Lead storage batteries

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1. LEAD

Lead was probably one of the first metals to be produced by man, being known since 3500 B.C., in agreement with archaeological discoveries done in Egypt. The oldest lead piece is in the British Museum and dates from 3800 A.D..

1.1 WHAT IS LEAD?

Lead is a metal. Its symbol is Pb and atomic number is 82. It is soft, malleable poor metal and also considered to be one of the heavy metals.

1.2History

Lead was probably one of the first metals to be produced by man, being known since 3500 B.C., in agreement with archaeological discoveries done in Egypt. The oldest lead piece is in the British Museum and dates from 3800 A.D..

The manner in which prehistoric people extracted lead from its minerals is not well-known. However, there are vestiges of very rudimentary furnaces, done of stone, where these people heated up the lead minerals with bonfires (that burned wood and coal) to extract the element. There is also evidence that the Chinese already produced metallic lead about 3000 B.C., and that Phoenicians had explorations close to deposits in Spain, in 2000 A.C.. In the 5th century B.C. the Romans made an extensive exploration of lead deposits in the whole Iberian Peninsula.

In the period 700 A.D. to 1000 A.D. the German mines of lead and silver, in the Rhine valley and in the Hartz mountains, were very important, just as those of Saxony, Silesia and Bohemia in the 13th century.

In the 17th century, the lead foundries flourished in Great-Britain, specially those located in Wales and Derbyshire.[1]

1.2 Characteristic

Lead has a bluish white colour when freshly cut but tarnishes to a dull greyish colour when exposed to air .It has a shiny chrome silver lustre when melted into a liquid.

1.3 OccurrenceLead is not very abundant, its relative rates being smaller than those of other metals as the aluminium, iron, magnesium, titanium, nickel, etc.. However, it is more abundant than cobalt, tin, cadmium or gold.

The more important lead minerals are galena (PbS), anglesite (PbSO4) and cerussite (PbCO3), respectively with 86%, 68% and 77% of lead. Other minerals that contain lead are linarite, pyromorphite, mimetite, vanadinite, crocoisoite and wulfenite.

The main deposits of lead minerals are located in the USA, Peru, Argentina, Bolivia, Australia, Zambia, South Africa, Germany, Spain, Sweden, Italy and Serbia.[1]

1.4 Uses

  1. It is used in building construction, lead acid batteries, bullets and shots, weights, and is part of solder, pewter, fusible alloys and radiation shields.
  2. It is used as a colouring element in ceramic glazes notably in the colours red and yellow.
  3. It is frequently used in polyvinyl chloride (PVC) plastic, which coats electrical cords.
  4. Lead is used as electrode in the process of electrolysis

1.5 Biological Action

Generally, lead compounds are noxious for the animals. The effect of the absorption of the element in plants does not seem serious. However, this accumulate lead will be absorbed by the animals in case of ingestion. That is why lead compounds are not used in pesticides or insecticides.

Lead and its sulphide are incapable of absorption, and are considered practically innocuous. However, the soluble salts, such as the chloride, the nitrate, the acetate, etc. are very poisonous. The main intoxication cause with lead is the exposure to vapors and dusts of its compounds. The intoxication symptoms are intestinal mal-function, strong abdominal pains, diarrhea, appetite loss, nausea, vomiting and cramps

1.6 Properties of Lead:

¯Density of Lead =11.3437gm/cm3

¯ Melting Point=327.35OC

¯Boiling Point=1515oC

¯Bulk Modulus =0.44×106Mbar ¯ Heat conductivity=0.081cal/cm-s-OC

¯Specific Heat =0.03046 cal/g-k

¯ Heat of fusion =6.26cal/gm

¯Electrical resistivity =20.648µΩ-cm

¯Tensile strength =2000psi

¯Young's Modulus = 2.56×106psi

¯Crystalline Form is Face-centred cubic,

¯Electronic configuration = 6s26p2

With Lattice constant 0.4939nm

¯Lead exhibit the oxidation state of +2 corresponding to loss of the two p electrons, in most of its common compounds. In this oxidation Lead is generally Basic the oxidation state of +4 also occurs and in this oxidation state Lead is more acidic.

1.7 Forms of Lead

Forms of Lead exist in both organic and inorganic forms.

