Assignment of Applied Chemistry

Submitted to: Mam
Submitted by: Farwa Jafar
Registration no.: 2014-EE-438

University of Engineering and Technology, Lahore(FSD campus)

Lead Acid Battery
Definition:
‘‘A lead-acid battery is a electrical storage device that uses a reversible chemical reaction to store energy’’.
It uses a combination of lead plates or grids and an electrolyte consisting of a diluted sulphuric acid to convert electrical energy into potential chemical energy and back again. The electrolyte of lead-acid batteries is hazardous to your health and may produce burns and other permanent damage if you come into contact with it. Thus, when dealing with electrolyte protect yourself appropriately
Lead acid battery diagram

Construction:
A battery consists of a combination of electro-chemical primitive cells. Cells are the building blocks of which batteries are constructed. A primitive cell normally consists of the following principal components:

* a positive electrode (anode) that receives electrons from the external circuit when the cell is discharged, * a negative electrode (cathode) that donates electrons to the external circuit as the cell discharges, * an electrolyte which provides a mechanism for charge to flow between positive and negative electrodes, and * a separator or spacer which electrically isolates the positive and negative electrodes. In some designs, physical distance between the electrodes provides the electrical isolation and the separator is not needed.

Since such a primitive cell will generate a voltage of about 2V, a battery will be assembled from a series connection of different primitive cells to obtain the required battery voltage. For commercial batteries, the components of the primitive cells and the cell connectors will be mounted and packed in a solid package for proper and save handling.

Machenical structure of lead acid battery:

The mechanical structure of lead-acid batteries consist of; * flat pasted plates (the electrodes) immersed in a pool of a sulphuric acid and water solution (the electrolyte). * The plates are packed together in a comb-like structure. * Two such plate sets (a positive and a negative) from the basic cell block of the battery. * Such a basic cell block thus consisting of a parallel connection of primitive cells. * The plates itself consist of a metal grid (current collector) used for the electrical connection, pasted with lead oxide (PbO2 for the positive plates or lead (Pb) for the negative plates). * This lead coverage is the chemical active layer of the cell. * In order to increase this active surface, the lead and lead oxide coverage is given a porous structure. * Plate thickness (of the positive plate) is related to lifetime of the battery because of a factor called "positive grid corrosion". Over time, the positive plate gets gradually eaten away, so eventually the complete material is consumed. The corroded material is deposed as sediment at the bottom of the battery. Thicker plates are directly related to longer life. * Staring batteries typically have plates about 1mm, while deep-cycle batteries may have plates of about 6mm thick.

A complete battery consists of a series connection of a number of basic cell blocks. A 12V battery will contain 6 basic cell blocks.
The electrolyte is a sulphuric acid - water solution (typically 30% acid -70% water at full charge).

Chemistry of the Lead-Acid Battery:
The heart of the lead-acid battery is a reversible electro-chemical reaction, that can produce a voltage difference between the involved metal electrodes.

