Batteries: types, characteristics and maintenance

Millions of batteries are used in America for automobiles, aircraft, portable lights, and emergency power installations. The ability to install, test, charge, and maintain storage batteries is an important asset to any well-qualified apprentice electrician.

CELLS

The basic unit of the battery is the cell (Figure 1). A battery is usually a group of separate cells connected in series. The number of cells used depends on the total voltage required.

Primary cells and secondary cells are types of cells widely used in the electrical field. Primary cells are commonly known as dry cells. This type can be used only once. When discharged, they are commonly discarded. The secondary or storage-type cell, when discharged, can be recharged by passing direct current through it in the proper direction.

Primary cells

Two common types of storage cells are the nickel-cadmium cell and the lead-acid cell. The lead-acid cell is used extensively.

TRADITIONAL BATTERIES

The internal features of the traditional lead-acid battery are shown in Figure 2. Two groups of coated lead plates, known as electrodes, are immersed in a dilute solution of sulfuric acid known as the electrolyte. One group of plates forms the positive electrode, whereas the other forms the negative electrode. Glass, rubber, or other insulating materials are used as separators to keep these electrodes from making contact. Each cell container is provided with a vent and vent cap. These devices permit gases to leave the cell while charging and the addition of distilled water that is lost by evaporation and during charging.

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Traditional storage battery

MAINTENANCE-FREE BATTERIES

Figure 3 illustrates a modern maintenance-free battery. This type of battery does not require the periodic addition of water to the electrolyte solution because of its design.

 An electrolyte reservoir eliminates the need for additional water, which is a feature not found in the traditional battery. The maintenance-free battery may be purchased with terminals located on the top or side to satisfy a variety of installation requirements.

Maintenance-free battery

BATTERY RATINGS

Storage batteries are rated for voltage and ampere-hour capacity. When each cell of a lead-acid storage battery is rated at 2.0 volts, with three cells connected in series (positive to negative), the total voltage of the battery is 6.0 volts. Higher voltage batteries contain more cells.

The current supplied by a storage battery depends on its voltage, physical condition, and the resistance of the load circuit.

The maximum current output is limited by the internal resistance of the cell. This resistance is determined by the condition of the electrolyte, the size of the plates, and the number of plates. Generally speaking, a larger cell is capable of supplying a higher current than a smaller cell. The voltage of a cell, however, is not dependent on the size or number of plates.

AMPERE-HOUR RATING

The time required to discharge a storage battery at a given load current is determined by its ampere-hour capacity. The ampere-hour rating is a measure of the total electrical energy the battery can deliver. The ampere-hour rating is a function of the size and number of plates in a battery. In general, a large battery has a high ampere-hour rating.

A battery rated at 100 ampere-hours will completely discharge in 100 hours at a rate of 1 ampere per hour, or in 50 hours at a rate of 2 amperes per hour. The number of hours a battery will last at a given load current can be determined from the following formula for ampere-hour capacity:

hours = ampere-hours / amperes

For example, how long will a fully charged battery deliver 10 amperes if it is rated at 60 ampere-hours?

hours = ampere-hours ÷ amperes

hours =  60 AH ÷ 10 A

hours = 6 h

STATE OF CHARGE

Discharging a lead-acid battery completely before recharging it is poor practice. A battery should be charged whenever its condition drops below the normal value. The condition of a battery, referred to as its state of charge, is measured by taking a reading of its specific gravity with a battery hydrometer. The student should have at least a general knowledge of the meaning of specific gravity to test a storage battery.

Specific Gravity

Specific gravity is the ratio of the weight of a volume of substance to the weight of an equal volume of fresh water. The equation that expresses this statement follows:

Specific Gravity = (Weight of a volume of substance) ÷ (Weight of an equal volume of fresh water)

For example, a pint of concentrated sulfuric acid weighs approximately 1.84 pounds. A pint of fresh water weighs approximately 1 pound. The specific gravity is determined as follows: 

Specific Gravity = 1.84 ÷ 1 = 1.84

The important part of a hydrometer, the instrument used to measure specific gravity, is the float on which a scale of specific gravities is marked. The float sinks in a liquid to a certain level, depending on the specific gravity. The lower the float sinks, the smaller the value of specific gravity. Therefore, in sulfuric acid, the float will sink until the surface of the liquid is at the 1.84 value.

LEAD-CELL ACTION

The liquid electrolyte in a fully charged storage cell is made up of sulfuric acid and water. When a cell discharges, acid leaves the electrolyte and combines with lead on the plates. As a result, the electrolyte becomes less dense and lower in specific gravity.

The specific gravity of a fully charged cell is approximately 1.28. A normally discharged cell has a specific gravity of 1.15. The decimal point is commonly omitted for convenience. Therefore, the numbers in this paragraph are usually referred to as 1,280 and 1,150.

BATTERY TESTING

The state of charge for a traditional battery is usually measured by opening a vent plug of the cell and drawing electrolyte into the barrel of the hydrometer (Figure 4). For maintenance-free batteries, the manifold cap is removed for hydrometer testing. The scale reading on the float at the level of the liquid is the specific gravity reading. 

