CELLSPrepared and presented by Doren Nedrick*

CellsThe purpose of an electric cell is to Change chemical energy into electrical energy.There are two types of cells: primary and secondary cell. Primary cells are non rechargeable while secondary cells are rechargeable. The symbol for a cell is A group of cells is called a battery and the symbol for a battery is *

Understanding Fundamental TermsEMF Electromotive force is the voltage directly across a source eg. generator or battery.Potential Difference (PD) - The difference in potential between two points in a circuit eg. voltage across a resistor.Terminal Voltage The voltage at the point of connection in a circuit, eg. At a plug (receptacle)*

Parts of a CellIf two electrode plates of different conducting material (e.g. copper and zinc) are placed in a solution (the electrolyte) of salts, acid or alkaline, a voltage will appear between them. An electrolyte is a liquid which changes its composition when current passes through it. An electrolyte can be liquid called a wet cell or a paste/ jelly called a dry cell.*

Chemical Action of a CellIn a cell, electrolysis occurs causing the electrolyte to break down or ionize to form negative and positive ions. At the same time, chemical action causes the atoms in oneelectrode to ionize. This causes electrons to be deposited on the electrode. Positive ions move from the electrode into the electrolyte solution. When a material looses proton it becomes negatively charged. Therefore this creates a negative charge on the electrode and leaves the area near it positively charged.*

Chemical Action ContdA surplus of positively charged ions is deposited on the other electrode, and at the same time loosing electrons. This causes the electrode to have more positively charged ions. Therefore, this electrode becomes positively charged (Fig. b).The chemical action causes one electrode to become positive while the other becomes negative. The opposite charges cause a voltage between the electrodes

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Chemical Action of a Cell*

Simple Leclanche Cell A simple leclanche cell consists of four components: (a) Glass container(b) Electrolyte (Sal ammoniac). (c) Carbon Plate (+ ve plate)(d) Zinc Plate (- ve Plate)The voltage between the plates of a leclance cell, without the connection of an external resistance , is 1.5V. This is termed the open circuit voltage, or electromotive force (e.m.f.) of the cell. The symbol for e.m.f. is E.*

Faults of the Leclanche CellThe simple leclanche cell has two main faults: Local action: The electrolyte attacks the impurities in the zinc, forming small cells and corroding the electrodePolarization: Bubbles of hydrogen gas (an insulator) form round the carbon electrode when the cell is in use and resist the flow of current.*

Improved Leclanche CellThe improved version of the leclanche cell contains two modifications:The zinc electrode (-ve) is coated with mercury to minimize local actionThe carbon plate (+ve) is surrounded with a depolarizing agent, which is contained in a porous pot*

Improved Leclanche Cell contdThe oxygen in the depolarizing agent combines with the hydrogen released at the carbon plate to form water (water consists of two parts of hydrogen and one part of oxygen). The depolarizing agent does not completely cure the effect of polarization. *

NICKEL CADMIUM CELL*

Nickel cadmium cell Designed for use in automobiles, in the secondary nickel cadmium dry cell, the electrolyte is potassium hydroxide. The anode is nickel hydroxide and the cathode is cadmium oxide. The open circuit voltage of nickel cadmium dry cell is 1.5V. These cells come in several sizes, including AA, C and D cells. The most common nickel cadmium batteries have voltages of 6, 9.6 or 12V. Nickel cadmium cells are rugged because they were originally intended as automobile cells, they work well under extreme conditions of shock, vibration and temperature. *

Mercury Cells The e.m.f. of a mercury cell typically varies between about 1.35 and 1.2 Volts in use. The anode is made from high purity zinc powder, and the cathode is a mixture of mercuric oxide and graphite; they are separated by an absorbent disc soaked in potassium hydroxide, a very strong alkali. A disadvantage is cost; mercury is not only toxic, it is expensive. *

Open and Closed Circuit VoltageEvery cell has an internal resistance due to the resistance of the plates and the electrolyte. This internal resistance (symbol r) becomes important when current flows through the cell. *

Cell calculationFigs. 9.3a and 9.3b show the cell off-load and on-load.*

Cell Calculation ContdThe voltage across the first cell is represented by the symbol E as this is the open circuit voltage. (E = 1.5V) In fig. 9.3b a resistor (R = 1) is connected across the external circuit of the cell and a current of 1A flows. But when a current of 1A flows through an internal resistance (r = 0.5) a voltage drop occurs.*

Cell Calculation ContdV (voltage drop across r) = I x r = 1A x 0.5 = 0.5VTherefore available voltage across terminals of cell on load: 1.5V 0.5V = 1VThis illustrates the difference between open circuit voltage (E) and closed circuit voltage (V). The open circuit voltage (E) is the voltage off-load. The closed circuit voltage (V) is the voltage on load.*

Closed Circuit VoltageThe closed circuit voltage (V) is often referred to as the potential difference (p.d.). The symbol is the same (V). Potential difference maybe defined as the voltage between two points when current is flowing.*

