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 interms 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.
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 and one of an aqueous polymeric binders such as polyvinylidene difluoride (PVDF), Polyvinyl acetate (PVA) etc in the range of 25-30ml diluted in 21it of DM water. The surface area of the said carbon black is around 1000-1050m /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 and binder 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), 0.14-0.15% of Modacrylic fiber, 0.15-0.2% of high surface area and conductive carbon black, 0.8-0.9%) of barium sulphate, 0.29-0.30%) of lignin, 10-11% of demineralised water. One of aqueous polymeric binders from PVDF/PVA in the range of 25-30ml diluted in 21it of DM water is also added at the end of the process. 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. The above said polymeric binder is added and mixed for 2-3min. 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 to make 12V-42Ah battery. 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 as per JIS 8702 as given below.

(a) Discharge the battery at 3.4x120 A for 3Hrs
(b) Charge at 14.1V for for 9Hrs(30% current limit)
(c) Continue (a) &(b) for 50 cycles which is considered as lUnit
After each unit, test discharge the battery for its C20 capacity and it shall be greater than or equal to 60%o of initial capacity. Repeat the above cycling until the capacity reaches less than 60%>. The requirement is to pass 200 cycles i.e. 4 units
The batteries prepared with above said process performed better than conventional batteries and there is improvement of about 12-15%> in cyclic life by avoiding negative active material degradation through the improved cohesion strength.
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.

CLAIM
1. A negative paste composition for valve regulated lead acid battery by weight percentage per
weight of lead oxide comprises of:
8.5-9% of sulfuric acid having specific gravity at 25°c of 1.395-1.405,
- 0.14-0.15% of Modacrylic fiber,
0.15-0.2%) of high surface area and conductive carbon black, 0.8-0.9%) of barium sulphate,
- 0.29-0.30% of lignin,
10-ll%o of demineralised water and
25-30ml of one of a polymeric binder such as PVDF/PVA diluted in 21it of DM water.
2. A negative paste composition for valve regulated lead acid battery as claimed in claim 1, wherein the percentage of unoxidized lead in 25-30%>
3. A negative paste composition for valve regulated lead acid battery as claimed in claim 1, wherein the surface area of the carbon black is 1000-1050m /g and particle size is 10-15nm.
4. The method of preparation of a negative paste composition for valve regulated lead acid battery as claimed in claim 1, wherein the method comprises the steps of
mixing the dry materials such as lead oxide, modacrylic fiber, carbon black, barium sulphate
and lignin in an automatic mixer for 3-5minutes,
adding deminerlized water into the mixer and wet mixing for 2-3 minutes;

adding Sulfuric acid is added to the mixer slowly for a period of 17-18 minutes and with
cooling system or exhaust switched on after 50°C;
mixing with exhaust and cooling system for about 5-7minutes and maintaining the paste peak
temperature at 62-64°C;
adding the polymeric binder and mixing for 2-3min;
measuring the apparent density of the paste and adjusting the same to 4.50 to 4.55 g/cm3.
5. A valve regulated lead acid battery made with negative electrode consisting of paste composition as claimed in claim 1 and the preparation thereof as per claim 4.