dc principles study unit conductors, insulators, and batteries · conductors, insulators, and...

DC PrinciplesStudy Unit

Conductors,Insulators, andBatteriesBy

Robert Cecci

You’ll start this study unit with an examination of the con-ductors that connect circuits together and the types ofinsulation used to cover conductors. Later in the text, you’lllearn about cells and batteries, the important storage devicesused as both power supplies and backup power sources formany industrial devices and systems.

Electronics Workbench is a registered trademark, property ofInteractive Image Technologies, Ltd., and is used with permission.

iii

Previe

wPrevie

wWhen you complete this study unit, you’ll beable to

• Describe the various types of conductors and discusstheir conductivity

• Explain the American Wire Gage system of sizing copperconductors

• Determine the size of conductor needed for an application

• Identify the various types of insulating materials and listtheir temperature ratings

• Explain the difference between a dry cell and a storagebattery

• Connect cells together to obtain more voltage, morecurrent, or more of both voltage and current

• Describe the proper safety precautions used when working with storage batteries

• Describe how to properly clean and care for storage batteries

• Discuss the instruments used for testing storage batteries

• Explain how NiCad, lithium, and other types of specialbatteries operate, and describe their ratings

You’ll see the symbol shown above at several locationsthroughout this study unit. This symbol is the logo ofElectronics Workbench, a computer-simulated electronics

laboratory. The appearance of this symbol in the text mar-gin signals that there’s an Electronics Work bench labexperiment associated with that section of the text. If yourprogram includes Electronics Workbench as a part of yourlearning experience, you’ll receive an experiment lab bookthat describes your Electronics Workbench assignments.When you see the symbol in the margin of your text, fol-low the accompanying instructions in the lab book tocom plete your Electronics Workbench assignment. If yourprogram doesn’t include Electronics Workbench, you maysimply ignore the symbol.

CONDUCTORS 1

What Is a Conductor?SuperconductorsConductor TypesWire Gage and AmpacityPractice Exercise: Wire Installation

WIRE INSULATION 11

Insulation Functions Insulation Types Voltage Ratings Testing Insulation

ELECTRIC CELLS AND BATTERIES 17

Cells and Batteries How Electric Cells Work Wet Cells Vs. Dry Cells Primary Cells and Secondary Cells Construction of a Dry Cell Connecting Dry Cells to Form Batteries Construction of a Wet Storage Cell Industrial Uses of Cells and Batteries Other Sources of DC Voltage

STORAGE BATTERIES 31

Storage Battery Construction Sealed Batteries Battery Straps and Cables Storage Battery Capacity Maintaining Storage Batteries Working Safely with Batteries

BATTERY TESTING AND CHARGING 42

Why Batteries Need Testing Specific Gravity Test Electrical Tests for Batteries Recharging a Storage Battery

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Contents

Contents

SPECIAL BATTERY TYPES 49

Nickel Cadmium (NiCad) Batteries Lithium Cells and Batteries Other Types of Batteries Environmental Responsibility

POWER CHECK ANSWERS 57

Contentsvi

1

CONDUCTORS

What Is a Conductor?

A conductor is any material that allows electrons to flowthrough it easily. The term conductor is also used to refer toobjects that are good conductors of electricity, such as cop-per wire. In order to be a conduc tor, a material must containatoms with loosely bonded electrons that can be removedeasily from the atoms.

Some examples of good conductors are copper, aluminum,silver, gold, and most other metals. Conductors are rated andcompared ac cording to several different qualities. One suchquality is conductivity. Conductivity is a measure of how eas-ily electric current passes through a material. Table 1 liststhe relative conductivities of several common metals used inelectric and electronic work.

No metal is a perfect conductor. All metals, even excellentconduc tors, offer some resistance to the flow of electricity.However, since silver has such a low resistance, it’s used as astandard of measure ment for the conductivity of other met-als. This is why silver is listed at 100 in Table 1. The rating of100 doesn’t mean that silver is a perfect conductor, but thatit’s the best conductor.

Table 1 lists copper as the second best conductor. Copper iscommonly used in industry for all types of conductors. Eventhough silver is a better conductor than copper, silver is sel-dom used in industrial ap plications because of its high cost.

Conductors, Insulators, andBatteries

Conductors, Insulators, and Batteries2

However, silver is used as a coating on relay contacts, circuitboard edge connectors, and other electronic parts whereextremely high conductivity is required.

