Lead Acid Accumulator – Working Principles, Care and Uses
The e.m.f of a freshly charged lead acid accumulator is about 2.2 volts, and the specific gravity of the acid is about 1.25. When the cell is being discharged its e.m.f. falls rapidly to about 2 volts, and then becomes steady; but towards the end of the discharge the e.m.f begins to fall again. When the terminal voltage load has dropped below about 1.9 volts, or the specific gravity of the acid below about 1.15, the cell should be recharged. If the cell is discharged too far, or left in a discharged condition, hard lead sulphate forms on its plates, and it becomes useless.
The internal resistance of a lead acid accumulator, like that of any other cell, depends on the area and spacing of its plates. It is much lower than that of any primary cell, however, being usually of the order of 1/10 to 1/100 ohm. The amount of electricity which an accumulator can store is called its capacity. It is a vague quantity, but a particular accumulator may give, for example, 4 amperes for 20 hours before needing a recharge. The capacity of this accumulator would be 80 ampere-hours. (One ampere-hour = 3600 coulombs). If the accumulator were discharged faster – at 8 amperes, say – then it would probably need recharging after rather less than less than 10 hours; and if it were discharged more slowly – say at 2 amperes – it might hold out for more than 40 hours. The capacity of an accumulator therefore depends on its rate of discharge; it is usually specified at the ’10-hour’ or ‘20-hour’ rate. Discharging an accumulator faster than at about the 10- hour rate causes the active material to fall out of the plates.
Lead acid accumulators are usually charged at about the ‘8-hour’ rate – say 5 amperes for the cell discussed above. The charging is continued until gas is bubbling freely off the plates. When the plates are gassing, the chemical reactions (9) and (10) have been completed, and the current through the cell is simply decomposing the water in it. Before the charge is started the vent- plugs in the cell- case must be removed to let the gases out; the gases are hydrogen and oxygen and naked lights near are dangerous. The water lost at the end of each charge must be made up by pouring in distilled water until the acid rises to the level marked on the case. If the specific gravity of the acid is then less than 1.25, the charging must be continued. Near the end of charging, the back-e.m.f of the cell rises sharply to about 2.6 volts. It never gives a forward e.m.f. as great as this: as soon as it is put on discharged, its e.m.f. falls to about 2.2 volts.
Efficiency Of The Lead Acid Accumulator
The number of ampere-hours put into a lead acid accumulator on charge is greater than the number which can be got out of it without discharging it too far. The ratio
Ampere-hour on discharge divided by Ampere- hours on charge is called the ampere-hour efficiency of the cell; its value is commonly about 90 percent. However, to judge an accumulator by its ampere- hour efficiency is to flatter it; not only does it taken in more ampere-hours on charge than it gives out on discharge, but it take them in at a higher voltage. The electrical energy put into a lead acid accumulator on charge is the integral of current, e.m.f. and time:
W= S I Edt.
For simplicity we may say
Energy put in = quantity of electricity put in X average e.m.f on charge.
If the quantity of electricity is measured in ampere-hours, the energy is in watt – hours instead of joules. Similarly, on discharge,
Energy given out = quantity of electricity given out X average e.m.f. on discharge.
The energy efficiency of the cell is:
Energy given out on discharge divided by Energy taken in on charge
= amp-hours x average e.m.f. on discharge/ Amp-hours x average e.m.f. on charge
= amp-hour efficiency X average e.m.f. on discharge divided by average e.m.f. on charge
= amp- hour efficiency x 2.0/2.2
The energy efficiency is more often called the watt-hour efficiency of the cell; it is about 80 per cent.
Lead acid accumulator are robust, cheap to construct and for certain applications able to sustain large currents. They are also relatively light in weight.
A battery may be revived with Epsom salts or EDTA. However, a battery treated in this manner will not perform normally during its normal charge-discharge cycle.
Working Principles of Lead Acid Accumulator
A lead acid battery is a secondary electrochemical cell. It has been designed to store electrical energy and emf (a flow of electrons). Unlike other sources of alternative energy for vehicles, it is reversible. The overall reaction during discharge is: PbO2 + 2H2SO4 – PbSO4+Pb. The battery consists of six cells connected in series. Each cell gives a current of about 2 volts. The battery is packed in a thick case to prevent leakage of the corrosive sulfuric acid inside.
The first lead acid batteries were based on the invention of a French engineer named Plante who found that platinum or silver wires electrolysing saline water produced a current for a short period of time. This was not developed into a working battery however. It took a further French engineer by the name of Faure to develop the basic lead acid battery. He made a great deal of progress in making the battery more efficient by adding strips of lead oxide to the plates to speed up the forming process.
Battery plates have a varied design but all consist of some form of grid which is made up of lead and the active material. The grid is essential for conducting the electric current and for distributing the current evenly on the active material. If the current is not distributed evenly then the active material will loosen and fall out.
Despite the low weight-to-energy ratio and good durability, the lead-acid battery has many limitations. The lead content and the corrosive nature of the electrolyte limit its use to stationary and wheeled applications. The self-discharge rate is also among the highest of all rechargeable batteries and requires regular recharging to maintain its capacity.
Care and Maintenance of Lead Acid Accumulator
Lead acid batteries work by chemically interacting with the electrolyte solution within their cells. As a result, they generate energy that can be used to power materials handling equipment. The battery’s lifespan and performance depend on how well it is maintained. To maximize a lead acid battery’s life and performance, companies should follow a few maintenance best practices.
