FIELD OF THE INVENTION:
The present invention relates to flooded flat plate type lead acid battery used in automotive applications, 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 being used throughout the world in various types of applications such as automotive, industrial, power back up etc. The recent advancements in automotive segment are forcing severe challenges to lead acid batteries from its rival storage technologies like lithium ion batteries. While lead acid batteries are highly recommended because of their cost benefit, their usage in advanced technologies has certain limitations under partial state of charge conditions and colder climates. The important parameters under consideration for automotive batteries are charge acceptance, cold cranking and cyclic life. The main failure modes of lead acid batteries in these conditions are negative plate sulfation and positive grid corrosion. While these parameters affect the cyclic life of the battery and charge acceptance, the negative active material composition greatly affect the cold cranking behavior and also initial charge acceptance.
Traditionally, the electrodes are prepared by four steps namely grid casting, paste mixing, pasting and curing. The precursors for negative active material called as paste are manufactured by mixing lead oxide, binder, carbon black, barium sulfate, vanisperse, sulfuric acid and demineralized water. The paste is then pasted onto the grids and cured & dried under controlled humidity and temperature. Then positive and negative plates are sandwiched with separator in between them and assembled. The electrolyte i.e. sulfuric acid is filled and initial charging i.e.

formation is done which converts cured material into electrochemically active materials lead dioxide in positive and spongy lead in negative.
US 5871862 discloses a battery paste composition and a lead-acid electrochemical cell which incorporates the composition. The composition paste include the basic ingredients, namely, (1) at least one particulate lead-containing composition; and (2) a paste vehicle. Many different materials may be employed in connection with these two ingredients, with the present invention not being restricted to any particular lead-containing compositions and vehicles. For example, the particulate lead-containing composition may involve the following materials: finely-divided elemental Pb, PbO ("litharge" or "massicot"), Pb3 04 ("red lead"), PbS04 ("lead sulfate" with the designation "PbS04 " being defined to also include its associated hydrates), and mixtures thereof. Water is the representative and preferred composition that is used as the paste vehicle. The composition may also include a number of additional (optional) ingredients. For example, H2 S04 ("sulfuric acid") therein.
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,

Several studies have been done to overcome the failure modes of lead acid batteries by means of active material additives, alternate grid materials, electrolyte additives. Very few are successful and the negative plate failure is still a challenging one for lead acid battery industry. Electrode preparation particularly negative electrode plays a vital role in the performance of the battery. The composition of the active material is one of the important factors to be considered for optimum battery performance. Thus there is a need for a novel composition for the negative active material which helps in improving performance of the battery especially charge acceptance and cyclic life without affecting cold cranking behavior.
SUMMARY OF THE INVENTION
As said in background of the art, an objective of the present invention is to improve the charge acceptance and cyclic life of the flooded plate lead acid battery used in automotive application. 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 1400-1500m2/g .
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 cold cranking behavior of the battery. Other embodiment of the present invention is to derive the paste mixing process with above said recipe to maintain the paste density and porosity.
DETAILED DESCRIPTION
One embodiment of the present invention provides composition of a paste used for making negative electrode of flooded plate type lead acid battery. A negative electrode paste formula consists of the following raw materials by weight percentage per weight of lead oxide

used: 8.2-8.5% of sulfuric acid (specific gravity at 25°c of 1.395-1.405), 0.195-0.205% of Modacrylic fiber, 0.15-0.2%) of high surface area and conductive carbon black, 0.55-0.65%) of barium sulphate, 0.25-0.35%) of lignin, 11.5-12.5%) 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 1400-1500m2/g.
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 60-64°c. Then the apparent density of the paste is measured and adjusted to 4.40 to 4.45 g/cm3.
The above paste is pasted on to lead-calcium alloy based grids are and cured by conventional 3BS low temperature curing method. The plates are assembled along with conventional positive plates. Diluted Sulfuric acid is filled and conventional formation is done. The control batteries are made with conventional carbon black and paste recipe. Then both of these batteries are subjected to electrical testing such as charge acceptance, cold cranking and Heavy load endurance cycling test.

The batteries prepared with above said process performed better than conventional batteries and there is improvement of about 10% in charge acceptance and cyclic life. There is no impact on cold cranking performance of the battery.
When the batteries are constantly under -charged or deep discharged 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 larger voids sizes may there by form and resulting in negative paste shrinkage, 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 black 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 shrinkages during cycling. It is to be noted that HSA carbon black of the following specification should be used in preparation.
Technical specification of High Surface Area Carbon black

High Surface Carbon Additives (HSCA) optimize NAM microstructure 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%o increased cycle life , comparing to the common one. High surface Carbon additive (HSCA) acts as porous-skeleton builder, electrolyte supplier and capacitive buffer, reduces acid stratification in a cells of battery. This gives consistent performance for longer life. Higher Surface area Carbon additive facilitates Pb nucleation and prevents lead sulfate (PbS04)

crystallization. The new negative paste formula with High Surface Carbon black 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 shrinkages during cycling. 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. It is to be noted that in order to get optimal performance, the recommended loading of High surface carbon additive shall be between 0.15% to 0.25% per 1000 kg of lead oxide mixture.
Due to its mesoporosity and hydrophilic in nature, the paste porosity is stable and consistently delivers good charge acceptance in dynamic situation. Because of good pore integrity between active materials ensures longer cycle life of about 15%-20% from normal activated carbon block.
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 flooded flat plate lead acid batteries comprises of
raw materials by weight percentage per weight of lead oxide used:
8.2-8.5% of sulfuric acid with specific gravity at 25°c of 1.395-1.405, 0.195-
0.205% of Mod acrylic fiber,
0.15-0.2%) of high surface area and conductive carbon black,
0.55-0.65%) of barium sulphate,
0.25-0.35% of lignin, 11.5-12.5% of demineralised water.
2. A negative electrode paste formula for flooded flat plate lead acid batteries as claimed in claim 1, wherein the percentage of unoxidized lead is 25-30%>
3. A negative electrode paste formula for flooded flat plate lead acid batteries as claimed in claim 1, wherein the surface area of the carbon black is 1400-1500m2/g
4. A method of preparation of A negative electrode paste formula for flooded flat plate lead acid batteries as claimed in claim 1, the method comprises the steps of
a. mixing in an automatic mixer dry materials such as lead oxide, mod acrylic fiber,
carbon black, barium sulphate and lignin for 3-5minutes;
b. adding de-mineralized water into the mixer and wet mixing is done for 2-3
minutes;

c. adding sulfuric acid the mixer slowly for a period of 17-18 minutes with cooling
system or exhaust switched on after 50°C.
d. maintaining the paste peak temperature at 60-64°C for final mixing done with
exhaust and cooling system for about 5-7minutes; and
e. measuring the apparent density of the paste and adjusting it to 4.40 to 4.45 g/cm3.
5. A negative electrode paste formula for flooded flat plate lead acid batteries as claimed in claim 1 used in the preparation of a lead acid battery.