Thursday 30 August 2012

Lead Acid Batteries

A lead acid battery consists of a negative electrode made of spongy or porous lead. The lead is porous to facilitate the formation and dissolution of lead. The positive electrode consists of lead oxide. Both electrodes are immersed in a electrolytic solution of sulfuric acid and water. In case the electrodes come into contact with each other through physical movement of the battery or through changes in thickness of the electrodes, an electrically insulating, but chemically permeable membrane separates the two electrodes. This membrane also prevents electrical shorting through the electrolyte. Lead acid batteries store energy by the reversible chemical reaction shown below.
The overall chemical reaction is:
At the negative terminal the charge and discharge reactions are:
At the positive terminal the charge and discharge reactions are:
As the above equations show, discharging a battery causes the formation of lead sulfate crystals at both the negative and positive terminals, as well as the release of electrons due to the change in valence charge of the lead. The formation of this lead sulfate uses sulfate from the sulfuric acid electrolyte surrounding the battery. As a result the electrolyte becomes less concentrated. Full discharge would result in both electrodes being covered with lead sulfate and water rather than sulfuric acid surrounding the electrodes. At full discharge the two electrodes are the same material, and there is no chemical potential or voltage between the two electrodes. In practice, however, discharging stops at the cutoff voltage, long before this point. The battery should not therefore be discharged below this voltage.
In between the fully discharged and charged states, a lead acid battery will experience a gradual reduction in the voltage. Voltage level is commonly used to indicate a battery's state of charge. The dependence of the battery on the battery state of charge is shown in the figure below. If the battery is left at low states of charge for extended periods of time, large lead sulfate crystals can grow, which permanently reduces battery capacity. These larger crystals are unlike the typical multicrystalline or porous structure of the lead electrode, and are difficult to convert back into lead.
lead acid charging voltage
Voltage of lead acid battery upon charging.
The charging reaction converts the lead sulfate at the negative electrode to lead. At the positive terminal the reaction converts the lead to lead oxide. As a by-product of this reaction, hydrogen is evolved. During the first part of the charging cycle, the conversion of lead sulfate to lead and lead oxide is the dominant reaction. However, as charging proceeds and most of the lead sulfate is converted to either lead or lead dioxide, the charging current electrolyzes the water from the electrolyte and both hydrogen and oxygen gas are evolved, a process known as the "gassing" of the battery. If current is being provided to the battery faster than lead sulfate can be converted, then gassing begins before all the lead sulfate is converted, that is, before the battery is fully charged. Gassing introduces several problems into a lead acid battery. Not only does the gassing of the battery raise safety concerns, due to the explosive nature of the hydrogen produced, but gassing also reduces the water in the battery, which must be manually replaced, introducing a maintenance component into the system. In addition, gassing may cause the shedding of active material from the electrolyte, thereby permanently reducing battery capacity. For these reasons, the battery should not regularly be charged above the voltage which causes gassing. The voltage across the battery and the gassing voltage are shown in the figure below.
Gassing voltage change with varying charge rates.
Lead sulphate is an insulator, and therefore the way in which lead sulfate forms on the electrodes determined how easily the battery can be discharged.
Self-discharge involves severn reactions. Give seperate reaction for H2 and O2 formation. Rate of oxygen evolution strongly influenced by imourties. Is O2 evolution a side reaction that contributes to self discharge? Anodic corrosion contributes to self-discharge. Negative self discharge is Pb + H2SO4 -> PbSO4 + H2. Formation of H2 gas is the slow step. High H2 overvoltage on lead, so process is slow with lead or lead calcium grids. Hoever, proceeds rapidly with SB. Sb Lowers O2 overvotlage. 1/2O2 + Pb+ H2SO4 -> PbSO$ + H20. IN open cells an issue. In closed cells can do an oxygen cylce.
Corrosion of lead and lead alloys. (what is reaction)

