Lead Acid Battery. Attacking Sulphate Passivation and Cyclability Problems

TABLE OF CONTENTS

INTRODUCTION
1.1 Timeline of Battery History:
1.2. Characteristics of Some batteries and achievable performance:
1.3. Lead-acid battery
1.3.1. Advantages of lead-acid system
1.3.2. Technical developments in Lead-acid battery:
1.3.3. Charging and Discharging Reactions:
1.3.4. Theoretical voltage and capacity:
1.3.5. Capacity of a cell:
1.3.6. Thickness of the plates and capacity:
1.3.7. Rate of Discharge
1.3.8. Electrolyte Temperature
1.3.9. Effect of Concentration of the electrolyte:
1.3.10. Manufacture of Lead-acid battery
1.3.11. Flow chart for the Manufacture of flooded lead-acid battery
1.4. Classification of Lead-acid battery:
1.4.1. SLI batteries:
1.4.2. Stationary batteries:
1.4.3. Motive power batteries:
1.4.4. Special purpose batteries:
1.4.5. Valve Regulated Lead-acid Batteries (VRLA)
1.5. Failures in Lead-acid batteries:
1.5.1. Sulphation is due to the following reasons:
1.5.2. Shedding of the positive mass:
1.5.3. Destruction of the positive grids:
1.5.4. Defects in the negative mass:
1.6. CHARGING OF LEAD-ACID BATTERY
1.6.1. Constant-current charging (CC)
1.6.2. Constant-Voltage charging (CV)
1.6.3. Taper charging
1.6.4. Pulse charging
1.6.5. Trickle Charging
1.6.6. Float Charging
1.6.7. Battery charger should have the following Characteristics
1.7. GRID MATERIALS:
1.7.1. Grid alloy properties
1.7.2. Ease of fabrication
1.7.3. Mechanical strength
1.7.4. Creep strength
1.7.5. Corrosion resistance
1.7.6. Conductivity
1.7.7. Compatibility with active material
1.7.8. High hydrogen and oxygen over potential
1.7.9. cost effective
1.7.10. Various Types OF Grid alloys:
1.7.11. Beneficial elements
1.7.12. Self discharge behaviour
1.7.13. Detrimental elements
1.8. GRID production methods:
1.9. VARIOUS TYPES OF GRIDS:
1. M C B-GRID
2. BOX -NEGATIVE PLATE
3. MONCHESTER GRID
4. IRONCLAD GRID
6. EXPERIMENTAL BATTERY GRID
References

LITERATURE SURVEY AND SCOPE OF THE WORK
2.1 LITERATURE SURVEY:
REFERENCES
2.2. SCOPE OF THE WORK

EXPERIMENTAL DETAILS
3.1. Chemicals and materials used
3.2. Weight loss Studies
3.3. Cyclic Voltammetry
3.4. Impedance measurements
3.5. Anodic polarisation studies
3.6. CHRONO AMPEROMETRIC STUDIES
3.7. XRD
3.8. Scanning Electron Microscope
3.9. Charge acceptance studies
3.10. cycle life test
1. cycle life test with low capacity battery
2. Heavy load endorsement test
REFERENCES :

