FIELD OF THE INVENTION:
The present invention relates to valve regulated lead acid battery and more particularly to composition of the paste used to make negative electrode in these batteries.
BACKGROUND OF THE RELATED ART:
Lead Acid battery has been used for more than 100 years since its invention. Even though other technologies such as Lithium ion batteries, Nickel metal hydride batteries perform well in terms of cyclic life with much higher energy densities, lead acid batteries are still used in many energy storage applications. The place of lead acid batteries especially in telecom and uninterruptable power supply (UPS) is very strong because of their cost and robustness.
The valve regulated lead acid batteries (VRLA) are special type of lead acid batteries in which the electrolyte is absorbed in Glassmat and it is completely maintenance free. These types of lead acid batteries are being extensively used in such type of applications where there is no feasibility for continuous maintenance.
A typical lead-acid battery manufacturing process includes grid casting, paste mixing, pasting, curing, drying, assembly and formation steps. The precursors for positive and negative active material called as paste are manufactured by mixing lead oxide with additives and sulfuric acid. Then these pastes are coated on the positive and negative electrode grids. The prepared so called plates are cured and dried at controlled temperature and humidity. The batteries are assembled with separator placed in between positive and negative plates. Then electrolyte i.e. sulfuric acid, is filled inside the battery and it undergoes a formation step in which a first time charge is applied in order to convert this cured positive

& negative paste into electrochemically active porous materials of lead dioxide and spongy lead respectively.
US 6733547 provides a composition and manufacturing process for lead acid batteries that produces negative plates with low shrinkage of the active mass. The plates have better performance and longer cycling life than plates produced by current technology. Battery discharge and recharge ability is improved and the battery self-discharge rate is decreased. To this end, and in accordance with the present invention, the negative active material composition includes a polymer addition. Due to the non-conductivity of the polymer, an amount of carbon black is optionally added to increase the conductivity of the negative active mass, if desirable or necessary. The polymer and optional carbon black may be mixed with the expander, leadoxide, sulfuric acid and water in any of a variety of manners,
Main disadvantages of lead acid battery compared to other technologies are higher charging time, lower active material utilization and lower cyclic life. There are so many efforts made in the past to overcome these drawbacks. But practically, very few are succeeded in this area due to lacking in production feasibility, additional cost incurred and adverse side effects on performance. Thus there is need for a specialized composition for paste recipe which can help in improving performance of the battery especially cyclic life without any adverse side effects.
SUMMARY OF THE INVENTION
As said in background of the art, an objective of the present invention is to improve the cyclic life of the valve regulated lead acid battery. This is achieved by introducing a high surface area and conductive carbon black of the concentration of about 0.15-0.2% in negative paste composition. The surface area of the said carbon black is around 1000-1050m2/g and particle size of about 10-15nm. The dispersability of the said carbon is superior to that of conventional carbon black.

The objective of this invention is to optimize the paste mixing recipe using the above said carbon in such a way that there is no impact on paste density and active material porosity which improves the cyclic life. The other objective is to derive the paste mixing process with above said recipe. The said battery prepared from this paste does not have any other side effects such as water loss.
DETAILED DESCRIPTION
One embodiment of the present invention provides composition of a paste used for making negative electrode of lead acid battery. The negative electrode paste formula consists of the following raw materials by weight percentage per weight of lead oxide used: 8.5-9% of sulfuric acid (specific gravity at 25°c of 1.395-1.405), 0.095-0.105% of Modacrylic fiber, 0.15-0.2% of high surface area and conductive carbon black, 1.1-1.2% of barium sulphate, 0.14-0.15%) of lignin, 10.5-11.5%) of demineralised water. The unoxidized free lead content of lead oxide powder is in the range of 25-30%. The surface area of the above said carbon black ranges from 1000-1050m2/g and particle size ranges from 10-15nm.
Another embodiment of the present invention provides the preparation of the negative active material paste consisting above ingredients. The dry materials such as lead oxide, modacrylic fiber, carbon black, barium sulphate and vanisperse are mixed in an automatic mixer for 3-5minutes, deminerlized water is added into the mixer and wet mixing is done for 2-3 minutes. Sulfuric acid is added to the mixer slowly for a period of 17-18minutes with cooling system or exhaust switched on after 50°c. Final mixing is done with exhaust and cooling system for about 5-7minutes and the paste peak temperature is maintained at 62-64°c. Then the cube density of the paste is measured and adjusted to 4.50 to 4.55 g/cm3.
Then lead-calcium alloy based grids are pasted with the above said paste and cured by conventional method. The plates are assembled along with conventional flat plate positive plates. The

