FIELD OF THE INVENTION:
The present invention relates to the lead acid batteries used in inverter application and solar application, and more particularly to composition of the paste used to make negativeelectrode in these batteries.BACKGROUND OF THE RELATED ART:Lead-acid battery is the only predominant back up energy source used in Inverter and Solar applications across the globe. It is also being used in Energy storage applications. In spite of widespread use the battery is not providing its intended life due to negative plate sulphation. This is mainly due to improper charging algorithms and inherent tendency of growth of PbSO4 crystal in negative plate under deficit charging (charge input < charge output) regime.Traditionally, the negative active material paste is prepared by mixing lead oxide, binder,expander (combination of carbon black, barium sulphate & vanisperse), sulfuric acid and dematerialized water under controlled temperature with cooling facility. The prepared paste is then pasted onto the grids made of lead alloy and then cured & dried under controlled humidity and temperature.US 6,733,547 discloses a paste composition for the negative plate, and a plate making process for a lead acid battery, which reduces shrinkage of the negative active mass during battery cycling. This document provides that a negative paste is produced by mixing an oxidized lead powder and Sulfuric acid with an expander, a polymer and optionally carbon black to produce a paste comprising tribasic lead Sulfate crystals, the expander and the polymer in which the reduction of Pb2+ to Pb during battery formation and cycling is facilitated by the polymer absorbed on the inactive lead Sulfate crystals. After mixing the negative paste composition, the resulting paste is then pasted onto a grid where the paste is dried and cured to form a negative battery plate of the lead acid battery.US 4,323,470 discloses lead-acid type storage battery paste which will provide light weight negative active material with improved electrode performance, i.e. increased material utilization efficiency over a wide range of temperature and discharge rates, and cycle life. Inclusion of such extender in the paste will also increase the mechanical strength and, in turn, the handling characteristics of the electrodes. EXAMPLE 1 of the said document discloses the method of 2
preparation of a typical lead-acid negative paste for an SLI automotive battery was prepared with a major amount of leady litharge, comprising lead oxide and approximately 30 weight % free lead. The litharge was mixed with minor additions of cellulose fiber, barium sulfate, lignosulfonate expander and channel black, in a total amount of 1.25 weight %, all of which are well known battery paste additives. In this standard paste, there was added 4.33 weight %, based on the weight of the litharge, of the above identified milled glass fibers. Into this mixture were blended distilled water and sulfuric acid in typical amounts and the resulting paste, with an apparent density of 65 to 68 grams per cubic inch (4.0 to 4.2 gm/cc) and a penetrometer reading of 34 to 38, was used to prepare pasted plates. The pasted plates were cured and formed in a conventional manner to produce the final negative sponge lead matrix. Several studies have been done to overcome the sulphation of the negative plate in lead acid batteries by means of active material additives. Very few are successful and the negative plate failure is still a challenging one for lead acid battery industry. This patent discusses how an appropriate negative paste will minimize growth of PbSO4 and in turn increases life of lead-acid batterySUMMARY OF THE INVENTIONAs said in background of the art, an objective of the present invention is to improve thecycle life of the lead acid battery used in inverter and solar application. This is achieved by introducing a special multi walled carbon nanotube material along with regular additives such as acrylic/polyester fiber& expander. The dosage of the said carbon derivative is around 0.15 to 0.25%. The other objective of this invention is to optimize the paste mixing recipe using the above said carbon to get the optimum performance.DETAILED DESCRIPTIONA negative paste formula consisting raw materials by weight percentage per weight of lead oxide used: 7-8 % of sulfuric acid (specific gravity at 25°c of 1.395-1.405), 10-11% of demineralized water, 0.09-0.12%% of acrylic/polyester fibre, 1.1-1.5% of expander (Barium sulphate, carbon black & Vanisperse). In this patent in addition to expander a special multi walled carbon nanotube material is also added to oxide during paste mixing operation at very low level in 3