Inorganic lead

The lead found in old paint, soil, and various products described below is inorganic lead. Leaded gasoline exhaust contributed to ambient inorganic lead contamination. For this reason, the focus of this document is on inorganic lead.

Organic Lead

Leaded gasoline contained organic lead before it was burned; however, since the elimination of lead from gasoline in the U.S. starting in 1976, exposure to organic lead is generally limited to an occupational context. However, organic lead can be more toxic than inorganic lead because the body more readily absorbs it. Potential exposures to organic lead should be taken very seriously. [2a]

2. BATTERY

2.1 Introduction

Much energy is stored by nature in chemical compounds (combinations of elements). Coal, wood, and oil have tremendous stores of energy which are released as heat when these compounds are burned. Oxygen and hydrogen have so much energy that they explode when they combine. The electrician is interested in these - substances only because he can get some of this energy as emf. [3]

Releasing electrical energy from chemical energy is surprisingly simple. If two dissimilar metals i.e. copper and zinc, for example - are placed in certain chemical solutions, an emf results. This is the principle employed in all cells and batteries - a battery is simply two or more cells connected together.

2.2 History

The name "battery" was coined by Benjamin Franklin for an arrangement of multiple Leyden jars (an early type of capacitor) after a battery of cannon. Strictly, a battery is a collection of two or more cells, but in popular usage battery often refers to a single electrical cell.

An early form of electrochemical battery called the Baghdad battery may have been used in antiquity. However, the modern development of batteries started with the Voltaic pile, invented by the Italian physicist Alessandro Volta in 1800. [4]

2.3 Definition

A battery is a device for converting chemical energy into electrical energy. Batteries can consist of a single voltaic cell or a series of voltaic cells joined to each other. (In a voltaic cell, electrical energy is produced as the result of a chemical reaction between two different metals immersed in a solution, usually a liquid.) Batteries can be found everywhere in the world around us, from the giant batteries that provide electrical energy in spacecraft to the miniature batteries that power radios and penlights.

The correct use of the term battery is reserved for groups of two or more voltaic cells. The lead storage battery found in automobiles, for example, contains six voltaic cells. However, in common usage, a single cell is often referred to as a battery. For example, the common dry cell battery found in flashlights is really a single voltaic cell.

A rechargeable battery (Storage battery) is a group of one or more secondary cells. Rechargeable batteries use electrochemical reactions that are electrically reversible.

A group of reversible of rechargeable secondary cells acting as a unit is called secondary battery. Storage battery:-A voltaic battery that stores electric charge.

2.4 Types of batteries:

Batteries are divided into two general groups:

Primary batteries:

A primary battery is one designed to be used just once. When the battery has run down (produced all the energy it can), it is discarded.

Secondary batteries (or cells):

Secondary batteries, on the other hand, can be recharged and reused. Primary batteries are designed to be used until the voltage is too low to operate a given device and then discarded. Secondary batteries have many special design features, as well as particular materials for the electrodes that permit them to be reconstituted (recycled). After partial or complete discharge, they can be recharged by DC voltage and current to their original state. While this original state is usually not restored completely, the loss per cycle in commercial batteries is only a small fraction of 1 percent even under varied conditions.

2.5 Uses of Rechargeable Batteries:

They are used for applications such as:

¯Automobile starters

¯Portable consumer devices

¯Light vehicles

¯Uninterruptible power Supplies

¯Emerging Application in hybrid electric vehicles and electric vehicles are driving the technology to improve cost, reduce weight, and increase lifetime

3. LEAD STORAGE BATTERY

3.1 Discovery of Lead Storage Batteries:

Gaston Plante (1834-1889) discovered the lead -acid battery

Observation of Gaston Plante:

When he had allowed the current to pass for some time and then disconnected the battery, the Lead plates acted like a battery on their own.

The plate that had been connected to the positive pole of the battery is called Anode and was at a higher potential than the plate that had been connected to the negative pole of the battery, called Cathode. Now current flowed in the opposite direction. The Lead plate observed to collect a layer of a white substance, Lead sulphate, as this happened. When the current finally stopped the cycle could be repeated by reconnecting the power source. [7]

3.2 Working of lead storage battery

Inside a lead storage battery is a series of plates. Half of the plates are made of lead dioxide and the other half are made of a spongy form of lead. The plates are bathed in a solution of sulphuric acid which serves as an electrolyte (a chemical solution that conducts electricity). Two posts extend to the outside of the battery through sealed openings in the battery wall. One of the posts---the negative post---is connected to the lead plates, and the other---the positive post---to the lead dioxide plates.

