FIELD OF THE INVENTION:
The present invention relates to tubular 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 batteries are one of promising energy storage technologies for so many years in various applications. Even though other technologies such as Lithium ion batteries, Nickel metal hydrides etc. are comparatively superior in terms of performance, the preference has been given to lead acid batteries because of their cost and robustness. But recent advances in new storage technologies clearly indicate that there is a threat for lead acid batteries. Hence, there is a need for improvement in the performance of lead acid batteries.
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. Even though the tubular batteries are superior in terms of cyclic life compared to flooded type lead acid batteries, it does not match with its rival lithium ion battery. 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 tubular lead acid battery. This is achieved by introducing a high surface area and conductive carbon black of the concentration of about 0.2-0.25% 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 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 charge acceptance and 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.
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: 7.5-8% of sulfuric acid (specific gravity at 25°c of 1.395-1.405), 0.14-0.15% of Modacrylic fiber, 0.2-0.25% of high surface area and conductive carbon black, 0.8-0.9% of barium sulphate, 0.29-0.30% of lignin, 10.5-11.5% 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 4.45 to 4.55 g/cm3.
Then lead-antimony alloy based grids are pasted with the above said paste and cured by conventional method. The plates are assembled along with conventional tubular positive plates to make 12V 150Ah tubular 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 perlS 13369.

The battery is charged continuously at a current 1 = 0.1 XC10 amperes for a period of 2000Hrs as given below:
(a) 2- Cycles of 300h charging followed by test discharges
(b) 3- Cycles of 200h charging followed by test discharges
(c) 4-Cycles of lOOh charging followed by test discharges
At the end of each period of charging (a), (b) and (c), the battery is subjected to test discharge (CIO) to an end voltage of 1.85V. The battery must provide not less than 9h of discharge to pass the requirement.
The batteries prepared with above said process performed better than control 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.

WE CLAIM
1. A negative paste formula composition by weight percentage per weight of lead oxide comprisies
of:
7.5-8% of sulfuric acid having specific gravity at 25°c of 1.395-1.405,
- 0.14-0.15% of Modacrylic fiber,
0.2-0.25%) of high surface area and conductive carbon black, 0.8-0.9%) of barium sulphate,
- 0.29-0.30% of lignin,
10.5-11.5%) of demineralised water,
25-30ml of one of a polymeric binder such as PVDF/PVA diluted in 21it of DM water.
2. A negative paste formula composition as claimed in claim 1, wherein the percentage of unoxidized lead in 25-30%)
3. A negative paste formula composition 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 formula composition 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 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 with paste peak temperature
maintained 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 4.45 to 4.55 g/cm3.
5. A tubular lead acid battery made with negative electrode consisting of paste composition as claimed in claim 1 and the said paste prepared as per claim 4