed to serve, in effect as a physical spacing means.

Preferably, the paste additives utilized should likewise function as bulking agents for the paste. Cellulose floc and sodium lignosulfonate both serve this function.

A mixture of 0.75% sodium lignosulfate and 0.6% cellulose floc, both percentage being based upon the weight of the dried unformed paste, is preferred. Use of this mixture appears to provide the desired cycle life.

The cured, but unformed, paste density for the negative active material should desirably be in the range of about 3.5 to 4.4 grams/cm.3. A density range of about 3.2 to 4.1 grams/cm.3 for the cured and formed negative active material is satisfactory. While the negative paste additives previously described serve to retard loss of porosity and thus retain the absorptive capacity, it is preferred to also incorporate in the negative paste an electrolyte-retaining agent such as colloidal silica. Amounts in the range of from about 0.05 to 1.0%, based upon the weight of the dried, unformed paste are suitable.

It has typically been believed essential, as set forth in U.S. 3,862,861, to utilize in a sealed system an excess of negative active material in relation to the positive active material so that the requisite oxygen cycle will be attained. According to one aspect of the present invention, it has been found that a satisfactory oxygen cycle will be achieved even though the amount of negative active material is less than that of the positive active material. It is accordingly preferred to utilize a negative active material to positive active material weight ratio of from about 0.75 to about 0.92, or so. This results in not only reduced weight but also in less cost without detrimental effects on performance.

The material used for the separators 20 should be stable in the sulfuric acid electrolyte used, resistant to oxidation by PbO2 and not release materials into the electrolyte which would deleteriously affect cell performance. In addition, the material should be highly porous, e.g.-at least 70 to 75%, desirably up to about 90% or so, and should be sufficiently compressible to at least subatantially conform to the changing shapes of the electrodes during assembly and service. Further, average pore diameter should be sufficiently small to prevent propagation of dendrites from the negative plate and shedding of the active material from the positive plate.

The average pore diameter should, however, be sufficiently large to be easily wetted by the electrolyte and not so small as to result in an unduly high internal impedance. The separator material must also be capable of wicking the electrolyte through the desired height of the separator.

Lastly, and importantly, the separator material must preferably provide, in service, a substantially uniform void volume throughout the separator. The separator thus preferably provides sufficient void volume to support the rate of oxygen transport necessary for the internal pressure desired for the cell. It is believed that the void volume is achieved through some of the pores having their walls covered with a film of electrolyte while the central portion of the pore is free from electrolyte. Satisfactory oxygen recombination efficiency for some applications may be achieved even when the separator is thoroughly wetted with electrolyte. Suitability in this respect can be determined by weight loss (water) determinations made during cycling. Unduly high water loss should not result if the material is suitable.

The thickness of the separator will, in general, be determined by the cell capacity and the expected operating rate for the particular application. In this respect, the separator thickness used does not materially differ from those found suitable for other types of lead-acid cells used for the particular end use application.

It has been found suitable to use a borosilicate glass material formed from glass microfibers and chopped strands. Materials of this type are commercially available and have been previously utilized for sealed, lead-acid cells. One such material (C.H. Dexter Division, The Dexter Corporation, Windsor Locks, Connecticut, "Grade 225 B") that has been found satisfactory has the following typical properties: nominal thickness of 300 microns, air permeability (ASTMD737-75) of 15.1 l/min/100 cm.2 at 12.7 mm. water A P (Frazier Permeometer), an average pore size of 12.6 microns and a porosity (by mercury intrustion) of 1.2 meters2/gram.

It is preferred to utilize at least one layer of the glass borosilicate material adjacent the electrodes. Additional layers of other materials may be used, as for example, for reinforcement to improve handleability in processing. As illustrative examples, commercially available, nonwoven, polyethylene, polypropylene and polyester sheets may be suitably employed. Such materials possess the desired electrolyte retention but both provide decreased cost as well as improved strength. Such a laminar separator system can be used to provide the necessary strength for relatively easy introduction of pocket-type and U-fold separators in conventionally used, high feed, SLI battery assembly techniques.

For a given application, the full charge specific gravity for the electrolyte needed can be readily computed. Typically, full charge specific gravities in the range of 1.255 to 1.320 will be satisfactory, 1.285 to 1.320 being preferred. Particular applications may make it desirable to use somewhat higher or lower acid gravities.