Superconductors

As we’ve mentioned, no material is a perfect conductor.However, scientists have discovered that when certain metalsare cooled to ex tremely low temperatures, they lose much oftheir resistance to elec tricity. This reaction is called supercon-ductivity, and the metals are called superconductors.

Engineers and scientists are currently searching for a mate-rial that will exhibit superconductivity at ordinarytemperatures. If found, this material could be used in powertransmission lines, power service ca bles, and even on com-puter circuit boards where the lack of resistance wouldincrease the computer’s speed.

Table 1RELATIVE CONDUCTIVITY OF METALS AT 2O°C

Metal Conductivity Value

Silver 100

Copper 98

Gold 78

Aluminum 61

Zinc 30

Platinum 17

Iron 16

Lead 15

Tin 9

Nickel 7

Nichrome 2

Mercury 1

Conductors, Insulators, and Batteries 3

Conductor Types

In your work with industrial circuits, you’ll probablyencounter con ductors made from several different metals.Depending on the type of circuit you’re working on and itsage, you may see conductors made from copper, aluminum,or nickel chromium. Let’s take a look at each of these metalsin more detail.

Copper Wire

Probably the most common conductor you’ll find in indus-trial circuits is copper wire. Copper wire is inexpensive andcan carry large amounts of current with little loss due toresistance.

There are two basic types of copper wire: solid and stranded.Solid copper wire (Figure 1) is used in the construction ofpanel boards for control systems. Thin solid wire is appropri-ate for this application because it can easily be bent intoangles that retain their shape, thus allowing for neat con-struction. However, solid copper wires with large diametersaren’t commonly used in industry.

Stranded copper wire (Figure 2) is used in most electrical cir-cuits. Stranded wire is much more flexible than solid wire ofthe same size. Therefore, stranded wire is much easier toplace in conduit or wire ways.

FIGURE 1—Shown here is asolid copper wire with athermoplastic insulatingjacket.

FIGURE 2—Shown here is astranded copper wire witha thermoplastic insulatingjacket.


The flexibility of stranded copper wire depends on the totaldiameter of the wire and the number of strands contained init. The more strands the wire contains, the more flexible itis. Average stranded wire contains between 16 and 19strands inside its insulating jacket. In contrast, very flexiblewelding wire may have more than 80 strands in a 3⁄4-inchdiameter wire.

Copper wire is made from high-quality copper. The copper isfirst extruded or pulled through a forming die to give it itsdiameter. Then, the extruded copper wire is annealed.Annealing is a process that heats and then cools the wire tomake it less brittle. The wire can then be used as it is orcombined with other strands to make stranded wire.

Before the wire is enclosed in an insulating jacket, it’s oftenrun through a bath of molten solder. This process is calledtinning. Tin ning has two advantages: it makes the wire easierto solder and it helps prevent oxidation. Without a tin coat-ing, copper wire can oxi dize and obtain the green coatingyou’ve probably seen on statues and other copper objects.

Copper Strips and Copper Bars

Copper strips or bars are used as grounding conductors forindustrial equipment. The strip or bar is bolted to the equip-ment with either a copper bolt or a screw and lockwasher. Theother end of the strip or bar is bolted or screwed to a ground-ing rail. This grounding rail may also be copper, and it’s oftenconnected to a thick grounding wire that leads to the build-ing’s electrical service. Or, a grounding rail may be connectedto a grounding rod. A grounding rod is generally driven abouteight feet or more into the soil beneath the building, or is con -nected to the structural steel of the building.

Copper strips or bars are also used as conductors for highfrequency signals. These signals are typically used in indus-try for inductive heating.

Aluminum Conductors

Aluminum wire is a good conductor of electricity, but it’s verysoft and crushes easily under the pressure of the connectorsused on ser vice panels or disconnect switches. When the wire


is first crushed or flattened by the connector, a good contactis made. However, after a period of time, this connectionloosens and oxidizes. This produces heat that can furtherdestroy the connection.

When aluminum wire is used, an oxide-prevention com-pound should be applied to the wire before it’s placed intothe connector.

Aluminum wire was popular many years ago when copperwire was very expensive. However, the use of aluminum hasbecome very lim ited in recent years. Some aluminum wire isstill used for service en trance cables in homes and smallbusinesses.

Nickel Chromium Conductors

Nickel chromium or nichrome wire is a special type of wireused for power resistors and heating applications. This wireis silver-gray in color and is much stiffer than copper or alu-minum wire. Stainless steel terminals are used to connectnickel chromium wire to circuits.