Batteries should be recharged as soon as possible after being discharged. The longer a battery stays in a discharged state, the more damage it will suffer. The battery will also become less effective over time. In addition, the battery will have a reduced lifespan due to sulfation. Using a charger with an automatic shut-off function helps prevent overcharging the battery.
Battery maintenance tasks include checking the voltage and specific gravity of the electrolyte solution with a voltmeter or hydrometer. A voltmeter will read the battery’s voltage, while a hydrometer will measure the concentration of sulfuric acid in the battery’s electrolyte solution. In order to get the most accurate reading, it is recommended that the multimeter be in a resting state before taking a voltage reading.
Checking the electrolyte level in wet-cell batteries is essential to proper battery maintenance. Workers should regularly inspect the electrolyte levels and top off the battery with distilled water. It is important to note that adding water to a fully discharged battery will cause the electrolyte to overflow.
In addition, companies should avoid deep cycling of lead acid batteries. This is because this can damage the positive plate grids. Furthermore, if a battery is discharged repeatedly, it may develop sulfation. While there are some commercial products that claim to reverse sulfation, these claims have not been independently verified. Ultimately, sulfation can be prevented by properly charging and maintaining batteries.
Applications of Lead Acid Accumulator
The lead acid accumulator is the most widely used heavy duty rechargeable battery. Students can investigate its operation by constructing a simple cell consisting of strips of lead and an electrolyte of dilute sulfuric acid. The cell can be charged for different lengths of time and then discharged through a light bulb. The duration of the bulb’s illumination can be measured, and a graph plotted to show the relationship between the electrical energy put into the cell and the electrical energy released.
It is important to point out that the electrode processes in a lead acid battery are reversible and the net reaction can be reversed by applying an external opposing e.m.f. greater than this battery’s e.m.f. This is why the battery is called a secondary cell, and it is also known as an accumulator or storage battery.
Lead itself is too soft to be used in battery cells, so small quantities of other metals are added to the lead to form various lead alloys. These are designed to reduce several disadvantages of lead, including its tendency to become brittle and its high specific gravity which affects the density of the cells and their ability to store energy.
The traditional “flooded” lead acid battery immerses the electrodes in liquid sulfuric acid, but newer types of sealed batteries (as well as recombinant fuel cells) use absorbed glass mats to separate the electrodes. Flooded batteries allow fluid to escape during stress charging and require more maintenance than sealed batteries. AGM batteries, on the other hand, can be charged and discharged without fear of leakage. However, all batteries should be charged in a well ventilated area because hydrogen gas can be produced during the process.
Uses of Lead Acid Accumulator
An electrochemical battery is a secondary cell that stores energy from an external source for later release during discharging. The lead acid accumulator (also known as the car battery) is the most familiar example of a heavy-duty rechargeable battery. The reaction within the battery is reversible, which allows the consumed reactants to be recycled for subsequent cycles of discharge and charging.
A lead-acid battery’s state of charge — the proportion of its capacity that remains available when charged — depends on how fast it is discharged and then recharged. Batteries that are left at low states of charge for long periods tend to develop large lead sulfate crystals that block the battery’s electrodes and prevent them from delivering current. This reduces battery capacity by an amount that correlates directly with the number of hours that the battery is at a low state of charge.
At the point when a battery’s sulfate crystals grow too big, they are converted to sulphur dioxide gas and sulfuric acid that is vented to the battery’s case and absorbed by the sponge-like mat of lead plates within. The sulphur dioxide then converts to water and the battery’s capacity is restored.
When a battery is charged too quickly, the reaction may take longer than it takes for lead sulfate to be converted to sulphur dioxide and the battery is ‘gassed’ — essentially blown apart by its own internal chemical processes. This raises safety concerns, reduces the battery’s capacity and can cause it to decompose internally.
A battery that is sealed to prevent the escape of hydrogen gas into the atmosphere is called a sealed lead acid or valve-regulated lead acid battery, or SLA or VRLA for short. Small packs of this type are used for UPS, emergency lighting and wheelchairs. Larger VRLA batteries are used as backup power for cellular repeater towers, Internet hubs, banks, hospitals and other high-availability applications. Some versions of this type are modified with additives such as antimony, tin or calcium to improve deep cycling performance and reduce maintenance requirements.
Recycling of the Lead Acid Accumulator
The lead and sulfuric acid in the accumulator are poisonous to humans and the environment. Overexposure can result in brain damage, kidney disease and other illnesses. This is why these batteries must be recycled rather than disposed of as trash or garbage.
Almost all battery retailers accept old lead-acid batteries for recycling, as required by state laws. Household hazardous waste facilities also take them, although you may need to call ahead to find out when the facility is open and if it charges fees for drop-off.
In the United States, more than 97 percent of these batteries are recycled to make new ones. This is the most successful battery recycling effort in history, and it has reduced the amount of lead that must be mined from the earth.
Reclaimers crush the batteries and separate their plastic components. The metal parts are melted to form ingots and used in the manufacture of new battery plates and posts. The plastic is reprocessed into new plastic products and sold to other industries.
A small fraction of the accumulators are recycled by hand, often in developing countries. These informal recycling operations are known to cause lead poisoning among workers in Africa. The symptoms are unspecific, and the exposures occur in areas where awareness of lead toxicity is low.
Fortunately, there are now many ways to recycle these batteries safely. This makes it much easier to ensure that they don’t end up in landfills, where they could leach lead into the ground and water. It also means that manufacturers don’t have to mine the raw materials for new batteries, reducing their carbon footprint. Moreover, it ensures that the toxic chemicals don’t seep into lakes and rivers, contaminating drinking water supplies and causing illnesses in children and adults.