Calculation of internal ResistanceThe internal resistance of a cell can be calculated by subtracting the closed circuit voltage (V) from the open circuit voltage (E) and dividing the answer by the circuit current (I). r = E V I(E V gives the voltage drop due to internal resistance).*

Example What is the internal resistance of a cell of e.m.f. 1.5V and a p.d. 1V? The circuit current is 0.5A?r = E V I= 1.5V 1V = 0.5V = 1 0.5A 0.5A *

ExerciseA 9V battery has an internal resistance of 3. How much current will flow if the battery has a short circuit?A 6V battery delivers a current of 960mA to a 6 load. What is the internal resistance of the battery?A leclanche cell has an e.m.f. of 1.5V and a closed circuit voltage of 0.5V with a current of 1A flowing. Find the internal resistance of the cell.*

Cells in series*

Cells in Series rT = r1 + r2 + r3E (total e.m.f.) = E1 + E1 + E2

REMINDER: A battery is two or more cells connected together*

ExampleSix leclanche cells of e.m.f. 1.5V are connected in series and an external resistor of 6 is connected across the combination. Calculate (a) the total resistance of the battery and (b) the current through the 6 resistor. The internal resistance of each cell is 2.*

SolutionrT = 6 x 2 = 12RT = rT + R = 12 + 6 = 18IT = E/RT where ET = 1.5V x 6 = 9V = 9V/18 = 0.5A*

Cells in Parallel Cells connected in parallel must have similar voltages or current will flow from one to another. As cells are taken to have a similar internal resistance, where all cells are of equal voltage. *

Calculating Total Internal Resistance and E.M.F.rT = Resistance of 1 cell Number of cells rT = 0.2 = 0.04 5E = E of any one cell*

ExampleA circuit consists of four lead acid cells connected in parallel, each cell having an e.m.f. of 2V and an internal resistance of 0.02. Find (a) the total resistance and (b) the current which will flow in the external circuit if a resistor of 0.995 is connected across the combination.*

SolutionrT = 0.02 4 = 0.005RT = rT + R = 0.005 + 0.995 = 1

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ActivityWhat is the internal resistance of a cell which has an open circuit voltage (emf) of 2V and closed circuit voltage of 1V when a current of 5A is flowing?Three identical cells, each having a no load voltage of 1.5V, are connected in series to form a battery with a 21 load connected, the voltage available from the battery is 4.2V. Calculate the ri of each cell in the battery.

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ChallengeFour leclanche cells are joined, two in series and two such groups in parallel. The e.m.f. of each cell is 1.5V and internal resistance of 3. The combination is connected to an external resistance of 3. Find (a) the total internal resistance; (b) the total current; the current through each cell; and (d) the voltage across the external resistance.Ans.: a= 3; b= 0.5A; c= 0.25A; d= 1.5V*

Why are Cells Connected in series and Parallel?Series 1. Because it gives greater electromotive force.

Parallel 1. Gives increased current capacity2. Decreases internal resistance*

LEAD ACID CELLThe lead acid cell is made up of the following:(a) the container;(b) the plates;(c) separators;(d) the electrolyte.*

A More Practical Lead Acid Cell*

Construction of the Lead Acid CellThe container is usually made of glass or a composition (ebonite or plastic).The plates of a lead-acid cell consist of a set of lead grids (fig 9.7) into which the active material has been pressed. The active material in the positive plate is lead peroxide (PbO2). The negative plate consists of spongy lead. The plates are interleaved to increase their effective cross sectional area and minimise internal resistance.*

ContdThe separators are made of glass, or, more commonly, wood. (Wood sheets which have been made porous by the removal of the resin.) The electrolyte in a lead acid cell is a mixture of sulphuric acid and distilled water.*

DischargeThe sequence is as follows:The sulphur in the sulphuric acid combines with the lead on the plates to form lead sulphate. (This is termed sulphation.)The oxygen in the lead peroxide (positive plate) combines with the hydrogen in the electrolyte to form water. The electrolyte is therefore weakened and the specific gravity falls.*

Discharge CellIndications of discharged cell are as follows:1. Specific gravity low.2. Cell voltage low (e.m.f. 1.8V to 2V).3. Whitish deposit on plates (lead sulphate).To determine the level of charge of secondary cells a hydrometer (measures specific gravity) or a high rate discharge tester is used. *

Charge The cell is charged by connecting a d.c. supply to the battery. The charging current flows in the reverse direction to the discharge current and the chemical action is reversed. The sulphur leaves the plates and re-enters the electrolyteThe water content of the electrolyte decreases as the oxygen released from the electrolyte combines with the lead of the positive plate to form lead peroxide.*

Lead Acid Cell on charge and Discharge*

Recharging a batteryA cell (or battery) is recharged by connecting it to a battery charger in "like-to-like" polarity as shown in Fig. below. The charger is a rectifier circuit that can produce a variable output voltage. Many battery chargers have a voltmeter and an ammeter that show the charging voltage and current.*

Indications of a fully charged CellHigh and constant specific gravityCharging voltage SteadyCharging current steadyColour of plates: Positive chocolate brown; negative bright grey*

