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Note for Applied Chemistry - CHEM By vtu rangers

  • Applied Chemistry - CHEM
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  • Visvesvaraya Technological University Regional Center - VTU
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MODULE – I BATTERY TECHNOLOGY Introduction • A battery is a portable energy source with four basic components-an anode (the negative part), a cathode (the positive part), electrolyte and an separator. As current is drawn from the battery, electrons start to flow from the anode through the electrolyte, to the cathode. • A device enables the energy liberated in a chemical reaction to be converted directly into electricity. • The term battery originally implied a group of cells in a series or parallel arrangement, but now it is either a single cell or group of cells. • Examples: It ranges from small button cells used in electric watches to the lead acid batteries used for starting, lighting and ignition in vehicles with internal combustion engines. 1. The batteries are of great importance based on the ability of some electrochemical systems to store electrical energy supplied by the external source. Such batteries may be used for emergency power supplies, for driving electric vehicles, etc. Page 1

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2. For the commercial exploitation, it is important that a battery should provide a higher energy, power density along with long shelf life, low cost and compatible rechargeable units. Battery Characteristics Characteristics of a battery: 1) 2) 3) 4) 5) 6) 7) Voltage Current Capacity Energy Efficiency Storage Density Cycle Life Self Life Page 2

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1. VOLTAGE (CELL POTENTIAL) Voltage is commonly used as a short name for electrical potential difference .its corresponding SI unit is Volts. The voltage between the two electrodes is denoted by (VC-VA), can be directly measured by voltmeter. The theoretical standard cell voltage can be determined using Eo values: Eo (cathodic) – Eo (anodic) = Eo (cell) The theoretical cell voltage is modified by the Nernst equation, Ecell=Eocell+ (2.303RT)/nFlog[P/R] (P=product, R=reactant) 1. If the difference in the standard electrode potential is more, higher is the emf of the cell. 2. As the concentration of the product increases, the Ecell decreases marginally. 3. As the temperature increases, Ecell decreases. 2. CAPACITY The energy stored in a battery, called the battery capacity. Battery capacity is a measure of the charge stored by a battery, determined by the amount of electrical energy the battery can deliver over certain period and is measured in Ampere-hour (Ah), capable of being provided by a battery during discharge. (One Ah = current of one Ampere flowing for one hour). The theoretical capacity may be calculated using faradays relation, C = WnF/M, (where W is weight of the active material and M is the molecular mass of the active material respectively) it is the number of moles of the electro active material associated with the complete discharge of the cell. Page 3

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The practical capacity (Qp) is the actual number of coulombs (or Ah) of electrical charge delivered, it is always lower than the theoretical capacity. The variation of voltage during discharge is measured by plotting a graph of voltage vs. time, is the flatness of the curve. ❖ When the capacity of a battery falls below 60% to 80%, it should be discarded. CURRENT Current is a measure of how many electrons are flowing through a conductor. Current is usually measured in amperes (A). and is the amount of charge flowing per second. (Current: I = q / t, with units of A = C s-1) ❖ Current flow over time is defined as ampere-hours (a.k.a. amp-hours or Ah), a product of the average current and the amount of time it flowed. POWER DENSITY OR SPECIFIC POWER Power density (or volume power density or volume specific power) is the amount of power (time rate of energy transfer) per unit volume. In energy transformers including batteries, fuel cells, motors, etc., and also power supply units or similar, power density refers to a volume. It is then also called volume power density, which is expressed as W/m3. Page 4

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