RESULTS & DISCUSSION
4.1. WEIGHT LOSS STUDIES
4.1.1. Dense Lead sulphate removal from the positive plate
4.1.2. Dense lead sulphate removal from the negative plate
SUMMARY:
4.2. Cyclic Voltammeteric Studies
4.2.1 .Cyclic Voltammetric Studies of the Positive Plate
CV STUDIES in electrolyte containing different acetates
CV STUDIES for the mixture of boric acid and ACETATES
CV STUDIES for the mixture of Phosphoric acid and ACETATES
Electrochemical Kinetic Parameters for the formation of lead sulphate in the absence and presence of sodium acetate and Phosphoric acid combined additive
4.2.2. Cyclic VOLTAMMETRIC STUDIES of the negative PLATE
CV STUDIES IN ELECTROLYTE CONTAINING DIFFERENT ACETATES
CV STUDIES FOR THE MIXTURE OF BORIC ACID AND ACETATES
CV STUDIES FOR THE MIXTURE OF PHOSPHORIC ACID AND ACETATES
ELECTROCHEMICAL KINETIC PARAMETERS FOR THE FORMATION OF LEAD SULPHATE IN THE ABSENCE AND PRESENCE OF SODIUM ACETATE AND PHOSPHORIC ACID COMBINED ADDITIVE
SUMMARY:
4.3. ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY STUDIES
4.3.1. EIS STUDIES on active / passivated POSITIVE PLATES in the absence and presence of additives
4.3.2. EIS STUDIES on active / passivated NEGATIVE PLATES in the absence and presence of additives
4.4. Studies on the passivation phenomena of lead (negative electrode) in the BATTERY ELECTROLYTE
4.5. Self-Corrosion of the electrodes in the battery electrolytes
4.6. Studies on the electro formation of Lead Sulphate with and with out the additives
4.7. SEM STUDIES
4.8. X-ray diffraction studies
4.9 CHARGE ACCEPTANCE STUDIES
4.1 CYCLE LIFE TEST
4.10.1. Slow rate cycle life test with low capacity Battery
4.10.2. HEAVY LOAD ENDORSEMENT TEST WITH HEAVY DUTY BATTERY
REFERENCES:

CONCLUSIONS

CHAPTER-1 INTRODUCTION

A battery is defined as an electrical storage device, which is able to convert the stored chemical energy into work of an electrical nature. The word battery was originally applied by Benjamin Franklin as a collective term to describe the apparatus obtained when several leyden jar capacitors were connected together. When the batteries were first invented, they had seen as merely laboratory curiosities.

Faraday distinguished electrodes into anode and cathode. These were derived from the Greek words ‘way up’ and ‘going down’ respectively as Faraday supposed that the anode releases the electrons which are consumed by the cathode.

It was just 200 years since the invention of the first battery; this has been ascribed to Alessandro Volta (1745-1827), Professor of Natural Philosophy (physics) at Pavia University, Italy. His name is commemorated at all time by the unit of electrical potential, the volt. Volta’s famous experiment, described in a letter to the Royal Society of London in 1800, involved the assembly of a pile of alternate silver (or brass or copper) and zinc (or tin) discs, with each pair of dissimilar metals separated from the next by a piece of cloth which was saturated with brine. One end of the pile was terminated in a silver disc and the other in a zinc disc, and a continuous current of electricity was produced as soon as a wire conductor connected the two. This was the first galvanic or primary battery and became known as ‘Volta’s pile’. Batteries have come a long way in 200 years!

The next significant step in the development of batteries was the invention of the ‘Daniell cell’ by John Daniell (1790 -1845), Professor of Chemistry at King’s College, London. In 1836, he took a copper vessel filled with copper sulfate solution and Zinc rod with zinc sulphate solution separated by gullet of an ox. This constituted a so called ‘cell’. Discharge of the cell caused the zinc electrode to dissolve and copper to be deposited at the positive electrode. The cell produced a voltage of 1.1 V.

This was possibly the first practical galvanic cell to give a continuous current of useful magnitude. Further modifications (Fig1.1a) included the use of porous ceramic pots (‘separators’) instead of animal membranes, substitution of sulfuric acid by zinc or magnesium sulfate, and the development of multi-cell batteries. Daniell cells were adopted by commercial telegraphic systems following a rapid expansion of such services in the early 1850s.

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Fig 1.1. a) Schematic of Plante`s lead acid cell b) early illustration of a battery of three Plante`s lead acid cell ¹

A subsequent major advance was made by the French chemist Georges Leclanche´ (1839-1882) who, in 1866, invented the primary cell, which bears his name. This cell consists of a zinc rod as the negative electrode and a carbon rod as the positive electrode, both immersed in a solution of ammonium chloride contained in a glass jar.

The positive electrode was housed in an inner porous ceramic pot and packed around with a mixture of powdered manganese dioxide and carbon. The cell, which has been extensively developed ever since, gives a voltage of 1.5 V. A major advance took place in the 19th century when the idea of using a zinc cane as both container and electrode was patented and came into general use. Before the invention of these galvanic cells, the only electricity known and available was static electricity, as produced by friction between dissimilar materials or in thunderstorms.