Sulphuric acid, chemically, is composed of two hydrogen atoms, a sulphur atom and four oxygen atoms. Inside the battery, the sulphuric acid molecules are in solution with water and so are dissociated. This means that the sulphur atom with the four oxygen atoms attached to it are in the water, separated from the hydrogen atoms. The sulphur with the four oxygen atoms is called a sulphate ion and has a double negative charge. The free hydrogen atom is called a hydrogen ion and has a positive charge. (An ion is simply an atom or a molecule with a positive or negative charge.)

The battery performs its function through a series of chemical reactions involving these ions. In the condition described in the introductory paragraph, the battery is charged and has the capacity to supply an electric current through cables connected to the two posts. As the battery supplies electric current, the sulphuric acid reacts by giving up its sulphate ions to the lead and lead dioxide plates. This forms lead sulphate that deposits on the plates. While this process occurs, the sulphuric acid concentration is decreasing, and the battery is discharging.

Supplying an electric current to the battery rather than drawing it out---such as what happens in an automobile when the engine is running---reverses the chemical reaction and the battery recharges. When recharging, the lead and lead dioxide plates give up sulphate ions to the electrolyte solution. This restores the amount of sulphuric acid in the electrolyte solution and restores the lead and lead dioxide plates to their charged condition.

[A] Fig1.1 Working of Lead storage battery

3.3 Chemical Reaction:

During discharging:

At Anode:

Pb(s) + SO42-(aq) à PbSO4(s) + 2e-

At Cathode:

PbO2(s) + SO42-(aq) + 4H+(aq) +2e- à PbSO4(s) + 2H2O

Overall Reaction: Pb(s) +PbO2(s) + 4H+(aq) + 2SO42-(aq) à2PbSO4(s) + 2H2O

[9]During Recharging:

At Anode:

PbSO4(s) + 2e- à Pb(s) + SO42-(aq)

At Cathode:

PbSO4(s) + 2H2O à PbO2(s) + SO42-(aq) + 4H+(aq) +2e-

Overall Reaction: 2PbSO4(s) + 2H2O à Pb(s) +PbO2(s) + 4H+(aq) + 2SO42-(aq)

3.4 Shortcomings of Lead Storage batteries:

The following is a short summary of the main performance shortcomings, or “bottlenecks” of existing lead acid batteries.