Wire Gage and Ampacity

The size of conductor used in an electrical installation is veryimpor tant for electrical safety. Wire that’s too thin providestoo much resis tance to current, and as a result it can over-heat and cause short circuits (or even fires). For this reason,the conductor used in a circuit must be the proper size tohandle the maximum amount of current the circuit can carry.

The size of an electrical conductor is called its gage. Anunderstand ing of how wire is sized or gaged is very importantto an industrial electrician or electronics technician. In orderto properly wire or rewire a circuit, you must use the properwire type and size for the circuit.

WARNING: NEVER use a wire gage tool to measure a serv-ice entrance cable or a conductor in a live circuit. Aserious electrical shock hazard exists! This tool is onlyused to measure new conductors during installa tion, or tomeasure conductors in a disconnected circuit.


In the United States, wire sizes are gaged and numberedaccording to the American Wire Gage (AWG) system. Wiregage numbers are often printed or stamped directly on thewire’s insulating jacket. In the AWG system, the smaller thegage number, the thicker the wire. Thus, No. 8 wire is thickerthan No. 14 wire. The thicker the wire, the more current itcan carry without overheating.

Figure 3 shows an AWG wire gage measurement tool. To usethe tool, simply insert a wire into the slots until you find thesmallest slot that the wire fits into snugly. The number nearthat slot indicates the gage of the wire. In Figure 3, a piece ofsolid wire is being measured by the tool. The number nearthe slot indicates that the wire is gage No. 0. Note that thistool can be used to measure either solid or stranded wire.

The wire gage tool shown in Figure 3 can be used to measurewire sizes 0 through 33. Larger wire is gaged with sizes 00,000, and 0000. The largest available wires are gaged in MCM

FIGURE 3—This measurement tool is used to determine the gage of solid and stranded wire accordingto the American Wire Gage (AWG) system.


values. The abbrevia tion MCM stands for millions of circularmils. A mil is a unit of measure equal to 0.001 inch. A circularmil is a unit used to measure the area of a wire. It’s equal tothe cross-sectional area of the wire. Wires in gages 0 throughthe MCM range are used for service entrance cables.

The current-carrying capacity of wire is called ampacity. Awire’s gage can tell you the ampacity of the wire. In general,the thicker the wire, the greater its ampacity. Table 2 liststhe approximate maximum ampacity of several popular gagesof copper wire used in industry. (Note: The ampacities listedin Table 2 are based on the National Electric Code for cablescontaining no more than three wires covered with one of thefirst four rubber insulations shown in Table 4 and operatedin an ambient temperature of 30°C or less.)

Thin wire has more resistance than thicker wire, and for thisreason, thin wire can’t carry large amounts of current. Acomparison of wire gage and resistance is shown in Table 3.Table 3 contains wire gages, wire diameters in mils, wire areain circular mils, and wire resistance in ohms per 1,000 feetat 25°C. This temperature value is given be cause a conduc-tor’s resistance can change with temperature. (Resis tance islower when the wire is cold.)

Table 2

WIRE GAGE AND AMPACITY

AWG Wire Gage Ampacity (amperes)

16 10 A

14 15 A

12 20 A

10 30 A

8 45 A

6 65 A

4 85 A

2 115 A

1 130 A

0 150 A


In short sections of wire, resistance has little effect on theoperation of a circuit. However, when a piece of equipment islocated a long distance from the power source, a longer wirecan act like a resistor connected in series in the circuit. Thewire’s resistance can lower the voltage in the piece of equip-ment and cause problems. Therefore, when installing longruns of wiring, it’s best to use a thicker wire than you woulduse for a short run.

Wire is also given a voltage rating that indicates the maxi-mum volt age it can safely carry. The voltage rating variesdepending on the type of insulation used on the wire. We’lldiscuss wire insulation in more detail shortly.

Practice Exercise: Wire Installation

Now, let’s look at a practical example of wire installation.Suppose you’re installing the armature wiring in a DC motor.The wire runs from the controller to the motor’s connectionbox. The distance be tween the motor and the controller isabout 10 feet. The motor’s nameplate indicates that the

Table 3

WIRE GAGE AND RESISTANCE

AWG Wire Wire Diameter Wire Area Resistance

Gage (mils) (circular mils) (ohms per 1,000 feet at 25°C)

18 40.30 1,624 6.5100 �

16 50.82 2,583 4.0940 �

14 64.08 4,107 2.5750 �

12 80.81 6,530 1.6190 �

10 101.90 10,380 1.0180 �

8 128.50 16,510 0.6405 �

6 162.00 26,250 0.4028 �

4 204.30 41,740 0.2533 �

2 257.60 66,370 0.1593 �

1 289.30 83,690 0.1264 �


armature draws 8 amps at 90 VDC (volts, direct current).What size of wire should be used in the installation? (Hint:Use Table 2 and Table 3 to help you in your calculations.)

