Batteries.ppt

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1、Batteries,Topics Covered in Chapter 12 12-1: Introduction to Batteries 12-2: The Voltaic Cell 12-3: Common Types of Primary Cells 12-4: Lead-Acid Wet Cell 12-5: Additional Types of Secondary Cells,Chapter,12, 2007 The McGraw-Hill Companies, Inc. All rights reserved.,Topics Covered in Chapter 12,12-6

2、: Series and Parallel Connected Cells 12-7: Current Drain Depends on Load Resistance 12-8: Internal Resistance of a Generator 12-9: Constant-Voltage and Constant-Current Sources 12-10: Matching a Load Resistance to the Generator ri,McGraw-Hill, 2007 The McGraw-Hill Companies, Inc. All rights reserve

3、d.,12-1: Introduction to Batteries,Batteries consist of two or more voltaic cells that are connected in series to provide a steady dc voltage at the batterys output terminals.The voltage is produced by a chemical reaction inside the cell. Electrodes are immersed in an electrolyte, which forces the e

4、lectric charge to separate in the form of ions and free electrons.,12-2: The Voltaic Cell (converts chemical energy into electric energy),A voltaic cell consists of two different metal electrodes that are immersed in an electrolyte (an acid or a base).The chemical reaction resulting from the immersi

5、on produces a separation of charges.The current capacity increases with large electrode sizes.The negative terminal is considered the anode of the cell because it forms positive ions in the electrolyte. The opposite terminal of the cell is its cathode.,The Voltaic Cell,Motion of electrons in ionic b

6、onding can be used to generate an electric current A device constructed to do just this is called a voltaic cell, or cell for short,12-1: Introduction to Batteries,A batterys voltage output and current rating are determined by The elements used for the electrodes. The size of the electrodes. The typ

7、e of electrolyte used.,12-1: Introduction to Batteries,Cells and batteries are available in a wide variety of types.,Fig. 12-1: Typical dry cells and batteries. These primary types cannot be recharged.,Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.,12-1: I

8、ntroduction to Batteries,Whether a battery may be recharged or not depends on the cells used to make up the battery. A primary cell cannot be recharged because the internal chemical reaction cannot be restored. A secondary cell, or storage cell, can be recharged because its chemical reaction is reve

9、rsible. Dry cells have a moist electrolyte that cannot be spilled. Sealed rechargeable cells are secondary cells that contain a sealed electrolyte that cannot be refilled.,12-3: Common Types of Primary Cells,There are several different types of primary cells in use today: Carbon-zinc dry cells. Alka

10、line cells. Zinc chloride cells. Mercury cells. Silver oxide cells.,12-3: Common Types of Primary Cells,Carbon-Zinc Dry Cell This is one of the most popular primary cells (often used for type AAA, AA, C, D). The negative electrode is made of zinc. The positive electrode is made of carbon. The output

11、 voltage of a single cell is about 1.5 V. Performance of the cell is better with intermittent operation.,12-3: Common Types of Primary Cells,Alkaline Cells The alkaline cell is another popular type also used for type AA, C, D, etc. It has the same 1.5V output as carbon-zinc cells, but they are longe

12、r-lasting. It consists of a zinc anode and manganese dioxide cathode in an alkaline electrolyte (potassium hydroxide). It works with high efficiency even with continuous use, due to low internal resistance.,12-3: Common Types of Primary Cells,Zinc Chloride Cells This cell is also referred to as a “h

13、eavy-duty” type battery. It is a modified zinc-carbon cell. It has little chance of liquid leakage because the cell consumes water along with the chemically active materials. The cell is usually dry at the end of its useful life.,12-3: Common Types of Primary Cells,Mercury Cells: This cell consists

14、of a zinc anode, mercury compound cathode, and potassium or sodium hydroxide electrolyte. It is becoming obsolete due to the hazards associated with proper disposal of mercury. Silver Oxide Cells: This cell consists of a zinc anode, silver oxide cathode, and potassium or sodium hydroxide electrolyte

15、. It is typically available as 1.5V, miniature button form. Applications include hearing aids, cameras, and watches.,12-3: Common Types of Primary Cells,Lithium Cells: This cell offers high output voltage, long shelf life, low weight, and small volume. It comes in two forms of 3V output in widesprea

16、d use: Lithium-sulfur dioxide (LiSO2). Lithium-thionyl chloride. LiSO2-type batteries contain methyl cyanide liquid solvent; if its container is punctured or cracked, it can release toxic vapors. Safe disposal of these cells is critical.,12-4: Lead-Acid Wet Cell,This cell is a widely applied type of

