FIELD OF THE INVENTION:
The present invention relates to lead acid battery, and more particularly to alloy for positive grid. The invention provides composition of a novel Pb-Sb-Se alloy superior to that of Pb-Ca-Sn alloy and also the process parameters to be maintained for casting of the grid from this alloy.
BACKGROUND OF THE RELATED ART:
Lead Acid batteries are widely used as power source in many applications such as automotive, telecom, electric vehicles, uninterruptible power supply (UPS), traction etc. Different types of lead based alloys are used in making grids of these batteries based on their application and requirement.
There are mainly two types of alloys used in lead acid batteries namely Lead (Pb)-Antimony (Sb) and Lead (Pb)-Calcium(Ca) based alloys. Pb-Sb based alloys are most widely used as positive grid material in majority of the deep cycle applications because of their ease of castability, good mechanical strength and higher cyclic life. However, higher water loss and lower shelf life when compared to Pb-Ca based alloys is limiting their usage in other applications.
EP application number 19800200095 provides for an antimony-lead alloy arms for producing mass support grids for lead accumulators. The said application states that It is customary in general to produce the grid of lead-acid batteries from a Pb-Sb alloy having an antimony content of 4 to 11%. Here, the antimony performs the task of giving the innately very soft and not usable for processing accumulators grids pure lead the required strength and moldability. The antimony in these alloys is an additive, however, increases the cost significantly the production of the lattice due to its high cost. In addition, the self-discharge of the battery increases with increasing antimony content. Furthermore, increases with increasing antimony content gas development, and the migration of antimony ions to the negative electrode of the battery during operation leads to poisoning of the electrode. For
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these reasons, efforts have been made to reduce the antimony content of the grid for lead storage batteries as much as possible.
It is well known in the art that Antimony is used to increase the strength and/or other physical properties of lead thereby facilitating various aspects of the battery manufacturing. Batteries are being produced in the industry with zero maintenance i.e. no addition of water is required to maintain the electrolyte. In order to achieve this maintenance free battery, it is required that the water losses must be reduced substantially and preferably to bare minimum. This requires that the grids used in such maintenance free battery generates minimal gases so as the water loss accompanying gases is bare minimum. There are antimony based grids available in the prior art wherein the content of Antimony is about 4.5% by weight. It is known that self discharge of a wet lead acid battery employing an antimony alloy is caused primarily by the dissolution of antimony from the positive grid and its subsequent deposition on the negative plates to cause electrochemical reaction. This electrochemical reaction leads to discharge the active lead to lead sulphate. Consequently when the antimony is the only alloying constituent used in a lead-based alloy, it is impractical to reduce the antimony level significantly below the 4.5% level due to a tendency of the grid cast from such low antimony based alloy to crack.
US 4,456,579 states that when antimony is used the only alloying constituent used in a lead-based alloy, it generally becomes impractical to reduce the antimony level significantly below the 4.5%level, due to a tendency of grids cast from such low antimony, lead based alloys to crack.
Mao and Lannoye, U.S. patent application Ser. No. 878,882, filed on Feb. 17, 1978, for: Cadmium-Antimony-Lead-Alloy for Maintenance-Free Lead-Acid Battery describes an alloy that has a reduced antimony content and is suitable for forming the grids of maintenance-free batteries. The lead-based alloys disclose therein contain from about 1.0 to about 1.9 wt.% antimony and from about 1.2 to about 2.0 wt.% cadmium, the cadmium being present in an amount at least equal to the antimony present. The addition of cadmium avoids the cracking phenomenon, and the resulting alloy provides superior grids
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for maintenance-free batteries. However, the toxicity of cadmium necessitates special handling precautions.
A number of patents suggest the use of lead-based, low antimony alloys containing selenium for grain refinement as well as several other alloying ingredients. These patents include the following: Great Britain No. 622,512; U.S. Pat. No. 3,801,310; U.S. Pat. No. 3,879,217; U.S. Pat. No. 3,912,537; U.S. Pat. No. 3,933,480 and U.S. Pat. No. 3,990,893. The amounts of selenium set forth vary significantly as do the levels of the other alloying ingredients proposed.