History of battery and development

History of battery invention and development

Our history of batteries begins with the Baghdad battery

Why was a battery required 2000 years ago?
In June, 1936, workers constructing a new railway near the city of Baghdad uncovered an ancient tomb. Relics in the tomb allowed archeologists to identify it as belonging to the Parthian Empire. The Parthians, although illiterate and nomadic, were the dominating force in the Fertile Crescent area between 190 BC to 224 AD. It is known that in 129 BC they had acquired lands up to the banks of the Tigris River, near Baghdad.
Among the relics found in the tomb was a clay jar or vase, sealed with pitch at its top opening. An iron rod protruded from the center, surrounded by a cylindrical tube made of wrapped copper sheet. The height of the jar was about 15 cm, and the copper tube was about 4 cm diameter by 12 cm in length. Tests of replicas, when filled with an acidic liquid such as vinegar, showed it could have produced between 1.5 and 2 volts between the iron and copper. It is suspected that this early battery, or more than one in series, may have been used to electroplate gold onto silver artifacts.
A German archeologist, Dr. Wilhelm Konig, identified the clay pot as a possible battery in 1938. While its 2000-year old date would make it the first documented battery invention, there may have been even earlier technology at work. Dr. Konig also found Sumerian vases made of copper, but plated with silver, dating back to 2500 BC. No evidence of Sumerian batteries has been found to date.
1747 -- Principle of the telegraph discovered, but not battery-powered.
In 1747 Sir William Watson demonstrated in England that a current could be sent through a long wire, using the conduction through the earth as the other conductor of the circuit. Presumably the current was from an electrostatic discharge, such as from a Leyden jar charged with high voltage. People at that time knew how to generate electrostatic voltages by rubbing dissimilar materials such as glass and fur together. Then in 1753 a certain C.M. in Scotland devised a signaling machine that used an insulated wire for each letter of the alphabet. At the sending end an electrostatic charge was applied to the selected wire, and a pith ball jumped at the receiving end in response to the voltage.
1786 -- Luigi Galvani notices the reaction of frog legs to voltage
He was remarkably close to discovering the principle of the battery, but missed it. He thought the reaction was due to a property of the tissues. He used two dissimilar metals in contact with a moist substance to touch dissected frog legs. The resulting current made the muscles in the frog legs twitch. Luigi Galvani made many more important discoveries later, when the relationship between magnets and currents became known. The galvanometer is named for him. It is a moving coil set in a permanent magnetic field. Current flowing through the coil deflects it and an attached mirror, which reflects a beam of light. It was the first accurate electrical measuring instrument.
1800 -- Alessandro Volta publishes details of a battery
That battery was made by piling up layers of silver, paper or cloth soaked in salt, and zinc. Many triple layers were assembled into a tall pile, without paper or cloth between zinc and silver, until the desired voltage was reached. Even today the French word for battery is ‘pile' (English pronunciation "peel".) Volta also developed the concept of the electrochemical series, which ranks the potential produced when various metals are in contact with an electrolyte. How handy for us that he was well known for his publications and received recognition for this through the naming of the standard unit of electric potential as the volt. Otherwise, we would have to ask "How many galvans does your battery produce?" instead of asking "how many volts does your battery produce?"
1820 -- The Daniell Cell