the range of 0.15% to 0.25%. Two sets of paste made one without special carbon additive and the other with special carbon additive. Batteries were made and subjected to green plate formation. The special multi walled carbon nanotube material reduces the recharging times having good charge acceptance and mitigate the deficit charging specifically in traction applications and exhibits good charge acceptance, and obtain 100% power output with less than 110% power input during deficit charging situation. This special multi walled carbon nanotube material prevents sulphation in negative plates in large extend and increases the life 20% higher than the conventional carbon additive due its high conductivity and reduces the current loss across the grid wires. It also prevents the negative paste shrinkage during its service life since the current utilization across the grid holding the paste will be higher than the conventional carbon additiveand delays the failures such as water loss, sulphation and Active material loss as compared with conventional carbon additive, this extends capacity retention more than 70% and as per Decentralized Rural Electrification (DRE) Test. The special multi walled carbon nanotube material exhibits phenomenal recharge efficiency at higher charge input without high water loss, as this is not possible with conventional carbon additives The formed batteries are subjected to various tests which are classified as short term performance test, charge acceptance test and life cycle tests. Short term performance tests are Capacity test at various rates from C-10 to C-1. Charge acceptance test is the time taken for a battery to reach 115% of discharge output under Constant voltage charge condition. Life cycle tests depend on application. In this study, the batteries are subjected to DRE (Decentralized Rural Electrification) test, which is one of the important tests for solar application.The following are the test results for the present invention DRE Test (Decentralised Rural Electrification) The Objective of this test is to accelerate the sulphation & stratification degradation mechanism in very few cycles and rehabilitate the battery also in few cycles using CNT as additive. The test is conducted for a tall tubular inverter battery having the capacity of 180Ah at 20 Hr rate with the test condition as follows: Stratification is established by applying 5 charge/discharge cycles with a recharge limited by a high voltage threshold (2.35 Vpc). Such a threshold voltage is insufficient for fully recharging the batteries. These cycles are followed by 5 cycles with a recharge coefficient of 1.2 C10. The applied overcharge thus enables the battery to recover from the sulphation induced during the 5 first cycles with low recharge. The development of the restored capacities allows an evaluation of the ability of the battery to recover its initial state. The series of 10 cycles is followed by a capacity 4
measurement and form a test sequence. Several test sequences are repeated until an "end of test" criterion is reached. A test sequence lasts about 12 days. The total duration ranges from 3 to 16 test sequences therefore from 1 to 7 months. The battery bath temperature is 25 °C. Test program comprises of the following steps: 1. Fully Charge the battery.2. Discharge the battery with 15A till 10.50V.3. Rest for 3hrs. 4. Recharge with 15A till 14.10V. 5. Rest for 3hrs. 6. Discharge the battery with 15A till 10.50V. 7. Rest for 3hrs. 8. Repeat 4 to 7 for 5 times. After completion of 8th step restart the cycling as follow 9. Recharge 15A for 12hrs.10. Rest for 3hrs. 11. Discharge the battery with 15A till 10.50V. 12. Rest for 3hrs. 13. Repeat 9 to 12 for 5 timesThe test results are provided hereinbelow: 5
The cycling test as per DRE corresponds to the 1 year of life in approximate 100 cycles of performance. More than 600 cycles of performance observed which is equivalent to the battery life 6+ years.The study proves the special carbon additive is not impacting Capacity tests (C-10 to C-1). It has reduced time taken to reach 115% of discharge output during charging by 25% to 30% and improves life of battery in DRE condition by 30%. After failure, it was observed that in control battery in negative plate PbSO4 has grown while in the battery with the additive there is no growth in PbSO4. 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 composition for negative electrode of lead acid battery, comprising:- 25 to 30 wt% lead; - 4-6 wt% sulfuric acid; - 10-11% of demineralised water; - 0.09-0.12 wt% of acrylic/polyester fibre; - 1.1-1.5 wt% of expander and - a multi walled carbon nano tube material in the range of 0.15% to 25%.
2.The composition as claimed in claim 1, wherein the lead is predominantly in the form of lead oxide.
3.The composition as claimed in claim 1, wherein the sulfuric acid has a specific gravity at 25 C of 1.395-1.405.
4.The composition as claimed in claim 1 wherein the expander consists of barium sulphate, carbon black and Vanisperse.
5.The composition as claimed in claim 1 wherein the unoxidized lead is in the range of 25-30%.