The formation of the pasted electrodes can be carried out by known techniques. Thus, prior to assembly in the container, the electrodes can be formed by conventional tank formation. When this technique is employed, the formed electrodes should be dried to remove the residual electrolyte.

Desirably, however, the unformed electrodes and separators are placed in the container, the necessary electrical connections made, the cover sealed to the container, the necessary electrolyte added through the aperture in the cover for the relief valve, and the valve then put in service position. Formation is then carried out using conditions suitable for conventional oneshot, lead-acid battery formation. It may be useful, however, to employ somewhat less severe formation finishing conditions than those conventionally used. It may also be desirable to initially chill the formation electrolyte to some extent.

It should likewise be noted that, when in situ formation is employed, initiation of formation should take place within about 1/2 to 1 hour or so after the electrolyte is added. Longer delays can result in conditions which may ultimately create internal shorts.

The amount of electrolyte employed should preferably not result in the absorbent materials of the battery being fully saturated, i.e.-the battery should be in an electrolyte-starved condition.

While the battery in service is self-regulating, a fully saturated condition may result in undue gassing during the initial stages of charging during cyclic operation. Further operation will reach an equilibrium state where an efficient oxygen recombination cycle is achieved. The somewhat higher gassing which occurs during the initial stages can be substantially eliminated by determining the particular void volume required for efficient oxygen recombination. However, it has been found suitable to add electrolyte sufficient to saturate the absorbent capacity of the system to a level of about 90%.

Care should be taken to avoid free electrolyte in the system. Significant quantities of free electrolyte (viz-not immobilized in the electrolyte and separators) can adversely effect performance.

In accordance with one aspect of the present invention, release venting is provided by a selfresealing relief valve which unseals at extremely low internal pressures. To this end, a valve is employed which will unseal at internal pressures of from about 0.5 to about 3.0 psig. Indeed, it is preferred to utilize internal operating pressures of no more than about 1 or perhaps 2 psig so that conventionally used, thinwall, plastic containers may be employed.

The following Examples further illustrate, but are not in limitation of, the present invention.

Unless otherwise specified, all percentages are by weight.

EXAMPLE 1 This Example illustrates the performance of the sealed lead-acid battery of the present invention in comparison to a conventional, flooded, maintenance-free battery.

Batteries of a nominal ampere-hour capacity of about 71 were assembled in hard rubber containers. Each battery contained 17 plates per cell. The alloys used for the grids of both batteries were calcium-tin-lead having a nominal composition of 0.09% calcium, 0.3% tin and the remainder lead. The paste densities employed were 4.0 gms./cm.3 for the positive and 4.4 for the negative, both based upon the dried, unformed paste weight. The pastes were cured by exposure at 140 F. for about 16 hours in a 100% relative humidity atmosphere, followed by exposure at 140"F. to a zero percent relative humidity atmosphere for a period of about 48 hours or so.Each electrode of the battery of the present invention was wrapped with a two-layer separator comprising an interior layer of a nominal 13 mil of the borosilicate glass material previously described and a 13 mil exterior layer of a nonwoven, point-bonded polypropylene material, U-folded about the electrode as depicted in Fig. 2. A conventional polyvinylchloride separator having a nominal thickness of 37 mils with a 12 mil back web was used for the conventional maintenance-free battery.

Other constructional details are set forth in Table 1.

Table 1 Sealed Conventional Parameter Battery Battery Positive grid thickness, inches 0.041 0.041 Negative grid thickness, inches 0.039 0.039 Dry positive paste weight per plate, gms. 110 110 Dry negative paste weight per plate, gms 101 101 Ratio of positive paste wt./grid wt., gms. 70/40 70/40 Ratio of negative paste wt./grid wt., gms. 62.5/38.5 62.5/38.5 Specific gravity of electrolyte 1.285 1.285 Interplate spacing, inches 0.022 0.037 Further details of the overall batteries are set forth in Table 2.