These low antimony, selenium, lead-based alloys require an alloying ingredient to provide the requisite strength characteristics (including instantaneous handling strength); and minor amounts of arsenic are employed for this purpose in several of the cited patents. Unfortunately, achievement of the requisite strength characteristics in this fashion is attained at the expense of the desired ductility. The use of arsenic in such alloys in the levels suggested thus results in grids which are too brittle to allow easy handling of the grids in further processing. This is particularly significant in direct gravity casting applications when either relatively thicker grids are needed or where the grid configuration involves the intersection of bars or wires of differing cross-sections. At such intersections, the susceptibility to breaks due to excessive brittleness is increased. Also, the presence of arsenic above critical limits in such alloys can detract from the characteristics desired for maintenance-free applications.
Three of the cited patents further suggest, as an optional alloying ingredient, the inclusion of silver in a level of from 0.025 to 0.1 wt.% Silver addition is said to stabilize the fine structure of such alloys and to improve corrosion resistance. Such addition is also desirable for batteries subject to relatively rigid requirements with respect to mechanical strength, ductility and electrochemical behavior of the grid alloys.
However, the inclusion of silver in such levels increases the alloy cost, which may be significant. Moreover, and importantly, such silver levels appear to detract from the characteristics required for maintenance-free applications.
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The prior art discloses low antimony, selenium, lead-based alloys which can be formed into battery grids having exceptional properties by minimizing the arsenic content of such alloys and adding a relatively low level of silver. Such alloys can be easily cast by direct gravity casting techniques at commercially acceptable rates, yet provide grids with superior ductility characteristics. Moreover, grids formed from such alloys exhibit reduced gassing and water loss characteristics in comparison to other alloys of this general type.
Thus there is a need for a specialized composition of Pb-Sb based alloy which reduces the water loss and improves shelf life to cater in the sensitive areas like automotive, UPS and VRLA segment where in customers expects low to no maintenance of the battery.
SUMMARY OF THE INVENTION
As said in background of the art, an objective of the present invention is to develop an ultra low antimony alloy which can replace the advanced Pb-Ca-Sn alloy in all the applications. The aim is to attain the water loss and shelf life of the batteries made with new alloy on par to that of Pb-Ca-Sn alloy by reducing antimony content to a maximum possible extent and adjusting the composition of other elements to attain the required mechanical strength and castability.
The invented alloy also exhibits superior cyclic performance compared to Pb-Ca-Sn because of the elimination of “antinomy free effect” besides will deliver low water loss as well high shelf life and hence can be used in both cranking and deep cycle applications. Thereby more benefits are achievable than Pb-Ca-Sn alloy.
Another objective of this invention is to derive the process parameters such as casting speed, melt pot, lead valve, upper mould, lower mould and mould cooling temperatures required for casting the above alloy.
DETAILED DESCRIPTION
The present invention provides a new ultra low antimony based Pb-Sb-Se alloy which replaces advanced Pb-Ca-Sn alloys. In one embodiment of the present invention, the
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antimony content in the alloy is minimized to a maximum possible extent of about 0.9-1.1 weight% which reduces the dissolution of antimony ions into the solution and preventing antimony migration, further reduction to Sb in the negative plate while charging process which in turn increases the hydrogen over potential of the alloy and further helps in reduction of water loss. Wherein the individual the amount of different alloying elements are given below:
S.No
Element
Symbol
Weight(%) range
1
Antimony
Sb
0.900-1.100
2
Tin
Sn
0.250-0.350
3
Selenium
Se
0.030-0.050
4
Arsenic
As
0.100-0.200
5
Bismuth
Bi
0.0150 Max.
6
Silver
Ag
0.0300 Max.
7
Manganese
Mn
0.0001Max.
8
Calcium
Ca
0.0005 Max.
9
Copper
Cu
0.0025 Max.
10
Nickel
Ni
0.0005 Max.
11
Iron
Fe
0.0010 Max.
12
Remaining
-
0.0010 Max.
13
Lead
Pb
Balance
In the above embodiment of the present invention, the mechanical strength of the casted grids are maintained during casting process due to the addition of tin content of about 0.25-0.35% and arsenic content of about 0.1-0.2%. The grain structure of the alloy is maintained due to the addition of selenium content of about 0.03-0.05%. The iron and other impurities contents are restricted to maximum of each 10 ppm.