The first effective demonstration of a secondary (rechargeable) cell was given in 1859 by the French chemist Gaston Plante´ (1834- 1889). This cell consisted of two concentric spirals of lead sheet, separated by porous cloth, immersed in dilute sulphuric acid within a cylindrical glass vessel .The ‘lead-acid battery’ thus constructed gave an output of 2 V, but very little current was initially gained because of the low surface area of the plates. By a series of discharges and charges, the chemical reactions at the surface of the plates resulted in the gradual build-up of deposits of higher surface area and the current was progressively improved. This became known as the ‘formation process’, a term still used today in the initial charging of lead-acid batteries. In March 1860, Plante´ presented a battery of ten cells (20 V) to the French Academy of Sciences in Paris; an illustration of an early battery of Plante´ cells is shown in Figure 1.1b.

An important advancement in the technology of the Lead-Acid battery was achieved by the French chemical engineer Camille Faure´ (1840-1898) who, in 1881, showed the change in level of electrical charge, or the ‘capacity’, of the system could be greatly increased by coating the lead plates with a paste of lead dioxide and sulphuric acid. This process also reduced the time required for plate formation from months to hours, and thus became part of the basic technology of the lead-acid battery industry.

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Fig 1.2. a) Schematic of Plante`s lead acid cell b) early illustration of a battery of nine Plante`s lead acid cell¹

The most important event in the history of Lead-acid battery was the invention of the electric self-starter by kettering in 1912. As a result, the battery market has grown with the growth of the automobile market throughout the last century.

1.1. Timeline of Battery History:

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A number of primary, secondary, reserve and fuel cells are also developed. Each one has special features and they have their own limitations. Performance and characteristic of few systems are presented in the following table.

1.2. Characteristics of Some batteries and achievable performance:

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1.3. Lead-acid battery

Lead - acid battery is the workhorse of the rechargeable battery systems. It is the single most used battery worldwide. Although many new systems may challenge its position, its reliability, low cost and good operational life, can’t so easily be substituted.

The first secondary battery (Lead-acid battery) was discovered and developed by Gaston plants [1-5] in 1859. Since then enormous developments have been taken place in the science and technology of the battery system. A number of references are available dealing with Lead-acid battery [6-20]. In principle, the Lead-acid battery consists of two electrodes immersed in a common electrolyte. The characteristic feature of such a cell is the conversion of electron conduction into ionic conduction at the phase boundary of the electrode/electrolyte. This change in conductivity is established by the electrochemical reaction, i.e., a chemical reaction accompanied with the exchange of electric charge.

1.3.1. Advantages of lead-acid system

Lead-acid battery is technically well-established electrochemical device and is produced in quantities for different applications. Its production and use continues to grow. The most attractiveness of the Lead-acid battery is due to the following reasons.

Well established technology of production.

- Popular low-cost secondary battery.
- Capable of manufacturing with simple methods.
- Manufactured in capacity ranges from smaller than 1 Ah to 1000 Ah.
- Good high-rate performance-suitable for engine starting.
- Good low and high temperature performance
- Easy state-of-charge indication.
- Electrically efficient .

1.3.2. Technical developments in Lead-acid battery:

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The electrochemical reactions specify the most important parameters of the cell. The cell voltage is determined by chemical affinity of the reacting substances and capacity is defined by the amount of electrode material that can be converted.

The reactions taking place in the Lead-acid battery are given below.

Positive Electrode:

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Negative Electrode:

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Overall reaction:

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Equation (1) and (2) represents reactions during discharge, where lead dioxide is reduced to lead sulphate at the positive electrode, while metallic lead is oxidized at the negative electrode. Reversal of the current reverses (1) and (2), and recharges the cell.

1.3.3. Charging and Discharging Reactions:

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1.3.4. Theoretical voltage and capacity:

To calculate the equilibrium cell voltage, the change in free energy (ΔG) is used, which is derived as the difference of the standard free energies of the substance involved in the reaction. For equation (3) this difference turns out to be ΔG = -372.5 KJ mole-¹ and the standard equilibrium cell voltage (emf) is E0 = 372.5 *1000 / 2*96500 = 1.930V

Since, free energy depends upon concentration of electrolyte, the equilibrium cell voltage changes with the concentration or activity of the reacting species except those present in solid state (activity = 1) according to the Nernst equation,

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where E0 represents the standard equilibrium cell voltage, ‘a’ the activity (moles/ litre). The Lead-acid cell can be represented as follows.