17、 secondary cell, used extensively in vehicles and other applications requiring high values of load current. The positive electrode is made of lead peroxide. The negative electrode is made of spongy lead metal. The electrolyte is sulfuric acid. The output is about 2.1 volts per cell. Cells are typica

18、lly used in series combinations of 3 (6-V battery) or 6 (12-V battery).,12-4: Lead-Acid Wet Cell,The secondary batteries used in vehicles have a reversible chemical process.,12-4: Lead-Acid Wet Cell,Current Ratings Lead-acid batteries are rated in terms of how much discharge current they can supply

19、for a specified amount of time.The Ah unit is amperes-hours. Generally, this rating is proportional to the physical size.,An automobile battery might have a 200 Ah rating. How long can this battery supply 20 amperes?The actual ampere-hours delivered varies with battery age and condition, temperature

20、 and discharge rate.,12-4: Lead-Acid Wet Cell,12-4: Lead-Acid Wet Cell,Specific Gravity Specific gravity is a ratio that compares the weight of a substance with the weight of water.The states of discharge (how much charge the battery has left) is checked by measuring the specific gravity of the elec

21、trolyte.,One cell of an automobile battery.Pb + PbO2 + 2H2SO4 2PbSO4 + 2H2O,12-4: Lead-Acid Wet Cell,12-4: Lead-Acid Wet Cell,Charging Lead-Acid Batteries Apply about 2.5 V per cell. Attach the terminal of a battery charger directly to the corresponding terminals of the battery. Positive terminal to

22、 positive terminal. Negative terminal to negative terminal. This process restores the batterys ability to deliver current and voltage to a load.,12-4: Lead-Acid Wet Cell,Charging an Automobile Battery (one cell shown).,Pb + PbO2 + 2H2SO4 2PbSO4 + 2H2O,As the cell,discharges, more,water is formed,low

23、ering the,specific gravity of,the electrolyte.,H,2,SO,4,+ H,2,O,Pb,PbO,2,H,2,SO,4,+ H,2,O,Pb,PbO,2,H,2,SO,4,+ H,2,O,Pb,PbO,2,charge,As the cell,discharges, more,water is formed,lowering the,specific gravity of,the electrolyte.,As the cell,discharges, more,water is formed,lowering the,specific gravit

24、y of,the electrolyte.,Charger produces 2.5 V (about 15 V for a 12 V battery),12-5: Additional Types of Secondary Cells,Nickel Cadmium (NiCd) Cells and Batteries This type of cell delivers high current. It can be recharged many times. It can be stored for long periods of time. Applications include Po

25、rtable power tools. Alarm systems. Portable radio and TV equipment.,12-5: Additional Types of Secondary Cells,Nickel Cadmium (NiCd) Cells and BatteriesThe electrolyte is potassium hydroxide (KOH) but does not appear above, as its function is to act as a conductor for the transfer of the hydroxyl (OH

26、) ions. Its specific gravity does not change with the state of charge.,12-5: Additional Types of Secondary Cells,Nickel-Metal-Hydride (NiMH) Cells These cells are used in applications demanding long-running battery performance (e.g., high-end portable electrical or electronic products like power too

27、ls). They offer 40% more capacity over a comparably-sized NiCd cell. They contain the same components as a NiCd cell, except for the negative electrode. They are more expensive than NiCd cells, self-discharge more rapidly, and cannot be cycled as frequently as NiCd cells.,12-5: Additional Types of S

28、econdary Cells,Nickel-Iron (Edison) Cells These cells were once used in industrial truck and railway applications. They are now almost obsolete due to lead-acid batteries. Nickel-Zinc Cells These cells were previously used in some railway applications. Their high energy density created interest in t

29、heir application to electric cars. They have limited life cycles for charging.,12-5: Additional Types of Secondary Cells,Fuel Cells A fuel cell is an electrochemical device that converts chemicals (such as hydrogen and oxygen) into water and produces electricity in the process.As long as the reactan

30、ts (H and O) are supplied to the fuel cell, it will continually produce electricity and never go dead, unlike conventional batteries.,12-5: Additional Types of Secondary Cells,Fuel Cells Fuel cells using methanol and oxygen are being developed. Fuel cells are used extensively in the space program as

