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Page 309

| crystallize and clog the pores of the plates, causing the battery to lose capacity and deteriorate and making it increasingly difficult to restore it to its normal condition. This procedure is not required for alkaline batteries. |

| Review. |

| 1. Control systems provide for the operation of relays, circuit breakers, pilot lights, alarms, recording meters, and other equipment and require a reliability of electric supply of the highest order; this is supplied by storage batteries. Control systems may operate at nominal voltages of 6, 12, 24, 48, or 120 volts, although 24-, 48-, and 120-volt systems are preferred. |

| 2. A storage battery is a device that may be used repeatedly for storing electrical energy at one time to use at another time. It does not directly store electrical energy as such, although energy is put into it in the form of direct-current electricity. Rather, the energy is stored as chemical energy by means of a chemical process. By a reversal of this process, the chemical energy is converted back to direct-current energy and delivered for utilization in this form. The process of putting energy into the battery is termed charging and of delivering energy, discharging. |

| 3. A storage battery consists of one or more cells connected in series, each cell having two plates (one positive and the other negative) immersed in a solution called the electrolyte (Fig. 15-1). When the battery discharges, the chemical composition of these three elements change; when it charges, they are restored to their original condition. The chemical reaction that takes place is different for each different type of battery. |

| 4. In the lead (or lead-acid) (Fig. 15-7), storage battery, the active material is lead on one plate and lead peroxide on the other, with sulphuric acid and water as the electrolyte. In the discharge, the materials change to lead sulphate and oxygen and hydrogen are developed; in charging, they return to their original state (Fig. 15-3), but some of the oxygen and hydrogen may escape as gases, constituting a possible explosive mixture. |

| 5. In the alkaline storage battery, the active materials are iron (or steel) on one plate and nickel dioxide on the other, with potassium hydroxide and water as the electrolyte. In the discharge, the materials change to iron oxide, nickel oxide and potassium; in charging, they return to their original state (Fig. 15-4). Since no gases are formed, the battery may be hermetically sealed. |

| 6. Batteries are rated in ampere-hours, usually based on the discharge rate, at a standard temperature of 25°C or 77°F. The capacities of all storage batteries decrease as the rate of discharge increases. |

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