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The electric potential difference is equal in sign and magnitude to the electrode potential of a metallic conductor attached to the right hand side electrode minus that of an identical lead on the left.

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1.3.5. Capacity of a cell:

Specific capacity (k) is defined as the ampere hours obtainable from a unit of the active material e.g. Ahkg-¹. A related parameter is the coefficient of utilization of active material .

The current density is expressed as

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The area expressed may be either the apparent geometric area (reckoned for the two sides of the plates) or the BET surface area, sometimes the c.d. is expressed as amperes per Kg of the active materials.

The capacity (C) is usually expressed in Ah (3.6 x 10³ Coulombs) specifying the flow of current per unit area in number of hours eg. C20 = 60 Ah implies that the battery can be discharged at 3 amperes over a period of 20 hours and it can deliver the declared capacity of 60 Ah before it reaches its end voltage (also called cut off voltage) usually about 1.75 to 1.70 V per cell. Sometimes the batteries are tested for their output in terms of unit power, energy for a unit time, watts.

In the electrochemical power source, the full utilization of the active material has not been realized. Thus in declaring the capacity, two types are distinguished, namely the theoretical capacity CTh and the practical capacity (C). The practical capacity depends on several parameters like discharge rate, Cell design, etc., and hence has to be declared very specifically under a given set of conditions.

The theoretical capacity is determined from the reaction according to Faraday’s laws and is equivalent to ZF Coulombs for one mole of reactants (here PbO2, Pb and H2SO4). The effective capacity equal to Current x Time is determined by discharging the battery at a definite rate over a period of time till it reaches the cut - off voltage.

From equation (3) it can be readily estimated that the theoretical amounts of the reacting materials required for 1 Ah are

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Similarly the weight of PbSO4 1 Ah is = 5.65 g

(Formed during discharge and consumed during charge )

Likewise the weight of Pb and H2SO4 required per Ah will be 3.86 and 3.66g respectively. Assuming the discharge voltage to be 2.0V, Specific energy per unit weight will be

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The value is only by the weight of the reacting substances. The inclusion of the weight of the inactive components such as grids, containers, and covers, poles and separators reduces the practical energy density to values as low as 30-40 WhKg-¹ (at 5- hour rate of discharge). Since the reaction product on both plates is PbSO4, which is a poor conductor of electricity, the realization of the theoretical energy density is made still more difficult.

Capacity of the battery depends on various factors such as the amount of active material, porosity of the plate, thickness of the plate, rate of discharge, electrolyte temperature and concentration of the electrolyte.

The theoretical requirements per Ah of capacity of the active materials are given below

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But the active materials cannot be discharged to the theoretical efficiency, ie, there is a practical limit for discharging these active materials. The ratio between the obtained capacity and the theoretical capacity (calculated from the amounts of active materials) is known as utilization co-efficient of active material. This is around 0.5 to 0.6.

Limited utilization of the active material is due to the following reasons.

(a) The reaction product PbSO4 is a poor conductor and increases the resistance of the active material to a very great extent.
(b) The lead sulphate formed during discharge hinders the diffusion of electrolyte through the pores because it is more voluminous than the materials from which it is formed.
(c) The resistivity of the electrolyte goes on increasing as the discharge proceeds further and further.
(d) Another reason is the limited contact between the active materials and the grids.
(e) Yet another reason is the ‘electrochemically inactive PbO2’ formed during the course of the cycling of the active material.

1.3.6. Thickness of the plates and capacity:

The diffusion of sulphate ions into the highly tortuous paths of the porous active materials becomes more and more impaired as the thickness of the plates increases, and this effect is more significant at high rate of discharge. At lower current densities a uniform loading of lead sulphate occurs in the interior of the electrode during entire discharge. At high current density the consumption of SO4-ions exceeds their replacement rate. The inner acid contributes more to capacity at a high rate discharge²¹.

If the interior of the plates is supplied with enough acid by the forced flow, the same capacity can be withdrawn even at high rate discharge as for discharges at low current density [22, 23].