31、 sources of dc power. They are very efficient; capable of providing hundreds of kilowatts of power.,12-5: Additional Types of Secondary Cells,Solar Cells Solar cells convert the suns light energy into electric energy.They are made of semiconductor materials.They are arranged in modules that are asse

32、mbled into a large solar array to produce the required power.,12-6: Series and Parallel Connected Cells,An applied voltage higher than the voltage of one cell can be obtained by connecting cells in series. The total voltage available across the battery of cells is equal to the sum of the individual

33、values for each cell. Parallel cells have the same voltage as one cell but have more current capacity. To provide a higher output voltage and more current capacity, cells can be connected in series-parallel combinations. The combination of cells is called a battery.,12-6: Series and Parallel Connect

34、ed Cells,Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.,Fig. 12-14: Cells connected in series for higher voltage. Current rating is the same as for one cell. (a) Wiring. (b) Schematic symbol for battery with three series cells. (c) Battery connected to lea

35、d resistance RL.,The current capacity of a battery with cells in series is the same as that for one cell because the same current flows through all series cells.,12-6: Series and Parallel Connected Cells,Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.,Fig.

36、12-15: Cells connected in parallel for higher current rating. (a) Wiring. (b) Schematic symbol for battery with three parallel cells. (c) Battery connected to lead resistance RL.,The parallel connection is equivalent to increasing the size of the electrodes and electrolyte, which increases the curre

37、nt capacity.,12-6: Series and Parallel Connected Cells,Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.,Fig. 12-16: Cells connected in series-parallel combinations. (a) Wiring two 3-V strings, each with two 1.5-V cells in series. (b) Wiring two 3-v strings i

38、n parallel.,To provide a higher output voltage and more current capacity, cells can be connected in series-parallel combination.,12-6: Series and Parallel Connected Cells,Fig. 12-16 cont. (c) Schematic symbol for the battery in (b) with output of 3 V. (d) Equivalent battery connected to lead resista

39、nce RL.,Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.,12-7: Current Drain Depends on Load Resistance,It is important to note the current rating of batteries, or any voltage source, is only a guide to typical values permissible for normal service life. The

40、 actual amount of current produced when the battery is connected to a load resistance is equal to: I = V/R by Ohms law.,12-7: Current Drain Depends on Load Resistance,Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.,Fig. 12-17: An example of how current drai

41、n from a battery used as a voltage source depends on R of the load resistance. Different values of I are shown for the same V of 1.5 V. (a) The V/R1 equals I of 200 mA. (b) The V/R2 equals I of 10 mA. (c) The V/R3 equals I of 600 mA.,I = V/R1 = 200 mA,I = V/R2 = 10 mA,I = V/R3 = 600 mA,A cell delive

42、rs less current with higher resistance in the load circuit.A cell can deliver a smaller load current for a longer time.,12-8: Internal Resistance of a Generator,A generator is any source that produces continuous voltage output.Internal resistance (ri) causes the output voltage of a generator to drop

43、 as the amount of current increases. All generators have internal resistance.Matching the load resistance to the internal resistance of the generator causes the maximum power transfer from the generator to the load.,12-8: Internal Resistance of a Generator,Measuring ri,Problem,Calculate internal res

44、istance of a generator if the output voltage drops from 50 V with zero load current to 45 V with 4 A of load current.,12-9: Constant-Voltage Source,Constant-Voltage Generator A constant-voltage generator has a very low internal resistance. It delivers a relatively constant output voltage in spite of

45、 changes in the amount of loading.,Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.,Fig. 12-21: Constant-voltage generator with low ri. The VL stays approximately the same 6 V as I varies with RL. (a) Circuit. (b) Graph for VL.,12-10: Matching a Load Resista

46、nce to the Generator ri,The power curve peaks where RL = ri. At this point, the generator transfers maximum power to the load. As RL increases, VL increases, I decreases, efficiency increases (less power lost in ri). As RL decreases, VL decreases, I increases. When ri = RL, maximum power yields 50%

47、efficiency. To achieve maximum voltage rather than power, RL should be as high as possible.,12-10: Matching a Load Resistance to the Generator ri,Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.,Fig. 12-24: Circuit for varying RL to match ri. (a) Schematic diagram. (b) Equivalent voltage divider for voltage output across RL. (c) Graph of power output PL for different values of RL.,Ri = 100 RL: variable from1 to 10, 000 ,ri = RL = 100 I = 200/200 I = 1 A,NOTE: PL is maximum when RL = ri = 100 ,