Table 2 Sealed Conventional Parameter Battery Battery Total wt., positive paste, Ibs. 7.40 7.40 Total wt., negative paste, Ibs. 7.44 7.44 Total wt., positive grids, Ibs. 4.23 4.23 Total wt., negative grids, Ibs. 4.58 4.58 Total wt., lead within element, Ibs. 23.65 23.65 Total wt. of electrolyte, Ibs. 5.75 15.30 The plates and separators were placed in the containers, the necessary electrical connections made, and formation acid of 1.200 specific gravity cooled to 0 F. added. A charge regime of 7 amps for 1 621 hours, followed by 4 amps for 3 hours and 2 amps for 2 hours, was used. The covers were then put in position. The sealed battery included a conventional Bunsen relief valve which unsealed at 0.5 psig.

The batteries were then tested and the performance is set forth in Table 3: Table 3 Sealed Conventional Test Battery Battery 0 F. Cold crank, voltage after 30 sec., 550 Amps 8.05 7.60 Relative electrical resistance, 0 F. 9.9 11.2 Reserve capacity, 25 Amp discharge, minutes 78 124 As can be seen, the sealed battery of the present invention provides better cold-cranking power than the conventional maintenance-free battery. Fig. 4 also shows this improved performance over the 60 second discharge to which each battery was subjected, the curve labelled 1 being the sealed system and curve 2 being for the conventional, maintenance-free battery.

However, as seen from Table 3, the reserve capacity of the sealed battery of this invention was substantially lower than that of the conventional, maintenance-free battery. It should be appreciated that the active material pastes were not modified as previously described, and the results thus highlight the need for suitable modification to achieve all of the advantages of this invention.

The sealed battery was subjected to a standard SLI 12 minute cycle, J240 regime and failed at about 4500 cycles which is substantially less than that typically achieved by the performance of commercially available, maintenance-free batteries. However, the failure mode was believed due to a design flaw in the particular positive grids used.

Example 2 This Example illustrates the life performance capable of being achieved by a sealed battery made in accordance with the present invention.

A test battery was assembled having a nominal ampere-hour capacity of about 67 which contained 13 plates per cell. The alloys used were the same composition as set forth in Example 1. The cured unformed paste densities were about 4.4 gms./cm.3 for the negative active material and about 3.9 for the positive active material. The negative paste was made from a formulation which included, in part, 1200 gms. of leady litharge, 50 grams of a conventional expander mixture and about 0.3% colloidal silica based upon the dry, unformed weight of the paste.

The pastes were cured as described in EXAMPLE 1. The positive plates were wrapped with 2 layers of the borosilicate glass material previously described and the negative plates were wrapped with a single layer of that material.

The plates and separators were placed in a standard SLI, thinwall, plastic container used for commercial, maintenance free batteries, modified to substantially reduce the height of the restups in the bottom of the container. Assembly and formation were carried out as generally described in EXAMPLE 1, the specific formation regime involving charging at 7 amps for 18 hours, followed by 4 amps for 6 hours and 2 amps for 2 hours.

Other constructional details are set forth in Table 4, the specifications for a conventional, maintenance-free battery being included as a general comparison: Table 4 Sealed Conventional Parameter Battery Specification Positive grid thickness, inches 0.075 0.064 Negative grid thickness, inches 0.051 0.047 Dry positive plate wt., gms. 148 149 Dry negative plate wt., gms. 104 107 Ratio of positive paste wt./grid wt., gms 89.8/58.5 87/62 Ratio of negative paste wt./grid wt., gms 63.5/40.2 66/41 Specific gravity of electrolyte 1.290 1.265 Table 5 sets forth details of the assembled battery, the specifications for the conventional battery referred to in conjunction with Table 4 also being included:: Table 5 Sealed Conventional Parameter Battery Specification Total wt., positive paste, Ibs. 7.12 6.90 Total wt., negative paste, Ibs. 5.88 6.11 Total wt., positive grids, Ibs. 5.42 4.92 Total wt., negative grids, Ibs. 3.73 3.78 Total wt. within elements, Ibs. 22.15 21.72 Total wt. of electrolyte, Ibs. about about 6.90 11.60 Total wt. of battery, Ibs. 32.2 37.8 The sealed battery was tested, and Table 6 sets forth the results in comparison to the specifications for the conventional battery described herein: Table 6 Sealed Conventional Test Battery Specification 0 F. cold crank, volts at 30 sec., 500 Amps discharge 7.84 7.2 Reserve capacity, 25 Amps, mins. 91 102 As can be seen from Table 6, the sealed battery of this invention provides the same improved cold cranking as shown in EXAMPLE 1.Also, while still not the same as the conventional battery, the modification to the negative grids together with the increased interplate spacing has provided improvement in relation to the comparison shown in EXAMPLE 1. Further improvement in this regard can be achieved by further modification of the active material pastes as previously described.