1.3.7. Rate of Discharge

Although a number of equations defining the dependence of Capacity (C),Current(I) and time (t) are available, the one proposed by Penkert has found the widest application. The relationship between capacity and discharge current ²⁴ is

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Where n and K are constants determinable from discharge data. The value of n approaches I for very small current densities (when I tends to Zero) and 2 for high current densities. Straight lines are obtained when current and duration are plotted on log-log scale in the current density range 0.02 - 0.2 A/cm-². The slope is identical for both positive and negative plates (value for n = 1.4). The polarity of the plates affects the value of k but not of n.

However when I tend to approach zero this equation fails and Liebenow equation is better applicable²⁵.

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Where A and B are constants here C = A/B i.e., the ratio between the constants is equal to the maximum capacity of the cell (at very low current density)

1.3.8. Electrolyte Temperature

The lower the temperature, lower will be the capacity. When the temperature is lowered below 0⁰ C there is a significant rapid decrease in the available capacity. At 0⁰ C the capacity is only 50% of that obtainable at 40⁰ C (C5 rate). Normally the temperature coefficient of capacity is 0.8 to 1% per⁰ C (around 20 - 30⁰ C). It is higher at lower rates of discharges for both types of polarity. The negatives are much more prone to lower temperature influences [26, 27].

Baikie et al have modified the Penkert equation as

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Where K0 is penkert constant at 0⁰ C, and it can also be taken as the temperature coefficient of capacity. The other terms have the useful significance. As stated earlier, n will be different for positive and negative plates.

1.3.9. Effect of Concentration of the electrolyte:

The concentration of the sulphuric acid affects the capacity of the Lead- acid battery. The concentration of the acid in the pores of the plates determines the potential of the plates. The resistance of the electrolyte, which in turn depends on its concentration, hinders the passage of electric current. The viscosity of the electrolyte depends partly on the concentration. These affect the rate of diffusion in the electrolyte in the pores of the plates and also affect the rate of diffusion outside. All these contribute to the variation in capacity of the battery. Various research workers have determined the concentration of the electrolyte at which maximum capacity is obtained and the value of the specific gravity varies in the range of 1.100 - 1.270 .

Generally the capacity of lead-acid battery increases as the concentration of the electrolyte increases, especially at high rates of discharge. The concentration of electrolyte is favourable for the positive plate but may be detrimental to the negative plate. The capacity of the negative plate in an electrolyte at 1.315 sp.gr is less than in the electrolyte of 1.140 sp.gr particularly at high rates of discharge and at low temperatures⁸.

1.3.10. Manufacture of Lead-acid battery

Grids cast from lead-antimony alloys or other lead alloys are pasted with a mortar like paste made of lead oxide (PbO), diluted sulphuric acid and water. The composition of the positive and the negative plate differs mainly in the inclusion of minor additives in negative plate. The plates are ‘cured’ in a humid atmosphere and later in a place where there is free current of air. When dry, they are electrochemically converted to the respective active material, namely PbO2 in the positive plate and Pb in the negative plate⁸.

1.3.11. Flow chart for the Manufacture of flooded lead-acid battery

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1.4. Classification of Lead-acid battery:

In a general way based on the applications, Lead-acid batteries can be classified in the following manner.

1.4.1. SLI batteries:

SLI (starting, lighting and igniting) is the common term for the battery used to start an internal combustion engine and to power the electrical system in emergencies, when the engine is not running. Most of the storage batteries are used for these applications.

1.4.2. Stationary batteries:

These types of lead storage batteries are applicable to communication systems, electrical utilities, computer systems, emergency lighting and railways to provide peak loads and emergency power.

1.4.3. Motive power batteries:

These Lead-acid batteries provide power for the propulsion of electrical lift, trucks, mining equipment, street delivery vehicles and other types of material handling.

1.4.4. Special purpose batteries:

Aircraft, submarine, marine, special military and small sealed batteries for consumer applications are the few examples of special purpose Lead-acid batteries.

1.4.5. Valve Regulated Lead-acid Batteries (VRLA)

These types of batteries are applicable to on-line UPS. And also the fast growing telephone sectors is major user of this type [39- 50].

