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Note for Applied Chemistry - CHEM By vtu rangers

  • Applied Chemistry - CHEM
  • Note
  • Visvesvaraya Technological University Regional Center - VTU
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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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Volume power density is sometimes an important consideration where space is constrained. In reciprocating internal combustion engines, power density power per swept volume is an important metric. ❖ Power density is a measure of how much power can be extracted from a battery per unit of battery weight and is expressed in W/Kg. ENERGY EFFICIENCY defined as the ratio of useful energy output to the total energy input. %Energy efficiency = Energy released during discharge x 100 Energy required during recharge ❖ Energy efficiency is using less energy to provide the same service, There are other definitions is a good operational one. (Energy efficiency is not energy conservation) CYCLE LIFE The greater is the average depth of discharge, the shorter the cycle life. The number of discharge-charge cycles the battery can experience before it fails to meet specific performance criteria. Cycle life is estimated for specific charge and discharge conditions. The actual operating life of the battery is affected by the rate and depth of cycles and by other conditions such as temperature and humidity. The higher the (Depth of Discharge) DOD, the lower the cycle life. ❖ Primary battery is non-rechargeable but secondary battery is rechargeable. The Cycle Life is a measure of how many charge and discharge cycles a battery can take before the battery is expected to collapse. SHELF-LIFE (EXPIRATION DATE) Shelf-life is the period of time a battery can be stored without significant deterioration. Aging is subject to storage temperature and state of charge. While Page 5

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primary batteries have a shelf life up to 10 years, lithium- based batteries are good for 2 to 3 years, nickel – based batteries are good for 5 years, etc. as shown in the table.1 CLASSIFICATION OF BATTERIES: Batteries are classified as primary (non-rechargeable), secondary (rechargeable) and reserve (inactive until activated): Primary battery Primary batteries are not rechargeable, and consequently are discharged then disposed of, in which the electrolyte is not a liquid but a paste or similar. Cell is operated until the active components in the electrodes are exhausted. Generally primary batteries have a higher capacity and initial voltage than rechargeable batteries. Example: Dry cell. Zn-MnO2 Secondary battery Secondary (rechargeable) batteries can be recharged by applying a reverse current, as the electrochemical reaction is reversible. The original active materials at the two electrodes can be re-constituted chemically and structurally by the application of an electrical potential between the electrodes to “inject” energy. These batteries are able to discharge and recharge many times. Example: Lead acid battery, NiMH battery, Ni-Cd battery Reserve battery A reserve battery is special purpose primary battery designed for emergency short service life and for long term storage. They are not rechargeable. The electrolyte is usually stored separately from the electrodes which remain in a dry inactive state and is activated only when required by assembling its internal parts. Page 6

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