The Voltaic Pile was not good for delivering currents for long periods of time. This restriction was overcome in the Daniell Cell. British researcher John Frederich Daniell developed an arrangement where a copper plate was located at the bottom of a wide-mouthed jar. A cast zinc piece commonly referred to as a crowfoot, because of its shape, was located at the top of the plate, hanging on the rim of the jar. Two electrolytes, or conducting liquids, were employed. A saturated copper sulphate solution covered the copper plate and extended halfway up the remaining distance toward the zinc piece. Then a zinc sulphate solution, a less dense liquid, was carefully poured in to float above the copper sulphate and immerse the zinc. As an alternative to zinc sulphate, magnesium sulphate or dilute sulphuric acid was sometimes used. The Daniell Cell was one of the first to incorporate mercury, by amalgamating it with the zinc anode to reduce corrosion when the batteries were not in use. We now know better than to put mercury into batteries. This battery, which produced about 1.1 volts, was used to power telegraphs, telephones, and even to ring doorbells in homes for over 100 years. The applications were all stationary ones, because motion would mix the two electrolyte liquids. The battery jars have become collectors items, with prices ranging for $4 to $44. Check them out on ebay.com.
1859 -- Lead Acid -- the Planté Battery
Raymond Gaston Planté made a cell by rolling up two strips of lead sheet separated by pieces of flannel, and the whole assembly was immersed in dilute sulphuric acid. By alternately charging and discharging this cell, its ability to supply current was increased. An improved separator was obviously needed to resist the sulphuric acid.
1866 -- The Leclanché carbon-zinc battery
The first cell developed by Georges Leclanché in France was a wet cell having its electrodes immersed in a liquid. Nevertheless, it was rugged and easy to manufacture and had a good shelf life. He later improved the battery by substituting a moist ammonium chloride paste for the liquid electrolyte and sealing the battery. The resulting battery was referred to as a dry cell. It could be used in various positions and moved about without spilling. Carbon-zinc dry cells are sold to this day in blister packages labeled "heavy duty" and "transistor power". The anode of the cell was zinc, which was made into a cup or can which contained the other parts of the battery. The cathode was a mixture of 8 parts manganese dioxide with one part of carbon black, connected to the positive post or button at the top of the battery by a carbon collector rod. The electrolyte paste may also contain some zinc chloride. Around 1960 sales of Leclanché cells were surpassed by the newer alkaline-manganese batteries.
1881 -- Camille Faure's Lead Acid Battery -- suitable for autos
Camille Faure's acid battery used a grid of cast lead packed with lead oxide paste, instead of lead sheets. This improved its ability to supply current. It formed the basis of the modern lead acid battery used in autos, particularly when new separator materials were developed to hold the positive plates in place, and prevent particles falling from these plates from shorting out the positive and negative plates from the conductive sediment.
1898 to 1908 -- the Edison Battery
Thomas Edison, the most prolific of all American inventors, developed an alkaline cell with iron as the anode material (-) and nickelic oxide as the cathode material (+). The electrolyte used was potassium hydroxide, the same as in modern nickel-cadmium and alkaline batteries. The cells were well suited to industrial and railroad use. They survived being overcharged or remaining uncharged for long periods of time. Their voltage (1 to 1.35 volts) was an indication of their state of charge.
1893 to 1909 -- the Nickel-Cadmium Battery
In parallel with the work of Edison, but independently, Jungner and Berg in Sweden developed the nickel-cadmium cell. In place of the iron used in the Edison cell, they used cadmium, with the result that it operated better at low temperatures, self-discharged itself to a lesser degree than the Edison cell, and could be trickle-charged, that is, charged at a much-reduced rate. In a different format and using the same chemistry, nickel-cadmium cells are still made and sold.
1949 -- the Alkaline-Manganese Battery
The alkaline-manganese battery, or as we know it today, the alkaline battery, was developed in 1949 by Lew Urry at the Eveready Battery Company Laboratory in Parma, Ohio. Alkaline batteries could supply more total energy at higher currents than the Leclanché batteries. Further improvements since then have increased the energy storage within a given size package.
1950 -- The zinc-mercuric oxide alkaline battery by Ruben
Samuel Ruben (an independent inventor) developed the zinc-mercuric oxide alkaline battery, which was licensed to the P.R. Mallory Co. P.R. Mallory Co. later became Duracell, International. Mercury compounds have since been eliminated from batteries to protect the environment.