In yet another embodiment of the present invention comprises the process parameters (melt pot, lead valve, upper mould, lower mould, mould cooling temperatures
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and casting speed) to be maintained during the continuous gravity casting of the grids from the alloy mentioned in the above embodiment. It is to be noted that the use of the continuous gravity casting process increases the productivity as well as other manufacturing benefits. Specifically, the production capability achieved with a single continuous casting machine should be equivalent to that of some 10 or more presently used direct gravity casting machines. Accordingly, labor costs and floor space requirements should be capable of being substantially reduced with continuous casting machines. Further, electrical energy costs to produce a given volume of grids with a continuous casting process should be significantly lower than what is required by present gravity casting techniques. Also, it is believed that a continuous casting machine should provide a more desired work environment due to less heat and other pollution in relation to present gravity casting machines.
In the above embodiments of the present invention, also includes a lead acid battery made with the said Pb-Sb-Se alloy grid exhibiting equivalent water loss & Shelf life and better cyclic life than that of the batteries made with Pb-Ca-Sn alloy. The said alloy also eliminates the so called “antimony free effect” which causes rapid capacity loss during cycling.
The composition of the novel alloy provides for an addition of Selenium in a specified amount as a grain refiner which helps to reduce the grain size which in turn aids in higher mechanical properties & corrosion resistance leads to improved cyclic life of the battery. It is to be noted that the alloy can be easily casted by Book Mould casting method which needs this stringent alloy composition. In case of alloys with still lower percentage than 0.9%Sb, it is difficult to book cast.
The present invention also provides for a lead acid battery made with novel alloy which has equivalent water loss & shelf life and better cyclic life than that of Calcium based alloys. IT is to be noted that where the alloys with higher percentage than 1.1%Sb are used, there is higher water loss & reduced shelf life. The novel alloy of the present invention can also be used to make valve regulated lead acid battery which requires zero maintenance or water addition during the entire life cycle of a battery.
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It may be noted that the above embodiments are only by way of example and that the invention could be exemplified in many other ways obvious to a person skilled in the art.

WE CLAIM
1. A lead acid battery comprising the continuous gravity casted positive grids made with the alloy containing greater than 0.9 weight% to less than 1.1 weight% antimony and greater than 0.25 weight% to less than 0.35 weight% Tin and 0.03weight% to 0.05weight% selenium.
2. A lead acid battery grid as claimed in claim 1 in which the alloy contains greater than 0.1 weight% to less than 0.2weight% of Arsenic
3. A lead acid battery grid as claimed in claim 1 in which the alloy contains not greater than 0.015 weight% of Bismuth
4. A lead acid battery grid as claimed in claim 1 in which the alloy contains not greater than 0.03 weight% of Silver
5. A lead acid battery grid as claimed in claim 1 in which the alloy contains not greater than 0.0001 weight% of Manganese
6. A lead acid battery grid as claimed in claim 1 in which the alloy contains not greater than 0.0005 weight% of Calcium
7. A lead acid battery grid as claimed in claim 1 in which the alloy contains not greater than 0.0025 weight% of Copper
8. A lead acid battery grid as claimed in claim 1 in which the alloy contains not greater than 0.0005 weight% of Nickel
9. A lead acid battery grid as claimed in claim 1 in which the alloy contains not greater than 0.0010 weight% of Iron
10. A lead acid battery grid as claimed in claim 1 in which the alloy contains not greater than 0.0010 weight% of any other elements that is not mentioned from claim 1 to claim 9 except lead.
11. A lead acid battery made with the said Pb-Sb-Se alloy grid as per claim 1 exhibiting equivalent water loss & Shelf life and better cyclic life than that of the batteries made with Pb-Ca-Sn alloy.
12. A lead acid battery made with the said Pb-Sb-Se alloy grid as per claim 12 wherein said battery is a valve regulate