chapter 12

• Battery


  A group of cells that generate energy from an internal chemical reaction.
• The cell consists of two different conducting materials as the electrodes that are immersed in an electrolyte.
• Electrolyte


  the solution that contains free positive and negative ions
• Electrode


  he contacting mechanism which combines the metallic parts to a non-metallic part in the circuit.
• The chemical reaction between the electrodes and electrolyte results in a separation of electric charges as ions and free electrons
• battery


  Provides a source of steady DC voltage of fixed polarity and is a good example of a generator or energy source.
• The voltage output and current rating of a battery are determined by several factors, including the type of elements used for the electrodes, the physical size of the electrodes, and the type of electrolyte
• there are two types of cell


  primary cell

  secondary cell
• Primary Cells


  This type cannot be recharged. After it has delivered its rated capacity, the primary cell must be discarded because the internal chemical reaction cannot be restored.
• Secondary Cells


  This type can be recharged because the chemical action is reversible
• Secondary Cells


  When it supplies current to a load resistance, the cell is discharging because the current tends to neutralize the separated charges at the electrodes
• secondary cell


  For the opposite case, the current can be reversed to re-form the electrodes as the chemical action is reversed. This action is charging the cel
• secondary cell


  The discharging and recharging is called cycling of the cell. Since a secondary cell can be recharged, it is also called a storage cell
• dry cells


  What we call a dry cell really has a moist electrolyte. However, the electrolyte cannot be spilled and the cell can operate in any position
• Sealed Rechargeable Cells


  This type is a secondary cell that can be recharged, but it has a sealed electrolyte that cannot be refi lled. These cells are capable of charge and discharge in any position.
• the voltaic cell 


  This device for converting chemical energy into electric energy is a voltaic cell. It is also called a galvanic cell, named after Luigi Galvani (1737–1798)
• The current through the electrolyte is a motion of ion charges
• This action represents the work being done by the chemical reaction to generate the voltage across the output terminals
• internal resistance


  Any practical voltage source has internal resistance, indicated as ri , which limits the current it can deliver
• For a good cell, ri is very low, with typical values less than 1 V.
• The voltage output of a cell depends on the elements used for the electrodes and the electrolyte.
• electrochemical series or electromotive series, gives the relative activity in forming ion charges for some of the chemical elements
• carbon-zinc


  The carbon-zinc dry cell is a very common type because of its low cost. It is also called the Leclanché cell, named after its inventor
• The electrochemical system consists of a zinc anode and a manganese dioxide cathode in a moist electrolyte.
• carbon-zinc


  The electrolyte is a combination of ammonium chloride and zinc chloride dissolved in water
• In operation of the cell, the ammonia releases hydrogen gas which collects around the carbon electrode. This reaction is called polarization, and it can reduce the voltage output
• However, the manganese dioxide releases oxygen, which combines with the hydrogen to form water. The manganese dioxide functions as a depolarizer
• Carbon-zinc dry cells are generally designed for an operating temperature of 708F. Higher temperatures will enable the cell to provide greater output. .
• alkaline cell


  It is available as either a primary or a secondary cell, but the primary type is more common
• The alkaline cell has many applications because of its ability to work at high effi ciency with continuous, high discharge rates
• alkaline cells have a higher energy density and longer shelf life
• Depending on the application, an alkaline cell can provide up to seven times the service life of a carbon-zinc cell
• The outstanding performance of the alkaline cell is due to its low internal resistance. Its ri is low because of the dense cathode material, the large surface area of the anode in contact with the electrolyte, and the high conductivity of the electrolyte
• Zinc Chloride Cells


  This type is actually a modifi ed carbon-zinc cell
• The zinc chloride cell is often referred to as a heavy-duty type.
• zinc-chloride cells


  It can normally deliver more current over a longer period of time than the Leclanché cell
• chemical reaction in the zinc chloride cell consumes water along with the chemically active materials, so that the cell is nearly dry at the end of its useful life. As a result, liquid leakage is not a problem
• Mercury Cell


  Mercury cells are available as flat, round cylinders and miniature button shapes
• mercury cell


  The mercury cell is used where a relatively flat discharge characteristic is required with high current density.
• mercury cell


  Its internal resistance is low and essentially constant