The sealed battery was subjected to the J240 life test described in EXAMPLE 1 and achieved about 8000 cycles. During this test regime, the battery was discharged at 500 amps; and the 5second and 30-second voltages were ascertained. Fig. 5 sets forth the results, curve 3 being the 5-second and curve 4 being the 30-second voltages. Failure occurred through positive grid corrosion, considered to be a design defect in such grids.

Significantly, over the life of the J240 test, the sealed battery of this invention lost only about 0.15 Ibs. of water (+ 0.05 accuracy in measurement) which is considered to be about an order of magnitude less than would be expected from a conventional, maintenance-free battery.

Thus, as has been seen, the battery of the present invention provides all of the advantages of a sealed system and particularly provides improved peak power at relatively high rates for the short times encountered in a wide variety of float applications. In an automotive starting, lighting and ignition application, this invention thus provides significantly superior cold cranking power. This performance is achieved while maintaining the other performance characteristics required. The efficient oxygen recombination rate even at the extremely low internal pressures provides superior maintenance-free characteristics. The use of low internal pressures likewise allows the utilization of conventional, thinwall plastic containers, a highly useful feature in automotive applications where reduced weight is becoming more and more important. Indeed, the relatively high volumetric energy density capable of being achieved can allow the use of even smaller sized containers with no sacrifice in performance.

Claims (16)

1. A maintenance-free, lead-acid battery having characteristics suitable for use in float applications comprising: a sealed container divided into a plurality of cells by internal partitions, a plurality of positive plates contained in each cell, each of said plates comprising a selfsupporting grid and positive active material pasted on said grid, a plurality of negative plates contained in each cell, each of said plates comprising a self supporting grid and negative active material pasted on said grid, at least one layer of an electrolyte absorbing and retaining separator material intimately contacting and separating said positive and negative plates, sulfuric acid electrolyte absorbed in said plates and separators, said plates and separators being sufficiently porous to retain sufficient electrolyte to provide a capacity of at least about 25 ampere hours, and said container having at least one normally closed relief valve capable of venting gases from the container to the atmosphere when pressure within said container is in the range of from about 0.5 to 3.0 psig.

2. The battery of claim 1 wherein at least one of the positive and negative grids are of a calcium-tin-lead alloy.

3. The battery of claim 1 wherein the positive active material has a density of from about 3.0 to 4.2 grams/cm.3.

4. The battery of claim 1 wherein the positive active material contains from about 0.05. to 1.0% by weight of an electrolyte-retaining agent.

5. The battery of claim 4 wherein said electrolyte-retaining agent is colloidal silica.

6. The battery of claim 1 wherein the negative active material has a density of from about 3.2 to 4.1 grams/cm.3.

7. The battery of claim 1 wherein the negative active material contains from about 0.05 to 1.0% by weight of an electrolyte-retaining agent.

8. The battery of claim 7 wherein said electrolyte-retaining agent is colloidal silica.

9. The battery of claim 1 wherein said negative active material contains an organic expander present in an amount of from about 0.3 to about 1.0 percent by weight.

10. The battery of claim 1 wherein said negative active material contains an additive which minimizes the consolidation of lead sulfate crystals during cycling and is present in an amount of from about 0.3 to about 1 percent by weight.

11. The battery of claim 1 wherein the weight ratio of negative active material to positive active material is less than 1.0.

12. The battery of claim 10 wherein said weight ratio is in the range of about 0.75 to 0.92.

13. The battery of claim 1 wherein said electrolyte retaining and absorbing separator material is a borosilicate glass material.

14. The battery of claim 12 wherein there are at least two layers of said electrolyte retaining and absorbing material and the layer adjacent said plates is a borosilicate glass material.

15. The battery of claim 1 wherein a common manifold connects a plurality of said cells to said relief valve.

16. The battery as substantially described herein.