Answer: Looking at Table 2, you can see that #16 AWG wirecan carry 10 amps of current. Since the distance between thecontroller and the motor is relatively short in this example(only 10 feet) the AWG #16 wire is sufficient.

Now, take a few moments to check your learning by complet-ing Power Check 1.


Power Check 1At the end of each section of your Conductors, Insulators, and Batteries text, you’ll beasked to check your understanding of what you’ve just read by completing a “PowerCheck.” Writing the answers to these questions will help you review what you’ve learnedso far. Please complete Power Check 1 now.

Fill in the blanks in each of the following statements.

1. Which metal is a better conductor of electricity: aluminum or platinum?

2. A is a metal that loses most of its resistance to electricity when cooled to a very lowtemperature.

3. What is the resistance of a 1,000-foot length of #14 AWG wire?

4. If a system draws 25 amps of current, what size copper conductor should be used?

Check your answers with those on page 57.


WIRE INSULATION

Insulation Functions

Insulation is a protective coating or sheath applied to a con-ductor. In sulation’s most important function is to preventelectrical shorts from occurring between a conductor and agrounded object, or between two conductors. However, insu-lation also prevents wires from break ing during installationand protects wires from the harmful environ mental condi-tions present in industry.

Insulation Types

Many different types of insulation are applied to wires. Thetype of insulation chosen for a certain application depends onthe environ ment the wire will be used in. For example, anenvironment that’s very warm or that contains oils or corro-sive gases will quickly break down insulation and cause wirefailure. Special insulation materials have been developed foruse in these conditions. Although these insu lation materialsare expensive, they ultimately save money by protect ingequipment and preventing circuit failure.

Now, let’s look at some different types of insulation in moredetail.

Rubber Insulation

A wide variety of rubber insulating materials are used oninsulated wire. Table 4 lists the insulation types, the identify-ing letters that ap pear on the wire spools or insulatingjackets, and the maximum tem perature the insulation canwithstand.

Except for the two moisture-resistant insulators listed, allother rub ber insulators must be used in dry conditions. Notethat the tempera ture ranges listed are from 60°C to 90°C(140°F to 194°F).


Polyvinylchloride Insulation (PVC)

Polyvinylchloride insulation (PVC) is a type of plastic used tomake plastic pipe, children’s toys, and lawn furniture. PVCinsulation can withstand higher temperatures than rubberinsulation. The average temperature that PVC insulation canwithstand is 105°C (221°F).

Some types of cable are covered with PVC jackets. This typeof cable contains two or more PVC-insulated wires within itsouter jacket. Some types of cable also contain a groundedmetal shield to protect the cable from electrical interference.

The standard PVC jacket can be improved by adding an outercoating of nylon. The nylon outer coating improves the wire’sresistance to abrasion and increases the temperature thewire can withstand to about 115°C (239°F).

A special type of PVC jacket is made of irradiated PVC.Irradiation (deliberate exposure to radiation during manufac-turing) increases the temperature that the cable canwithstand to about 125°C (257°F).

Table 5 lists the different types of PVC insulation, the identi-fying letters for each, and the maximum temperature theinsulation can withstand.

In industry, most of the single wires used are the MTW,THHN, THW, or THWN types. These wires appear in circuitboxes and panel boards and are run through conduit.

Table 4

RUBBER INSULATION TYPES

Insulation Identifying Maximum

Type Letters Temperature

Heat-resistant rubber RH 75°C (167°F)

Heat-resistant rubber RHH 90°C (194°F)

Moisture and heat-resistant rubber RHW 75°C (167°F)

Heat-resistant latex rubber RUH 75°C (167°F)

Moisture-resistant latex rubber RUW 60°C (140°F)


Fluorinated Ethylene Propylene Insulation

Fluorinated ethylene-propylene (FEP) can withstand greatertempera tures than PVC insulation can. Standard FEP insula-tion with no outer covering can withstand temperatures150°C (302°F). If an outer coat ing of fiberglass braid is usedover the FEP insulator, the maximum operating temperatureincreases to 250°C (482°F). Another type of FEP insulationcalled extruded polytetra-fluoroethylene (TFE) is capable ofwithstanding 250°C (482°F) without an outer braid of fiber-glass. FEP insulation is often used for data transmissioncables where pro tection from heat is a necessity.