Milliampere Hours (mAh)


Milliampere Hours (mAh)
Milliampere Hours (mAh) is an important figure because the easiest way to determine the capacity of a battery. It is also a way to compare battery run times on batteries with the same voltage. The higher the mAh, the longer the battery will last. Batteries with same footprint and different mAh ratings are usually interchangeable. If your battery is rechargeable then the mAh rating is how long the battery will last per charge.
Milliampere Hour is 1/1000th of a Amp Hour, therefore 1000mAh = 1.0Ah
It is similar to a car fuel tank. Voltage is how much fuel is being used, and mAh is the size of the tank. The bigger the fuel tank (mAh) rating the longer the device will run. If your battery is rechargeable, then think of the fuel tank as refillable (rechargeable).
 ampere-hour

(Mathematics & Measurements / Units) a practical unit of quantity of electricity; the quantity that flows in one hour through a conductor carrying a current of 1 ampere. 1 ampere-hour is equivalent to 3600 coulombs Abbreviation a.h
 
 am·pere-hour (mpîr-our)
 A.h. or amp hr
The electric charge transferred past a specified circuit point by a current of one ampere in one hour.
 The table below shows the battery volume for cell sizes AAA, AA, C, and D. The volume does not change with the type of battery, but the weight does, as shown here.
Battery
Cell Size
Volume
ml
Alkaline
Weight g
Carbon-Zinc
Weight g
NiCad
Weight g
NiMH
Weight g
Lithium Ion
Weight g
AAA 3.9 12 9.7 11 12  
AA 8.3 24 19 29 26 24
C 26.5 65 48 85 82  
D 55.8 135 98 200 170  

Energy storage in AAA batteries



Battery
Type
Avg. voltage
During discharge
milli-Amp
hours (mAh)
Watt-hours
Wh
Joules
J
Alkaline
Long-life
1.225 1150 1.41 5071
Carbon-zinc 1.1 320 0.35 1268
Nickel-Cadmium 1.2 300 0.36 1296
NiMH 1.2 800 0.96 3456

Energy storage in AA batteries



Battery
Type
Avg. voltage
During discharge
milli-Amp
hours (mAh)
Watt-hours
Wh
Joules
J
Alkaline
Long-life
1.225 2122 2.60 9360
Carbon-zinc 1.1 591 0.65 2340
Nickel-Cadmium 1.2 1000 1.20 4320
NiMH 1.2 2100 2.52 9072
Lithium Ion 3.6 853 3.1 11050

Energy storage in C batteries



Battery
Type
Avg. voltage
During discharge
milli-Amp
hours (mAh)
Watt-hours
Wh
Joules
J
Alkaline
Long-life
1.225 7800 9.56 34398
Carbon-zinc 1.1 2172 2.39 8600
Nickel-Cadmium 1.2 2500 3.00 10800
NiMH 1.2 4500 5.40 19440

Energy storage in D batteries



Battery
Type
Avg. voltage
During discharge
milli-Amp
hours (mAh)
Watt-hours
Wh
Joules
J
Alkaline
Long-life
1.225 17000 20.83 74970
Carbon-zinc 1.1 4733 5.21 18743
Nickel-Cadmium 1.2 5000 6.00 21600
NiMH 1.2 9500 11.40 41040

Lithium-Thionyl Chloride -- Special Mention

Although non-rechargeable,Lithium-Thionyl Chloride has the highest energy storage per kilogram of any battery type. Lithium-Thionyl Chloride cells do not supply high discharge currents, typically 1/10 the current of other cell types. However they last for 20 years, and withstand temperature extremes of -55 to +150 degrees C.

Lithium-Thionyl Chloride
(non-rechargeable)
Avg. voltage
During discharge
milli-Amp
hours (mAh)
Watt-hours
Wh
Joules
J
AA cells 3.6 2400 8.64 31104
C cells 3.6 8500 30.60 110160
D cells 3.6 19000 68.40 246240
Finally, if you would like a D-size battery that can supply 20 amperes of current for a short time (5 seconds), that will last for 10 years, check out the Ultracapacitor at Maxwell Technologies Inc. It is not really a battery at all, it is a 350 farad capacitor rated 2.5 volts. Yes, we said farads, not microfarads. As such it can store 277 Joules at 2.5 volts. This is not a great amount of energy storage, hence the limited discharge time. However, it can deliver more peak current than a lead-acid battery of the same size, and it recharges easily more than 500,000 times. The cost compares favorably with other D-size rechargeable batteries, but the cost per Watt-hour is not too good, $195.