  1. Life: Lead acid batteries suffer from a limited useful life due primarily to two factors:
    1. Corrosion of the positive grid b) Sulfation on the negative grid.
  2. The corrosion failure mode is primarily due to the fact that the positive battery plates are required to perform their function as “electron collectors” in an incredibly harsh, acidic environment.But most of batteries are frequently required to operate in high-temperature conditions, which have the effect of accelerating corrosion. To mitigate the effects of corrosion, battery manufacturers have focused their research efforts on developing corrosion-resistant lead alloys and grid manufacturing processes. Although improvements have been attained in this manner, corrosion remains one of the most common failure modes of lead acid batteries.
  3. Sulphation failures result from a lead acid battery being kept in a discharged state for a period of time. In this situation, the lead sulphate formed in the normal chemical discharge reaction, re-crystallizes and hardens. This non-conductive lead sulphate blocks the conductive path required for recharging. Once they are in this crystalline state, the sulphate crystals are very difficult to convert back to the charged lead and lead oxide required to produce the battery's energy-producing chemical reaction. Even a well-maintained battery will, over time, lose some of its capacity due to the continued growth of large sulphate crystals that are not entirely reabsorbed during the charging cycle. The sulphate crystals are also larger in volume than the original paste, so they can actually mechanically deform the plate or grid by pushing the material apart. Sulfation is a common problem in recreational vehicle applications where extended off-season storage leads to dead batteries that will not accept a recharge.
  4. Cycle Life: Cycle life is a term that refers to the number of deep discharges that a battery can endure without significantly diminishing its useful life. As users have become more familiar with rechargeable batteries in cell phones and laptop computers, they have become comfortable with bringing these batteries down to an almost totally discharged state and bringing them back to full capacity with a recharge of just a few hours. In contrast, conventional lead acid batteries, because of inherent design and utilization limitations, are only capable of handling discharges down to 20 to 30 percent of full capacity. The number and frequency of these deep discharges can lead to a drastic reduction in the battery's overall life span. A motorist who forgets to turn his or her headlights off and has to have the battery recharged because it's totally dead most often never realizes that the battery has suffered a deep-discharge “injury” that will significantly shorten its useful life span. Many new products that have historically used lead acid batteries are now being requiring a significant increase in cycle life. A notable example is the hybrid electric vehicle, which requires high-rate discharges at mid to low state-of-charge conditions. Such conditions are a nightmare for designers of conventional lead acid batteries, as their products simply do not possess sufficient overall longevity under such conditions. This has left car companies no alternative but to go with much more expensive alternatives such as Nickel-Metal Hydride, and even to begin experimentation with Lithium Ion technology.
  5. Vibration: Having worked for years at CAT, the premier manufacturer of heavy equipment, Kurt Kelley was particularly sensitive to developing a design that could minimize the adverse effects of vibration and rugged use. Conventional lead grids generate a great amount of “back and forth” force when subjected to vibration and jarring. Although steps (such as anchor-bonding) have been taken by battery manufacturers to reduce these effects, the root cause is the mass of the heavy lead grids. Batteries subjected to continuous, severe vibration literally tear themselves to pieces, internally, over time.
  6. Recharge Time: Typically a lead-acid battery product will require a recharge time significantly longer than the advanced materials seen in portable products. A complete charging of a lead-acid battery, such as found in electric vehicles, can take from 8 to 16 hours. In the case of Uninterrupted Power Supplies (UPS), a rapid charge rate is essential to quality performance, as well as reducing the related capital expenditures for back up equipment while charging takes place on initial batteries put into service.
  7. Size and Weight: Although lead-acid batteries are the cheapest energy storage products in the world to manufacture, the extensive use of lead gives them an exceptionally large footprint and weight. Again, like other performance aspects of this industry's products, this limits their form factors and overall utilization in new product designs. In addition, a traditional lead battery plate (there are over 100 of these in a typical automotive lead acid starting battery) on average only utilizes 30% to 40% of its surface area over the life of the battery. This creates even more inefficiencies of power-to-weight ratios.

3.5 Modification:

For better output

  1. An Sn or Pb/Sn alloy heat-treated at 170° C. or higher for a given period of time is applied to the surface of a collector to make a lead storage battery which is improved in terms of its chargeability upon left over discharged.
  2. In accordance with a new method of casting electrode grids for electric lead storage batteries in a mold, premature solidifying of the melt is prevented before the end of the mold filling period by an additional heating pulse applied to the melt during the mold filling process, as well as by the use of a good heat conducting mold material. The cooling down to the unmolding temperature is also accelerated. Because of the short dwell time of the lead within the mold, there simultaneously results a short machine cycling period. The separate pulse heating of the melt is preferably carried out by an induction heating apparatus, the alternating field of the inductor located within the mold walls producing heat through eddy current production within the molten molded body.

3.6 New formula:

Time for discharging a battery

Mr Peukert first devised a formula that showed numerically how discharging at higher rates actually removes more power from the battery than a simple calculation would show it to do. For instance discharging at 10 amps does not remove twice as much power as discharging at 5 amps. It removes slightly more. Therefore a 100 amp hour battery (at the 20hr rating) could provide 5 amps for 20 hours, but it could not provide 10 amps for 10 hours. The available time would actually be slightly less.

"Mr Peukert first devised a formula for....". This is because he is generally regarded as being the man who first discovered the phenomenon. This is incorrect. The effect had been known for many years beforehand and was first noted by a certain Mr Schroder several years before Peukert devised his formula. Mr Peukert simply quantified it in a way that had never been done before. However the effect is now known as Peukert's effect, the formula for calculating it is known as Peukert's equation, and the important number, unique to each battery type that is put into the equation in order to perform the calculation, is known as Peukert's exponent. Note that Peukert's exponent changes as the battery ages.

So Peukert's equation is: T = C/ (I/(C/R))n * (R/C)

Where: I = the discharge current

T = the time

C = capacity of the battery

n = Peukert's exponent for that particular battery type

R = the battery hour rating, i.e. 100 hour rating, 20 hour rating, 10 hour rating etc.

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