The HydrometerA hydrometer, shown in figure 2-12, is a glass syringe with a float inside it. The float is a hollow glass tube sealed at both ends and weighted at the bottom end, with a scale calibrated in specific gravity marked on its side. To test an electrolyte, draw it into the hydrometer using the suction bulb.*

The HydrometerWhat is the purpose of the hydrometer? Which electrolyte has more active ingredient? Electrolyte A, specific gravity 1.015? Electrolyte B, specific gravity 1.125? *

The High Rate Discharge Tester:this tester works by rapidly discharging a battery. This is a very dangerous test to carryout, and it is very hard on thebattery. If you can avoid carrying out this type of test and can carry out a no-load test instead, you should do so. With the high rate discharge test, the length of time the battery can maintain a sufficiently high voltage decides if the battery is serviceable or not.*

Capacity of a BatteryThe capacity of a battery is measured in ampere hours (Ah). If a battery has a capacity of 40 Ah it should give a current of 4A for 10 hours; this is termed the 10 hour rate.Low temperature has an adverse effect on a batterys efficiency.*

Maintenance of a Lead Acid BatteryThe lifespan of a lead acid battery can be lengthened by proper maintenance.1. Re-charge battery when voltage or specific gravity fall.2. Recharge battery regularly even if not in use.3. Do not add electrolyte to the battery unless there has been a spillage.*

Maintenance Contd4. Keep battery topped up with distilled water above plate level, otherwise air will attack the plates, causing hydration.5. Do not over charge, boost charge, or over-discharge a battery. This makes the plates shed (a common fault in car batteries)6. Keep the terminals free from corrosion by smearing with petroleum jelly.*

ApplicationThe lead acid cell is used in car batteries and stand by supplies.*

Alkaline CellsAlkaline cells can be either primary or secondary (rechargeable). Primary alkaline cells are similar to the secondary cells and have the same open-circuit voltage.The secondary alkaline cell was a major advance in portable energy sources. In this dry cell, the electrolyte is potassium hydroxide. The anode is zinc. The cathode is manganese dioxide.*

Alkaline Cells contdSecondary alkaline cells have an open-circuit voltage of 1.5 V. They commonly come in AA, C, and D sizes. They can be used as direct replacements for carbon-zinc cells of the same sizes. The most common secondary alkaline batteries have voltages of 4.5, 7.5, 13.5, and 15 V.

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Alkaline Cells contdAlkaline cells last longer than carbon-zinc cells of the same size when used in the same way. Both primary and secondary alkaline cells (and batteries) give good service when used with high current loads. For this reason, they are widely used as sources of energy in radios, television sets, tape recorders, cameras, and cordless appliances *

Comparison of lead acid Cell and Alkaline CellLead Acid Cell AdvantagesCheaperHigher voltage per cellLower internal resistance

DisadvantagesLacks mechanical StrengthSelf-DischargingPlates shed easily*

Alkaline CellAdvantagesVery robustNot self-dischargingCapable of withstanding heavy charging and discharging currentsLighter for given capacityLonger life*

DisadvantagesExpensiveRequires more cells per voltContainers are liveHigher internal resistanceMust be kept sealed as air attacks electrolyte*

Precautions Necessary Alkaline and acid batteries must be charged separatelyNeutralizing agents should be available in the event of burns or spillage (boric acid for alkalines and washing soda for sulphuric acid). *

CXC Past Paper Question, 08 Q.7*

*A mercury battery is a cell that stores and provides energy for a wide range of applications. Mercury batteries achieved a high degree of popularity due to their large storage capacity, long shelf life and stable voltage output. These batteries were commonly used in devices such as cameras and radios from their introduction in the 1940s through the mid 1990s when many nations chose to ban or limit the production of mercury batteries. Batteries containing small amounts of mercury are still manufactured for use in watches, hearing aids, and calculators today. Mercury is a toxic chemical that can affect and damage parts of the body such as the brain, nervous system and gastrointestinal tract of those who are exposed to it. Some forms of mercury are caustic and may cause damage to human and animal tissue if come into contact with. * A hydrometer, shown in figure 2-12, is a glass syringe with a float inside it. The float is a hollow glass tube sealed at both ends and weighted at the bottom end, with a scale calibrated in specific gravity marked on its side. To test an electrolyte, draw it into the hydrometer using the suction bulb. Draw enough electrolyte into the hydrometer to make the float rise. Do not draw in so much electrolyte that the float rises into the suction bulb. The float will rise to a point determined by the specific gravity of the electrolyte. If the electrolyte contains a large amount of active ingredient, its specific gravity will be relatively high. The float will rise higher than it would if the electrolyte contained only a small amount of active ingredient.* To read the hydrometer, hold it in a vertical position and read the scale at the point that surface of the electrolyte touches the float. Refer to the manufacturer's technical manual to determine whether or not the battery's specific gravity is within specifications. Note: Hydrometers should be flushed with fresh water after each use to prevent inaccurate readings. Storage battery hydrometers must not be used for any other purpose.*