Structure and Operation Mechanism of Lead-Acid Battery - Research Paper Example

LEAD –ACID BATTERY By Department Summary The report discusses the lead-acid battery in detail. The structure,mode of operation and its thermal properties are discussed in detail. The advantages and disadvantages of the battery are also listed in comparison to other types of batteries. Introduction The lead acid battery has been in existence from as early as the 1859 by Gaston Plante. It was the first battery that was rechargeable. Initially, it was mainly used for lighting rail rods and train stations but it was developed for domestic usage (Glaize and Genies, 76). Lead acid batteries can be classified into three categories: starter, industrial and deep cycle batteries. Starter batteries are mainly used in applications that require short bursts of high power and are, mainly used in vehicles. Deep cycle batteries tolerate deep discharge; hence produce steady power for a considerate amount of time. Industrial batteries produce low but steady power for longer periods (Leadacidbatteryinfo.org, 2015). Theory Structure of the lead ion battery Figure 1-Basic structure of a lead acid battery Source: (Radio-electronics.com, 2015) A normal lead acid battery consists of a combination of electro-chemical cells. The cells form the building blocks of the battery. The basic components of the cell are the anode, cathode, electrolyte and a separator. The anode consists of metal mesh that is pasted with lead oxide while the cathode is covered with lead. The anode, which is the positive electrode, receives electrons from the circuit when the cell is discharged. The cathode, which is the negative electrode, donates electrons while the electrolyte provides a mechanism for the charge to flow from the cathode to the anode. The electrolyte is usually a solution of sulphuric acid (30%) and water (70%). The separator ensures the electrodes are electrically isolated. The lead and lead and lead oxide grids are given a porous structure to increase the active surface. Due to positive grid corrosion, the thickness of the anode affects the lifetime of the battery; the thicker the plate, the longer the lifespan. Operation Mechanism Overall chemical reaction: PbO2 + Pb + 2H2SO4­­­­2PbSO4 + 2H2O At the cathode: Pb + SO42-PbSO4 + 2e- At anode: PbO2 + SO42- + 4H +2e-PbSO4 + 2H2O Thermodynamic Calculation of the battery Capacity. In the reaction Pb + PbO2 + 2H2SO4 + 2e = 2PbSO4 + 2H2O: Heat Content = 90500 cal. Battery Capacity, C = - (4.18* H)/ 3600 [E – T (dE/dT)] where; H = heat content E = voltage, T = temperature in Kelvins, and (dE/dT) = 0.0004 (mean value of 21 – 29% by weight sulphiric acid) (Crompton, 2000). If a battery has a Voltage of 2.5V at a temperature of 298K then C = - (4.18* 90500)/ 3600[2.5V – 298(0.00004)] = 105 Coulombs. Heat Radiation of a battery dQ/dt = where Stefan-Boltzmann constant (5.67*10-8),emission ratio of the material (0.95 for battery containers) is temperature in Kelvins. Example when T = 298K (Kiehne, 2003), dQ/dT = (5.67*10-8*0.95* 2984) = 424.78 W/m2 Heat flow by Conduction dQ/dT = f*(dT/d), where f = surface area in m2 specific heat conduction and T is temperature (Kiehne, 2003) . is 0.2 for plastic materials f = 1000 m2 dT = 0.005 K Source: (Kiehne, 2003) Table showing Heat Conductance of common materials. The Heat conduction for a wall thickness (d) of 4mm of the battery: dQ/dT = (0.2 * 1000/ 0.04) 0.005K = 25 W/m2 Discharging process The anode receives electrons from the external circuit. The electrons then react with active materials at the anode through the reduction reaction, which then continue the charge flow through the solution of the electrolyte. The lead oxide is converted to lead sulphate and absorps HSO4 and H+. Water is hence generated from the process. Oxidation occurs at the cathode. The lead material is oxidized to lead sulphate, absorbing HSO4- and releasing H+ in the process. (Erikdeman.de, 2015) As the discharge continues, the electrolyte loses the sulphur and the active materials reduce as they absorb lead sulphate. When the sulphur concentration decreases in the electrolyte, the chemical reactions slowly reduce and finally stop when the battery is incapable of supplying any further electrons (Dell & Rand, 103). Recharging process During the recharging process, the above process is reversed. The flow of electron is reversed to flow from the anode to the cathode; the chemical reactions are thus reversed and the active material is restored at the electrodes (Erikdeman.de, 2015). THERMAL PROPERTIES The battery capacity of a lead acid battery is dependent on the battery’s temperature; an increase in temperature increases the capacity. For example, if the rating is 49% at 250C, at -250C, the rating decreases to while at 500C the rating increases to 118% (Erikdeman.de, 2015). The figure below shows the relationship between temperature and the retention capacity of a lead acid battery. Figure 2- Relationship between temperature and the capacity of a lead acid battery At high temperatures, charging in a lead acid battery is limited. This is because increasing temperature reduces the gassing voltage up to a point where more charges go into gassing rather than charging the battery. If this continues over time, the oxygen is reduced and the temperature keeps increasing, thus thermal runaway occurs (PAVLOV, 179). The figure shows the relation between the charging efficiency and temperature. Figure 3-The relationship between charging efficiency and temperature Source: (Basytec.de, 2015) The self-discharge rate of a lead acid battery depends on its temperature. At high temperature, side reactions including gassing increase, resulting in higher self-discharge. An unused battery that is kept in a hot and moist environment will have almost double the self-discharge rate when it is finally used. Thus, unused lead acid batteries are to be kept in dry and cool environments (Erikdeman.de, 2015). The life span of the led acid battery is inversely proportional to temperatures; increasing temperature lowers its lifespan. Moreover, low temperature decreases the conductivity of the ionic conductors in the electrolyte. The reduction in conductivity reduces the performance of the battery. High temperatures increase the conductivity of the ionic conductors; hence the performance of the battery is increased. ADVANTAGES AND DISADVANTAGES They are simple and affordable in manufacture They are durable and stable compared to other batteries. They have a low self-discharge. Their discharge rate can be modified to suit the available needs. For example, there are three types of lead acid batteries for different requirements. They are environmentally friendly and can be recycled when they wear out. Disadvantages The battery cannot be stored in a discharged condition. CONCLUSION The lead acid battery has been in existence for over two centuries and it will continue to be used considering their superior qualities. Compared to other batteries, the lead acid battery is easier to use and produce, it is easily flexible in use and environmental friendly. Temperature changes affect the battery in different ways; increase in temperature increases the perfomance, self-discharge rate and reduce the lifespan. BIBLIOGRAPHY Basytec.de, (2015). Willkommen bei BaSyTec. [online] Available at: http://www.basytec.de/Literatur/temperature/DE_2002.htm [Accessed 22 May 2015]. DELL, R. M., & RAND, D. A. J. (2001). Understanding batteries. Cambridge, Royal Society of Chemistry. Erikdeman.de, (2015). Battery Charging and Maintenance. [online] Available at: http://www.erikdeman.de/html/sail080e.htm [Accessed 22 May 2015]. Glaize, C. and Genies, S. (2012). Lead and nickel electrochemical batteries. London: ISTE. Leadacidbatteryinfo.org, (2015). Lead-Acid Battery Info. [online] Available at: http://www.leadacidbatteryinfo.org/resources.htm [Accessed 22 May 2015]. PAVLOV, D. (2011). Lead-acid batteries science and technology : a handbook of lead-acid battery technology and its influence on the product. Amsterdam, Elsevier Science Ltd. http://www.books24x7.com/marc.asp?bookid=44707. Radio-electronics.com, (2015). Lead Acid Battery | Rechargeable Cells | Tutorial. [online] Available at: http://www.radio-electronics.com/info/power-management/battery-technology/lead-acid-battery-tutorial.php [Accessed 22 May 2015]. Kiehne, H. (2003). Battery technology handbook. New York: Marcel Dekker. Crompton, T. (2000). Battery reference book. Oxford, England: Newnes. Read More