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WEEK 03: GENERATION: ALTERNATIVE SOURCES: BATTERIES Sections: Operation | Primary vs Secondary | Types of Batteries Electrochemical Principles and Reactions. A battery is a device that converts the chemical energy contained in its active materials directly into electrical energy by means of an oxidation-reduction electrochemical reaction This type of reaction involves the transfer of electrons from one material to another. In a nonelectrochemical reaction, this transfer of electrons occurs directly and only heat is involved. In a battery, the negative electrode or anode is the component capable of giving up electrodes, being oxidized during the reaction. It is separated from the oxidizing material, which is the positive electrode or cathode, the component capable of accepting electrodes. The transfer of electrons takes place in the external electric circuit, connecting the two materials. Transfer of charge is completed within the electrolyte by movement of ions, not by electron flow. Components of Batteries The basic unit of the battery is the cell. A battery consists of one or more cells, connected in series or parallel depending on the desired output voltage and capacity. The cell consists of three major components: the anode (the reducing material or fuel), the cathode or oxidizing agent, and the electrolyte which provides the necessary internal ionic conductivity. These electrolytes are usually liquid, but some batteries employ solic electrolytes which are ionic conductors at their operating temperatures. In addition, practival cell design requires a separator material (which serves to separate the anode and cathode electrodes mechanically), electriclaly conducting grid structures or materials added to each electrode to reduce internal resistance, and suitable containers. Theoritical Cell Voltage and Capacity The theoritical capacity (ampere-hours) of a battery system is determined by its active materials. The maximum electrical energy (watthours) corresponds to the free-energy change of the reaction. The voltage is determined by the active materials selected, while the ampere-hour capacity is determined by the amount (weight) of available reactants. One gram-equivalent weight of material will supply 96,480 coulombs, or 26.805 Ah of electrical charge. The theoritical voltage and specific energy ratings of a number of electrochemical systems are given in the table below:
Values in parentheses are nominal Factors Influencing Voltage and Capacity are the following:
a) Voltage Level When a battery is discharged in use, its voltage is lower than the theoritical voltage due to 1) IR losses due to cell resistance; and 2) by polarization of the active materials during dicharge. Polarization is the losses or reduction in voltage from the ideal. Ampere-hour Capacity is equal to the number of Ampere-hour delivered divided by the battery weight or battery volume. Thus Energy Density is equal to the ah capacity x cell voltage. Sections: Operation | Primary vs Secondary | Types of Batteries Primary versus Secondary Batteries Primary Batteries have a nominal voltage from 0.85 to 3.50. Nominal specific energy is about 45 to 340 Wh/kg. Primary batteries are used in civilian, industrial and metering applications. They are relatively inexpensive, lightweight, good shelf life, high energy densities, offer little maintenance and ease of use. Secondary Batteries have a nominal voltage from 1.00 to 1.80. Nominal specific energy is about 40 to 150 Wh/kg. Secondary batteries are basically rechargeable, with high power densities, discharged at high rates, with flat discharged curves. They are applied in small hand-held devices and as energy storage devices. Reserved Batteries are experimental in nature. They have a nominal voltage from 1.50 to 1.60. Nominal specific energy is about 25 to 80 Wh/kg. High-Temperature Batteries are used pirmarily for military and space exploration use.. They have a nominal voltage from 1.20 to 2.40. Nominal specific energy is about 80 to 1000 Wh/kg. Sections: Operation | Primary vs Secondary | Types of Batteries Although batteries are classified into primary and secondary, batteries are further classified into four types, namely:
a) Automotive They are of moderate capacity, high rate of discharge, and operates at low temperature. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