 Hydrometer

A battery can also be tested with a high-current discharge tester. This is simply an ammeter combined with a load circuit. A high reading indicates a fully charged battery, and a low reading indicates a need for charging. The ammeter in this instrument is usually calibrated in terms of the state of charge. 

BATTERY CHARGING

A battery used for emergency power should be charged once a month or whenever its specific gravity falls to 1,150. Low specific gravity readings result from normal discharge or because the battery has been allowed to remain inactive. Completely discharged batteries must be recharged immediately. A permanent reduction of the ampere-hour capacity, due to hardening of chemicals on both electrodes, results from letting the battery stand discharged.

Charging Rate

The normal charging rate, in general, is the current specified on the nameplate or in the manufacturer’s literature. For a quick charge, a current value a few times higher than the normal value can be used if the temperature of the electrolyte is kept below 110°F.

Charging Current

Either DC or pulsating DC may be used to charge batteries. In either case, the direction of the charging current (electron movement) must be opposite to the current during discharge as shown in Figure 5(A). A charging current is produced by connecting the battery to a charger with electrical polarities as marked in Figure 5(B).

Battery current

The charging rate depends on the voltage difference between the battery voltage and the voltage of the charging source. In all instances, the voltage of the charger must be greater than the total battery voltage. If the charger voltage were lower than the battery voltage, the battery would discharge by driving electrons through the charger.

In engine-driven vehicles, batteries are charged by an alternator that is mounted in the vehicle. When a high-voltage DC supply is available, batteries may be charged directly from the source by using suitable current-limiting circuitry. When an AC supply is used, the voltage must be rectified, that is, changed to DC before being applied to the battery.

Charging Systems

Battery chargers operate on the constant-current or constant-potential system. In the constant-current system, the charging rate remains the same regardless of battery condition.

In a constant-potential system, the voltage of the charger is held constant at a value slightly above the battery voltage. As the battery charges, its voltage increases slightly, thus reducing the voltage differential between the battery and charger. The result is a high charging rate in the beginning and a low charging rate near the finish, in other words, a tapering charge. This is very desirable because the charging rate is dependent on battery condition.

BATTERY MAINTENANCE

The life of a lead-acid storage battery depends on the use to which it is put and on the care it receives. With good care, it will last several years; with little or no care, it may be ruined in a month. The important rules for battery care are as follows:

1. Test storage batteries periodically. Always wear eye and clothing protection to shield yourself from battery acid. 

2. If a battery is completely discharged, recharge it immediately. 

3. When charging a battery, select a charging rate consistent with the time available for charging. When time is available, use the normal rate indicated in the product manufacturer’s literature. 

4. If it is necessary to charge a battery at a very high rate, keep a careful check on the temperature of the electrolyte and never let it exceed 110°F. If cells release gas freely, reduce the charging rate to the normal rate. 

5. Never try to charge batteries to a definite specific gravity. Maintain the charge until the same specific gravity reading is indicated at three successive half-hour intervals. 

6. By the regular addition of distilled water only, maintain the level of the electrolyte above the top of the separators according to the manufacturer’s specifications. Rapid deterioration of a battery will result if the electrolyte level is allowed to remain below the top of the separators. Usually, maintenance-free batteries do not require the addition of water. 

7. Add distilled water immediately before recharging a lead-acid battery. In the process of charging a traditional battery, the water in the electrolyte is changed into hydrogen gas and oxygen gas that escape through the vent holes. This water must be restored so that the level of the electrolyte is maintained. Maintenance-free batteries do not experience this electrolyte loss. 

8. Never use a match to provide light when checking the electrolyte level. Hydrogen and oxygen mixed together are highly volatile. The area used for recharging must be well ventilated. 

9. Never disconnect the leads to a battery while it is on charge. The spark that occurs at the terminals may ignite the gas and cause an explosion. Many times, a battery is to be charged while permanently mounted in position, such as in an automobile, where the negative terminal may be connected to a frame or an engine. To reduce the chance of an explosion, the negative lead of the charger should be connected to the frame instead of to the terminal. 

10. Never take a specific gravity reading just after adding distilled water to a battery. Addition of distilled water dilutes the electrolyte and lowers the specific gravity. A reading then would indicate a state of charge below the actual condition of the battery. 

11. Avoid spilling electrolyte when testing a battery with a hydrometer. 

12. Never add acid or electrolyte to a battery unless it has been definitely determined that some electrolyte has been lost. If it is ever necessary to prepare electrolyte, remember that acid must be added to water, and must be added slowly. 

13. When placing a battery on charge, do not remove the vent plugs. The plugs prevent acid spray from reaching the top surface of the battery and allow the gases to escape as noted in number 7 previously. 

14. Remove deposits that may form on the terminals of a storage battery so that the metal will not be eaten away. The presence of a greenish-white deposit on battery terminals indicates corrosion. Remove this material by thoroughly cleaning the affected parts with a wire brush. Apply a strong solution of baking soda and water to all corroded parts to neutralize any acid that remains. Wash the battery with fresh water and dry with compressed air or a cloth. Finally, coat the terminals with petroleum jelly or other suitable material. 

15. Do not draw a heavy discharge current except for short intervals of time. If high current is needed for a long period, use additional batteries connected in parallel. 

16. Test storage batteries more frequently in very cold weather than in warm weather. A discharged battery freezes easily.