1.5. Failures in Lead-acid batteries:

The reasons for the failure of a battery in the particular field depends on the type of application and maintenance [51-60] Most of the failures are caused by loss of active components (active material and grid) and by separators. Other reasons such as mechanical destruction or maintenance failure are mostly very rare.

The reasons for most frequent failures are listed below.

1. Passivation of active materials during service life.
2. Shedding of the positive active material (PAM).
3. Destruction of the positive grid.
4. Defects in the negative active material (NAM).
5. Defects in separators
6. Combination of these causes.

Also at the end of life of the battery several defects may exist.

1.5.1. Sulphation is due to the following reasons:

1. Long standing in discharge condition.
2. Too high acid concentration.
3. Prolonged charging.
4. Increased self-discharge.
5. Continuous operation between 45 - 50 oC.

1.5.2. Shedding of the positive mass:

Shedding of the positive active mass occurs especially in charge/ discharge service. During the service of the battery a weight loss of active material parallel to their diminishing capacity is observed. Shedding occurs preferentially at the end of charge and at the beginning of discharge. Strong gas evolution during overcharge facilitates the shedding [61-65].

1.5.3. Destruction of the positive grids:

The grids of the electrodes, which serve as carriers for the active masses and conductors for electric current are manufactured from lead and lead alloys by casting. Other methods such as punching or stretching are not common.

The process of disintegration of a metal grid structure starting at the surface is caused by corrosion. The following reaction takes place at the grid surface.

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The growth of the plates is expressed as the relative increase in the linear dimension.

Often values differ for expansion in height and width. The expansion depends on the thickness of the layer formed by the corrosion and on the tensile strength. The expansion is not caused by volume changes of the active masses during charge and discharge.

1.5.4. Defects in the negative mass:

The most important defect of the negative mass is sulphation, leading to solidification of sponge lead, a sintering process resulting in loss of capacity during the normal operation of discharge when lead sulphate which is not easily rechargeable, forms on both the plates. Under sulphation the lead sulphate has lost its rechargeability.

1.6. CHARGING OF LEAD-ACID BATTERY

Proper recharging is important to obtain optimum life from any type of Lead-acid battery under any condition of use³.

- The charge current at the start of recharge can be any value that does not produce an average cell voltage in the battery system is greater than the gassing voltage
- During the recharge the current should be controlled to maintain a voltage lower than the gassing voltage.
- The charge should be finished at a constant current of 5A per 100Ah of rated capacity.

Following methods are meeting the above conditions to charge leadacid batteries.

1.6.1. Constant-current charging (CC)

Constant - current charging is used in the laboratory because of the convenience of calculating ampere-hour input and because constant-current charging can be done with the help of simple inexpensive equipment.

For initial battery formation process constant current charging is widely used. CC Charging at the half of the 20-h rate can be used in the field to decrease the sulfation in batteries which have been overcharged and /or undercharged.(This treatment may diminish the battery life)

1.6.2. Constant-Voltage charging (CV)

CV charging is used on-the-road vehicle, uninterruptible power system and where the charging circuit is tied to the battery. In this case the charging circuit has a current limit and this value is maintained until the cell reaches the preset voltage, then the preset constant-voltage is maintained on the cell.

1.6.3. Taper charging

A charge regime delivering moderately high rate charging current when the battery is at a low state of charge and tapering the charging current to lower rates as the battery is charged. Taper charging is a variation of the modified constant-potential method using less sophisticated controls to reduce equipment cost.

1.6.4. Pulse charging

Pulse charging is used for traction application. In this case the charger is periodically isolated from the battery terminals and the OCV of the battery is automatically measured. If the open circuit voltage decays below that limit, the charger delivers direct current pulse for a fixed period or the battery SOC is very low, charging current is delivered to the cell until the cell reaches its preset voltage.

1.6.5. Trickle Charging

Trickle charge is a continuous constant current charge at a low rate about C/100, which is used to maintain the battery in a fully charged condition. This method used for SLI and similar type battery recharging to its losses due to self-discharge as well as to restore the energy discharge during intermittent use of battery.

1.6.6. Float Charging

Float charging is a low - rate constant - potential charging used to maintain the battery in a fully charged condition. This method is mainly used to Stationary Batteries.