Special Insulation Types

Many other types of special insulation materials are used toprotect wires. Teflon® is used as an outer jacket on manytypes of wire and cable. Teflon® is a high-temperature insula-tor (up to 200°C or 392°F) and has good chemical resistance.In addition, a material called varnished cambric can be usedon single wires. Silicon and silicon mixtures are also used.Some very high-temperature wires use nickel chromium wirecovered with multiple layers of braided fiberglass. This type ofwire should be used in large industrial electric heaters.

Table 5

PVC INSULATION TYPES

Insulation Type Letter Temperature

Moisture-, heat-, and oil-resistant thermoplastic MTW 60oC–90°C (140oF–194°F)

Thermoplastic T 60°C (140°F)

Heat-resistant thermoplastic THHN 90°C (194°F)

Moisture- and heat-resistant thermoplastic THW 75°C (167°F)

Moisture- and heat-resistant thermoplastic THWN 75°C( 167°F)

Moisture-resistant thermoplastic TW 60°C (140°F)


Voltage Ratings

The voltage rating of a conductor depends upon the type ofinsulator used and the thickness of the insulator. Normally,wires such as MTW, THHN, THW, and THWN have a maxi-mum voltage rating of 600 V. This means that these wires aresuitable for 480 V circuits and other circuits up to 600 V.Beyond 600 V, the insulation can puncture due to the poten-tial difference between the conductor and the ground.

Typically, the voltage a conductor can safely carry is printedon the wire along with the wire’s gage and identifying letters.On other types of wires, such as those covered by fiberglassbraid, this informa tion is printed on a label on the spool.

Testing Insulation

The first test that should be performed on insulation is avisual test of the insulation’s integrity. Look for obvious signsof burn marks or for physical decay. Look for cuts or forchafing where the wire touched or scraped a sharp corner ofa connector body or wireway.

As wire ages, insulation can become dry and crack. This isespecially true of the rubber-based insulation commonly usedon power supply cords for machinery. Any sign of insulationdamage is cause for the replacement of the wire.

A special test device is used to check the quality of wire insu-lation. This test device is called a high-pot tester or a megohmmeter.

Figure 4 shows how wire insulation is tested with a high-pottester or a megohm meter. One of the test clips on the high-pot tester or megohm meter is connected to the wire or cableto be tested. The other test clip is connected to the systemground. The wire to be tested is now disconnected at bothends of the circuit.

Next, the testing device is energized, placing a very high volt-age on the wire. The device then measures the leakagecurrent or resistance between the wire and the ground. Anyimperfection in the insulation will show up as leakage cur-rent or a lower-than-normal resistance.


Now, pause for a moment to review what you’ve learned bycomplet ing Power Check 2.

FIGURE 4—A high-pottester or megohm metercan be used to test thequality of wire insulation.

WARNING: Caution is required when performing such atest because of the high voltages the test equipment uses.Personal injury is possible due to the high voltages. It’salso possible to puncture and destroy a wire if too muchvoltage is used during the test,


Power Check 2 1. What material is often placed over standard PVC insulation to improve the insulating quality?

2. What is the maximum operating temperature of type RUW wire?

3. What material is type RUW insulation made from?

4. Special test devices used to check the quality of wire insulation are .



ELECTRIC CELLS AND BATTERIES

Cells and Batteries

Electric cells and batteries convert chemical energy into elec-tric en ergy to produce a flow of electrons (a current). What’sthe difference between a cell and a battery? A single cell is abasic source of voltage and current; a battery is a group ofconnected cells. Cells are often mis takenly referred to as bat-teries; the common flashlight battery, for ex ample, is actuallya single cell.

As an electrician or electronic technician, you’ll need to knowabout the many different types of cells and batteries used inindustry. You’ll need to know how to maintain batteries andhow to recognize and troubleshoot problems when theyoccur. In the following sections of the text, we’ll examine thestructure, operation, and uses of cells and batteries in detail.

How Electric Cells Work

To understand how a basic electric cell works, look at thediagram in Figure 5. The cell shown is a container of elec-trolyte solution with two electrodes suspended in it. Theelectrolyte is a weak solution of acid, base, or salt in water.The electrodes are two strips of metal; the nega tive electrodeis lead and the positive electrode is lead dioxide. A chemicalreaction occurs between the metal electrodes and the elec -trolyte solution, and as a result, particles in the electrolytebreak down into ions. Ions are atoms that have lost or gainedelectrons.