The lead-acid battery is made up of plates (electrodes): * the negative electrode (cathode) is covered with lead (Pb) * and the positive electrode (anode) is covered with lead oxide (PbO2). * Both electrodes are immersed in a 35% sulphuric acid and 65% water solution (H2SO4 + H2O). In the electrolyte the sulphur-oxide crystals H2SO4 are split into free H+ and HSO4- ions.
Discharging:
During the discharge process, the positive electrode receives electrons from the external circuit. These electrons will react with the active materials of the positive electrode in a "reduction" reactions that continue the flow of charge through the electrolyte to the negative electrode. In this reduction reaction, PbO2 of the active material will be converted to PbSO4 absorbing HSO4- and H+ from the electrolyte and electrons from the external load. As a by-product water is produced.
At the negative electrode, an "oxidation" reactions between the active materials of the negative electrode and the charge flowing through the electrolyte results in surplus electrons that can be donated to the external load closing the electrical loop. In this oxidation reaction, the Pb active material is oxidized to PbSO4 absorbing HSO4- from the electrolyte and giving H+ ions to the electrolyte while loading electrons to the negative electrode.
Remember that the system is electrically closed: for every electron generated in an oxidation reaction at the negative electrode, there is an electron absorbed in a reduction reaction at the positive electrode. At the same time H+ ions are generated in the electrolyte at the negative electrode and absorbed at the positive electrode. So for the external electron flow through the load, there is an internal opposite H+ flow through the electrolyte. The electrolyte is always a part of the electrical circuit!
As the discharge process continues, the electrolyte loses sulphur and the active materials become depleted absorbing this sulphur as as PbSO4. As the sulphur concentration is the electrolyte decreases, the chemical reactions slow down until finally, the battery is no longer capable of supplying electrons to the external load. At this point the electrolyte is primary water and the battery is discharged.
Both electrodes are now covered with PbSO4. This PbSO4 tends to crystallize, building a hard crystalline coating, which is hard to break down and this may destroy the electrodes. Therefore discharged batteries should be recharged as soon as possible to avoid this sulphation of the electrodes. When the battery is recharged, the sulphur is returned to the electrolyte and the electrodes are chemically converted back into lead oxide (anode) and lead (cathode).
Recharging:
The chemical reactions in the lead-acid cell are reversible. By reversing the flow of electrons i.e. by putting electrical current in rather than taking it out, the chemical reactions are reversed to restore active material that had been depleted.
Chemical Reactions:
The scheme below shows the detailed chemical electrode reactions:

Discharge | Charge | Positive Electrode
Anode (+) | Negative Electrode
Cathode (-) | Positive Electrode
Anode (+) | Negative Electrode
Cathode (-) | PbO2 + HSO4- + 3H+ + 2e- PbSO4 + 2H2O | Pb + HSO4- PbSO4 + H+ + 2e- | PbSO4 + 2H2O PbO2 + HSO4- + 3H+ + 2e- | PbSO4 + H+ + 2e- Pb + HSO4- | Overall Cell Reaction | Overall Cell Reaction | Pb + PbO2 + 2H+ + 2HSO4- 2PbSO4 + 2H2O | 2PbSO4 + 2H2O Pb + PbO2 + 2H+ + 2HSO4- |

Electrical Characteristics
A battery is an accumulator for electric energy: it can be "loaded" (charged) with energy when e.g. the engine is running (alternator) and can later release the energy previously stored in it's chemical contents. The efficiency of this charge-discharge cycle depends on the quality of he electrolyte-electrode system of the battery and on the efficiency of the charger system.
Two parameters are commonly used to describe battery performance: voltage : voltage is the force driving each of the electrons coming out of a battery . capacity: capacity is the number of electrons that can be obtained from a battery.

The voltage of a battery cell is determined by the chemical materials used in it. The reduction and oxidation reactions mentioned above, each produce a fixed potential. The sum of the reduction and oxidation potentials determines the voltage of the cell.
Advantages:
* Inexpensive and simple to manufacture — in terms of cost per watt hours, the SLA is the least expensive. * Mature, reliable and well-understood technology — when used correctly, the SLA is durable and provides dependable service. * Low self-discharge —the self-discharge rate is among the lowest in rechargeable batterysystems. * Low maintenance requirements — no memory; no electrolyte to fill. * Capable of high discharge rates.

Limitations: * Cannot be stored in a discharged condition. * Low energy density — poor weight-to-energy density limits use to stationary and wheeled applications. * Allows only a limited number of full discharge cycles — well suited for standby applications that require only occasional deep discharges. * Environmentally unfriendly — the electrolyte and the lead content can cause environmental damage. * Transportation restrictions on flooded lead acid — there are environmental concerns regarding spillage in case of an accident. * Thermal runaway can occur with improper charging.
Applications
* Automotive and traction applications. * Standby/Back-up/Emergency power for electrical installations. * Submarines * UPS (Uninterruptible Power Supplies) * Lighting * High current drain applications. * Sealed battery types available for use in portable equipment. * Grid scale energy storage