control batteries are made with conventional carbon black and paste recipe. Then both of these batteries are subjected to electrical testing such as capacity and cycling test.
The batteries prepared with above said process performed better than conventional batteries and there is improvement of about 10% in cyclic life by avoiding negative active material degradation.
It is to be noted that when the batteries are constantly High rate partial state of charge (HRPSoC) in application, a portion of discharge product Lead Sulfate (PbSo4) become crystalize and cannot be reversible at negative plate, the crystal size increases further during cycling and thus high dense lead sulfate may there by form and resulting in negative paste expansion, this will lead to degradation of the structure of the negative active mass, eventually the battery will lose its capacity to a point where it is no longer useful as a result of shrinkage.
The new negative paste formula with High Surface Carbon additive controls the crystal growth due to it BET value more than 1000 increases the negative active material (NAM) surface area and optimizes its microstructure enhance its electrode reaction kinetics, and thus reduces negative paste expansion during HRPSoC charging. It is to be noted that the HSA carbon black of the folloinwg specitiion should be used in the preparation
Technical speciation of HSA Carbon black

S.No Parameters Units Requirement
1 Moisture 0/
/o 2 max
2 Fixed Carbon 0/
/o >90
3 Ash Content 0/
/o <5
4 pH (5% Solution) pH 7-8
5 BET Surface Area m2/g 1300-1550
6 Oil absorption ml/lOOg 140-200
7 Acetone / Benzene Extract Visual Colorless

8 Retention in 63 micron mesh 0/
/o 0.1
9 Appearance Visual Uniform black
10 Wettability Visual Good & Hydrophilic
11 Dispersion Visual Immediate and no settlement
Impurities (MAL)
1 Iron (Fe) 0/
/o 0.0050max
2 Manganese (Mn) 0/
/o 0.0005max
3 Chloride (cl) 0/
/o O.OlOOmax
Due to its mesoporosity and hydrophilic in nature, the paste porosity is stable and consistent delivers good charge acceptance in dynamic situation. The good pore integrity between active materials ensures longer cycle life of about 15%-20% from normal activated carbon block. High Surface Carbon Additives (HSCA) optimize NAM micro structure and enhance electrode reaction kinetics reduces the sulfate crystal growth enhance its service life. Negative paste made with high surface carbon additive (HSCA) battery exhibits 20% improved charge acceptance and 10% -15% increased cycle life , comparing to the common one. Higher Surface area Carbon additive facilitates Pb nucleation and prevents lead sulfate (PbS04) crystallization. The new negative paste formula with High Surface Carbon additive (HSCA) controls the crystal growth increases the negative active material (NAM) surface area and optimizes its microstructure enhance its electrode reaction kinetics, and thus reduces negative paste expansion during partial state of charge operation. Further, due to its mesoporosity and hydrophilic in nature, the paste porosity is stable and consistent delivers good charge acceptance in dynamic situation. High Surface Carbon additive negative paste aiding depolarization of negative plate

enhances its charge acceptance at partially state of charge conditions. In order to achieve the optimal performance, the recommended loading of High surface carbon additive shall be between 0.15% to 0.20% per 1000 kg of lead oxide mixture.
It may be noted that the present invention is not limited to the above embodiments and the invention could be exemplified in many other ways obvious to a person skilled in the art.

WE CLAIM
1. A negative electrode paste formula for valve regulated lead acid battery comprises of raw
materials by weight percentage per weight of lead oxide used:
8.5-9% of sulfuric acid with specific gravity at 25°C of 1.395-1.405,
0.095-0.105% of Modacrylic fiber,
0.15-0.2%) of high surface area and conductive carbon black,
1.1-1.2% of barium sulphate,
0.14-0.15% of lignin,
10.5-11.5% of de-mineralized water.
2. A negative electrode paste formula for valve regulated lead acid battery as claimed in claim 1, wherein the percentage of un-oxidized lead in 25-30%>
3. A negative electrode paste formula for valve regulated lead acid battery as claimed in claim 1, wherein the surface area of the carbon black is 1000-1050m2/g and particle size is 10-15nm.
4. A method of preparation of a negative electrode paste formula for valve regulated lead acid battery as claimed in claim 1, the method comprises the steps of
mixing in an automatic mixer dry materials such as lead oxide, modacrylic fiber, carbon
black, barium sulphate and lignin for 3-5minutes;
adding deminerlized water into the mixer and wet mixing is done for 2-3 minutes;

adding Sulfuric acid in the mixer slowly for a period of 17-18 minutes with cooling system or
exhaust switched on after 50°C;
finally doing mixing with exhaust and cooling system for about 5-7minutes maintaining the
paste peak temperature at 62-64°c;
measuring the apparent density of the paste and adjusting it to 4.50 to 4.55 g/cm3.
5. A negative electrode paste formula for valve regulated lead acid battery as claimed in claim 1 used in the preparation of valve regulated lead acid battery.