The negative ions and the positive ions float in the electrolyte.Some ions have too many electrons and are negativelycharged. Other ions have too few electrons and are positivelycharged. The positive ions are attracted to the negative elec-trode and the negative ions to the positive electrode (Figure 5).The ions then combine, or react chemi cally, with the elec-trode substance and deposit their charges on the electrodes.

As one electrode takes on an excess of electrons, it becomesnegatively charged. At the same time, the other electrodeloses electrons and be comes positively charged. Note that thenegative and positive elec trodes are both connected to termi-nals outside the cell. That explains why a cell has twoterminals: the positive terminal (�) is connected to the posi-tive electrode, and the negative terminal (�) is connected tothe negative electrode.

When one electrode is negatively charged and the other ispositively charged, a voltage, or difference of potential, existsacross the elec trodes. So, when a conductor and a lamp areconnected between the two electrodes as shown in Figure 5,electrons flow between the two electrodes and the lamplights. Since electrons are negatively charged, current flowsfrom the negative electrode through the lamp to the positiveelectrode.

Any given cell generates a fixed voltage value over most of itsuse ful life. The amount of voltage generated across the elec-trodes in a cell depends on the materials the electrodes andthe electrolyte are made of. However, the voltage doesn’tdepend on the size of the cell and its parts.

FIGURE 5—In an electriccell, a chemical reactiontakes place between theelectrodes and the elec-trolyte solution. This actionproduces a voltage. Thearrows indicate the direc-tion of electron flow.



A cell’s electrodes and electrolyte change chemically as thecell pro duces current. After some time, the electrodes are sochanged that the cell can no longer supply current. Largecells that contain larger elec trodes and greater amounts ofelectrolyte will generally last longer and produce more cur-rent than smaller batteries. Some cells may be recharged andused over and over; others must be replaced when they runout of power.

Wet Cells Vs. Dry Cells

Cells can be classified as either wet cells or dry cells. Wetcells contain a liquid electrolyte (usually dilute sulfuric acid)and produce a little over 2 V. Wet cells are used in forklifts,material handling equipment, and in automobile storage batteries.

In contrast, dry cells contain an electrolyte in paste form. Inthe com mon flashlight cell, the electrolyte is ammonium chlo-ride paste. A typical dry cell produces about 1.5 V.

Dry cells are convenient to use, but they run out of powerquickly. In contrast, wet cell storage batteries require moremaintenance, but they can be recharged to provide years ofservice. Figure 6 shows both a dry cell and a wet cell storagebattery.

FIGURE 6—The cell shownin 6A is a dry cell, likethose used in flashlightsand transistor radios. Thebattery shown in Figure 6Bis a wet cell storage battery.


Primary Cells and Secondary Cells

Some cells use up their electric charge quickly and can’t becharged again. These cells are called primary cells. Primary cellsare thrown away after use. Most dry cells are primary cells.

In contrast, cells that can be recharged are called secondarycells or storage cells. The electrodes and the electrolyte inthese cells can be re charged or restored to their original stateof chemical activity. A cell is recharged by passing currentthrough the cell in the direction oppo site to the dischargecurrent.

Construction of a Dry Cell

Figure 7 shows the construction of a basic dry cell. The posi-tive elec trode is a carbon rod; the negative electrode is thezinc can. The elec trolyte is ammonium chloride, and anabsorbent lining surrounds the electrolyte paste inside thecan. A steel cover at the top protects the cell. Directly under-neath the steel cover is an expansion chamber that allows

FIGURE 7—Shown here is acutaway view of a typicaldry cell.


the electrolyte to expand (under certain conditions) withoutexploding the battery. A gasket insulates the steel cover fromthe negative electrode (the zinc can). A washer insulates thesteel cover from the positive electrode. The positive and nega-tive electrodes con nect to the binding posts of the positiveand negative terminals. To protect the zinc can from damageand corrosion, the can is shielded by a cardboard jacket onthe outside.

Connecting Dry Cells to Form Batteries

Recall that a battery is made up of a group of cells connectedtogether. Cells can be connected in series, in parallel, or inseries-parallel ar rangements depending on the needs of thecircuit. When cells are con nected in series, they produce ahigher voltage than just one cell can produce. When cells areconnected in parallel, they produce a greater current thanjust one cell can produce. When cells are connected in series-parallel, they provide higher voltage and higher current.