1.6.7. Battery charger should have the following Characteristics

- Light weight, small size, low cost.
- Resistance to the environment, temperature extremes, corrosion & vibration.
- Ease of maintenance and repair.
- Compatibility with available power supply (AC) and battery specifications.
- Safety against electrical shock and other equipment malfunction.

1.7. GRID MATERIALS:

1.7.1. Grid alloy properties

The over all function of the grid of a Lead-acid battery is to retain the active material and to act as a conductor during operation of the battery²⁹.

1.7.2. Ease of fabrication

Grid must have a high open pore area so that the active material grid ratio can be maximized. The conventional production method is casting and any alloy must therefore have good cast ability in order to found application as grids.

Alternatively, wrought methods of manufacture can be considered in which case the alloy must be capable of being rolled into a sheet form and converted into a lattice by perforation or by expanding the sheet form and converted into a lattice by perforation. The latter is the preferred method as it produces little scrap and gives a high open pore area and results in a structure, which has good paste retention.

1.7.3. Mechanical strength

The grid must not suffer from under distortion or fracture during the manufacture process like cutting, pasting, handling. The alloy must have sufficient mechanical strength and must not be brittle.

1.7.4. Creep strength

The positive grid is corroded during the charge process and the corrosion product lead dioxide has a higher specific volume than the original lead alloy material. The tensile stresses on the grid so created during service cause the positive plate to grow. The positive grid alloy must therefore have a high creep resistance to restrict the amount of growth in service.

1.7.5. Corrosion resistance

The positive grid alloy must have good corrosion resistance so that there is sufficient grid material to act as retainer and conductor through out the life of the battery. Since the grid is both corroded and stressed during the charge cycle, it is important that the corrosion should take a general form with no preferential attack on specific areas like grain boundaries.

1.7.6. Conductivity

Lead has not a particularly high conducting and any alloying is found to decrease the conductivity. The detrimental effect should be kept as far as possible to a minimum level.

In large grids or in grids when very high discharge rates are required, consideration can be given to incorporating a metal or alloy of higher conductivity.

1.7.7. Compatibility with active material

For a battery plate to function well there must be a good active material interface. Further more the active material must be retained and must not shed during service. (an effect which is more prevalent during deep discharge / charge cycling). The behaviour of an alloy in this respect can normally be discovered only from actual battery tests.

1.7.8. High hydrogen and oxygen over potential

Lead-acid battery is rechargeable because of high H2 & O2 over potential on the plates. Ideal alloying elements should not cause a decrease in the over potential.

1.7.9. cost effective

Cost is obviously important for most applications and the cost of any alloy (both material and manufacture cost) must always be taken into account.

1.7.10. Various Types OF Grid alloys:

The addition of small amounts of other elements is necessary to prevent grid fabrication defects and grid brittleness²⁹. The grid metal may be pure lead in some designs, but in the great majority of cases lead is alloyed selective metals to provide both mechanical strength and corrosion resistance. Some of the alloying elements fall into two broad classes.

1.7.11. Beneficial elements

The pure lead has been hardened, traditionally by the addition of antimony metal.

Antimony is helpful in the manufacturing process by improving the fluidity of the metal and reducing the volume change on setting.

During operation in batteries antimony has the following disadvantages.

1. Sb induces the self-discharge reactions.
2. Because antimony has a lower over potential than lead for hydrogen evolution, water loss is increased and charging efficiency is decreased.

Antimony also reduces the conductivity of lead. For example conductivity of 8% antimony alloy is about 20% lower than that of pure lead. Arsenic and selenium are also added up to 0.5 to 0.4% respectively. Arsenic improves corrosion resistance and selenium improves the cast ability and prevent cracking.

Lead operates synergistically with antimony and arsenic to improve metal fluidity and cast ability. Silver is reported to improve corrosion resistance. Cobalt also improves the corrosion resistance.

Now a days Calcium 0.03 to 0.1% + Tin 0.5 to 1 % has been used in place of antimony. This alloy provides low self-discharge, low gassing, low resistance, and low water loss than antimony alloys. Tin controls the grain size and structure and prevents intergranular corrosion. The amount of antimony has been varied between 5 and 12 % by weight. The antimony content should be kept below 4 % to reduce the maintenance problems and other environmental effects.

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