Cells & batteries


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Cells & batteries

  1. 1. Energy & Environment
  2. 2. For electricity to flow, there has to besomething to push the electrons along. EMFThis is called an electromotiveforce (EMF) EMF is measured in volts A battery releases EMF to a circuit which induces current. A battery is a group of cells which transfer electrical energy from chemical energy
  3. 3. Because of the Electrochemical way a battery Deviceworks, the power supply is DC - - Chemical energy Electrical energyChemical reactionbetween electrodes Creates a voltage& electrolytes
  4. 4. Number of electrons = Number of protons29-18=11 29-36=-7 -ve charge as Cu 11+ +ve charge as electrons been Cu 7- electrons gained. been lost. 29x Protons18x Electrons 29x Protons 36x Electrons
  5. 5. Other cells areExample of a Wet made this way Cell using different electrodes and electrolytes Electrochemical Device + - Sulphuric acid Hydrogen & Zinc Sulphate Ions Copper Negative ions Positive ions (lots of electrons) (missing electrons)
  6. 6. Zn ZnNormally zinc contains many impuritiese.g. copper & iron Zn ZnThese react with the zinc like the carbon Fe Znelectrode CuThis takes ions away from the current Zn Znand electrical power is lost. Fe ZnThis is minimised by coating theelectrode in mercury or use pure zinc Znwithout impurities Zn Cu Zn Zn Zn Zn Zn Zn Zn Zn Fe Zn Zn Zn Cu Zn Zn Zn Zinc Zn Fe Zn Zn Zn Zn Zn Cu Zn Zn Zn
  7. 7. Cu H CuHydrogen ions are produced in the Cuchemical reaction between the electrodes Cuand electrolyte Cu Cu CuBecause of its charge it is attracted to the Cu Hcopper electrode Cu Cu H H Cu CuHydrogen is a bad conductor of electricity. HThis increases the resistance of the copperelectrode.
  8. 8. H H H H This can eventually stop the flow of electrons HThis is called polarizationThis can be reduced by using a depolarizer which converts theH back into H2O. Or the rods can be cleaned with a brush.
  9. 9. Electrolyte is a paste Positive electrode rather than liquid.Unlike a wet cell the Negative electrodebattery can work in any position Electrolyte Ammonium Suitable for small portable devices chloride Manganese oxide
  10. 10. Do you think a potato can act like a battery? How could we increase the voltage of this circuit?
  11. 11. Disposable batteries Used once and thrown awayUsed in:Portable device Devices away Devices onlywith low current from a power used source sometimesPrimary cells cannot be recharged as the chemical reactions arenot easily reversible
  12. 12. Rechargeable batteriesMust be charged before useRecharged by applying electric current, which reverses thechemical reactions that occur during its useThe oldest form of rechargeablebattery is the lead-acid batteryIt is a wet cell so must be keptupright and ventilatedHeavy & BigLow cost Used as carHigh current batteries
  13. 13. Made up of a series of identical cells eachcontaining sets of positive and negativeplatesThe voltage of lead acid cellis normally 2 voltsSo for a car battery that needs12volts 6 cells are added inseries and encased in a plasticbox
  14. 14. Should never run flat. Charge when 25% of charge leftShould regularly be charged even if not is use. The internalresistance will cause it to dischargeA battery can go completely flat without ever having been putinto serviceA battery should never be storeddirectly on the ground or on concrete.A wooden pallet is best as it does notconduct or allow damp paths but doesallow good air circulation
  15. 15. Fully Charged + -1. The colors of the positive & negative plates become dark brown & grey respectively2. The electrolyte has a relative density of 1.25 to 1.303. Plates form of bubbles4. The EMF or a cell is 2.0 voltsUncharged 1. - + plates become The colors of the positive & negative white due to formation of lead sulphate. 2. The electrolyte becomes dilute & its relative density lies below 1.18. 3. The e.m.f. of the cell becomes 1.8 volts.
  16. 16. The plates of flooded batteries must always be fully submergedin electrolyte.Fill battery with distilled or de-ionized water to cover the platesif low. Never add electrolyte.The frequency of watering depends on usage, charge method andoperating temperatureA new battery should be checked every few weeks to determinethe watering needFill water level to designated level aftercharging. Overfilling when the battery isempty can cause acid spillage.
  17. 17. Charge in a well-ventilated area. Hydrogen gas generated duringcharging is explosive.Do not charge at temperatures above 49°C (120°F).Over charging is bad for the batteryAlthough batteries can be recharged, deteriorationoccurs after each cycle
  18. 18. Now the hydrometer floats Once squeezed this results in a in the acid. The density of vacuum in the glass tube. the acid can then be read on the floating hydrometerAcid is then pushedinto the glass tubedue to atmosphericpressure. When the battery is fully charged the cell should have a relative density of 1.25 to 1.30. A reading of less than 1.18 means that recharging is necessary
  19. 19. Batteries can be very dangerous if not used correctly e.g.Recharging a primary batteryShort circuiting a batteryWhen a battery is recharged at anexcessive rateOverchargingDisposing of a battery in fire
  20. 20. EMF1. What is EMF?2. What type of energy is stored in a battery? What type of energy is this transferred to?3. What is the difference between a wet and dry cell?4. What can you do to a secondary battery that you cannot do to a primary battery?5. Should you top up the water of a lead acid battery before or after charging?