Connecting Dry Cells in Series

Figure 8A shows a single dry cell connected to a DC volt-meter, an in strument for measuring voltage. The voltmeterindicates that the out put voltage of this cell is 1.5 V.

Figure 8B shows five dry cells connected in series to form abattery. A voltmeter is then connected to the battery. The posi-tive terminal of the voltmeter connects to the positive terminalof the first cell. The negative terminal of the first cell connectsto the positive terminal of the second cell. The negative termi-nal of the second cell connects to the positive terminal of thethird cell, and so on. The negative termi nal of the fifth cell con-nects to the negative terminal of the voltmeter. The voltmeterindicates that the battery has a total voltage of 7.5 V.

When cells are connected in series to form a battery, the totalvoltage of the battery is equal to the sum of the voltages ofthe individual cells. In the battery shown in Figure 8B, eachof the five cells has a voltage of 1.5 V. So, the total voltage ofthe battery is equal to the sum of the five voltages, as follows:

1.5 V � 1.5 V � 1.5 V � 1.5 V � 1.5 V � 7.5 V

A fairly high voltage can be produced by connecting a num-ber of cells in series. However, a battery made up of cellsconnected in series can only supply as much current as anyone cell in the battery can pro duce. Thus, if a cell with a volt-age of 1.5 V has a current of 1 A (ampere), then a battery of7.5 volts also has a current of only 1 A.

Connecting Dry Cells in Parallel

Cells can also be connected in a parallel arrangement to forma battery (Figure 9). In a parallel arrangement, the positiveterminals of all five cells are connected together and to thepositive terminal of the ammeter. The negative terminals ofall five cells are connected together and to the negative termi-nal of the ammeter.

When cells are connected in parallel to form a battery, thetotal am perage of the battery is equal to the sum of theamperages of the indi vidual cells. (However, note that the


FIGURE 8—As shown in 8A,one dry cell has a voltageof 1.5 V. In 8B, five cellsare connected in series,and the total voltage of thebattery is equal to the sumof the individual voltages(7.5 V). Note how the terminals of the cells arelinked together in a posi-tive (�) to negative (�)chain.


total voltage of the battery is the same as the voltage of justone of the cells.) So, if each of the cells in Figure 10 can sup-ply 1 A, the total amperage of the battery is as follows:

1 A � 1 A � 1 A � 1 A � 1 A � 5 A

Thus, a battery of cells connected in parallel can supply ahigher cur rent than a battery connected in series. However, a battery connected in parallel has a lower voltage than abattery connected in series.

Connecting Dry Cells in Series-Parallel

Figure 10 shows a battery of cells connected in a series- parallel forma tion. This type of battery can supply both highcurrent and high volt age. The battery shown consists of twogroups of four cells connected in series (positive to negativeterminal connections). The two groups of series-connectedcells are then connected in parallel; note the positive-to- positive connection at one end of the group of cells and thenegative-to-negative connection at the other end. The nega-tive ter minals of the two cells on the left end of the battery

FIGURE 9—The cells in thisbattery are connected inparallel to provide high cur-rent. Note that the positiveterminals are connectedtogether and the negativeterminals are connectedtogether.


are connected to the negative terminal of the voltmeter. Thepositive terminals of the two cells on the right end of the bat-tery are connected to the positive terminal of the voltmeter.

Each cell in the battery has a voltage of 1.5 V and an amper-age of 1 A. The voltage of the entire battery is equal to thevoltage of one of the series-connected groups of four cells.Thus, the voltage is 6 V, as shown on the voltmeter (1.5 V �1.5 V � 1.5 V � 1.5 V � 6 V). The cur rent of one group offour series-connected cells is 1 A. Then, since the two groupsof four cells are connected in parallel, the total current of thebattery is equal to 2 A. The battery has twice the current-car-rying capacity of either series-connected group of cells.

Note that for practical uses, you’ll probably never have toconnect cells together to form a battery. Manufacturers pro-vide complete batteries in which the necessary connectionshave already been made for standard voltage ratings.

FIGURE 10—In 10A, twogroups of four cells areconnected in series. Then,in 10B, the cell groups areconnected in parallel. Aseries-parallel arrangementof cells makes it possiblefor a battery to provideboth high current and highvoltage to a circuit.


Construction of a Wet Storage Cell

Figure 11 shows the two basic parts of a wet storage cell—thepositive electrode and the negative electrode. Each electrode ismade up of a group of plates or grids. A group of plates islined up with spaces between the individual plates. Then,each group of individual plates is welded together and con-nected by a conducting plate strap. A cell terminal isconnected to each plate strap.

