Topics Covered in Chapter 12 Carbon-Zinc Dry Cell The Voltaic Cell Alkaline Cell General Features of Batteries Lithium Cell
Lead-Acid Wet Cell Series and Parallel Cells Current Drain Depends on Load Resistance Internal Resistance of a Generator Topics Covered in Chapter 12 (continued)
Why the Terminal Voltage Drops with More Load Current Constant-Voltage and Constant-Current Sources Matching a Load Resistance to the Generator r i Topics Covered in Chapter 12 (continued)
Cells and batteries are available in a wide variety of types. 9 V battery AA 1.5 V cell AAA 1.5 V cell 12 V Battery (type 21/23)
The Voltaic Cell A voltaic cell consists of two different metal electrodes that are immersed in an electrolyte (an acid or a base). The current capacity increases with large electrode sizes. A primary cell cannot be recharged. A secondary cell, or storage cell, can be recharged.
Batteries Current and Voltage Ratings Batteries consist of two or more cells that are connected in series. Cells are connected in series to increase the voltage rating of the battery. Identical cells and identical batteries can be connected in parallel to increase current capacity and operating time.
Cell and Battery Capacity The Ah unit is amperes times 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? Time = = 10 hours Capacity Load current = 200 Ah 20 A The actual ampere-hours delivered varies with battery age and condition, temperature and discharge rate.
Carbon-Zinc Dry Cell One of the most popular primary cells. The negative electrode is made of zinc. The positive electrode is made of carbon. The output voltage of a single cell is about 1.5 V.
Lead-Acid Wet Cell A widely applied type of secondary cell (used extensively in vehicles). 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.
Discharge: the battery reacts by producing current flow in an external load circuit and produces lead sulfate and water. Charge: the battery reacts to a reverse current from an external energy source and produces lead, lead peroxide and sulfuric acid. The secondary batteries used in vehicles have a reversible chemical process. Pb + PbO 2 + 2H 2 SO 4 2PbSO 4 + 2H 2 O D C
H 2 SO 4 + H 2 O PbPbO 2 One cell of an automobile battery. + - discharge Pb + PbO 2 + 2H 2 SO 4 2PbSO 4 + 2H 2 O Specific gravity decreasing
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 positive terminal. Negative terminal to negative terminal. This process restores the battery’s ability to deliver current and voltage to a load.
H 2 SO 4 + H 2 O PbPbO 2 Charging an Automobile Battery (one cell shown). Charger produces 2.5 V (about 15 V for a 12 V battery) charge Pb + PbO 2 + 2H 2 SO 4 2PbSO 4 + 2H 2 O Specific gravity increasing
2 Ni(OH) 3 + Cd 2 Ni(OH) 2 + Cd(OH) 2 D C The electrolyte is potassium hydroxide but does not appear above as its function is to act as a conductor for the transfer of the hydroxyl (OH) ions. Nickel Cadmium Cells and Batteries
Constant-Voltage Generator A constant-voltage generator delivers a relatively constant output voltage in spite of changes in the amount of loading. The generator must have a very low internal resistance to maintain the constant voltage output.
A typical voltage source has a low internal resistance. riri 12 V 0.01 This one will provide a relatively constant voltage to loads of 0.1 or more.
12 V 0.01 Measuring r i riri V NL = 12 r i = 10 A V L = 11.9 V NL – V L ILIL 12 – 11.9 10 = = 0.01
Constant-Current Generator A constant-current generator delivers a relatively constant output current in spite of changes in the amount of loading. The generator must have a very high internal resistance to maintain the constant current output.
R 1 M 1 A A real current source has a high internal resistance. This one will provide a relatively constant current to loads of 100 k or less.
Generator Internal Resistance Internal resistance (r i ) causes the output voltage of a generator to drop as the amount of current increases. Matching the load resistance to the internal resistance of the generator causes the maximum power transfer from the generator to the load.
0 1 2 3 4 5 6 7 8 9 1 2 3 4 R L in k Power in R L in mW riri 10 V 5 k RLRL 5 Power Transfer The power curve peaks where R L = r i. 5 k 2 mA riri RLRL