Both sets of plates are then sandwiched together to form acell. The two groups of plates are insulated from each otherby separators made of insulating material. The positive groupof plates (the positive electrode) is connected to the positivecell terminal. The negative group of plates (the negative elec-trode) is connected to the negative cell terminal.

Groups of plates are used to form the electrodes in a wetstorage cell because a group of plates can produce more cur-rent for a longer pe riod of time than a single rod or plate can.

CAUTION: Cells should never be connected in parallelwithout using protection circuits. Cells have a very lowinternal resistance, and a small voltage difference betweenthe cells can cause a large current to flow between thecells. If a large enough current flows between the cells,they could explode.

FIGURE 11—ln a wet storage cell, each electrodeconsists of a group ofplates or grids.


This is because a group of plates contains more of the chemi-cally active materials needed to produce current. The voltageof a typical wet cell is between 2.0 and 2.2 V.

A cutaway view of a wet cell is shown in Figure 12. For sim-plicity, the view shows only one positive plate and onenegative plate. The posi tive plate is made of lead peroxideand the negative plate is made of spongy lead. The plates areset in an electrolyte solution of sulfuric acid and water. Thetwo plates are separated by a separator made of insulatingmaterial.

The plates and electrolyte are contained in a molded plasticcase. The case cover is insulated from the posts (the cell ter-minals) by post gaskets. The vent cap at the top of the wetstorage cell can be removed so that water can be added tothe electrolyte if necessary during main tenance. A hole in thevent cap prevents the buildup of gas pressure when the cellis recharging or discharging.

FIGURE 12—This figureshows a cutaway view of awet storage cell.


Industrial Uses of Cells and Batteries

Cells and batteries are an important source of backup powerin many industrial plants. Batteries can provide power tointercom and paging systems, emergency telephone and light-ing systems, and to security locks and alarms if the mainsource of power fails. Batteries are also used to power material-handling machines such as electric fork trucks.Additionally, most industrial computer systems (such as pro -grammable controllers) contain batteries. These batteries areused to maintain computer information when the main ACpower service is turned off during shutdown periods or onweekends.

Other Sources of DC Voltage

While batteries are an important source of backup power inindustrial settings, direct current can be produced in a vari-ety of other ways for industrial use. You should be familiarwith some of these other DC power sources, so let’s take amoment to examine them.

The most common DC source is the power supply. A powersupply converts standard AC line voltages of 120 VAC or 240VAC to the low voltages needed for computers and electroniccircuits. Power supplies can produce positive and negativevoltages of 5, 10, 12, 15, or 24 VDC.

When larger amounts of direct current are needed, or when asource of DC voltage is needed for special applications, a DCgenerator (also called a dynamo) may be more economicalthan a power supply or a battery. DC generators producecurrent by converting magnetic energy into electric energy(Figure 13A). Small DC generators were used in older modelautomobiles to provide electricity for lights and bat tery charg-ing. Newer model automobiles use alternators or ACgenerators that contain their own built-in power supplies toprovide electricity.

Other DC voltage sources include devices that convertmechanical, light, and heat energy into electric energy. Figure13B shows a phono graph needle. In this device, pressure isapplied to certain crystals and ceramic materials, generating


a voltage. Thus, the device converts mechanical energy intoelectric energy, generating signal voltages to pro duce soundfrom a phonograph disk.

A more modern example of a device that converts mechanicalenergy to electricity is an accelerometer. An accelerometer is adevice used to detect and measure vibrations in industrialmachinery. An acceler ometer uses a crystal (much like theone in a phonograph needle) to create electricity.

An accelerometer contains a precision weight suspended insidethe sensor on special springs. When the sensor is applied to amachine bearing, the precision weight moves and excites thecrystal. The crystal is calibrated so that a force of 1 g will cre-ate a voltage of 1 V. (The g is a unit used to measure the forceof gravity exerted on an object when the object is moving. Oneg is equal to the force of gravity exerted on an object at rest.)

The voltage generated by the crystal is fed back to a hand-held ana lyzer or computer for analysis. If a machine bearingis defective, it will emit too much vibration, and the voltagegenerated by the accel erometer will fall in a certain range.

FIGURE 13—Shown hereare four different methodsof creating electric energy.


Figure 13C shows a photovoltaic cell. A photovoltaic cell (orsimply photocell) generates a voltage when exposed to light.These cells are used in photographic light meters and inindustrial automation equipment. They’re also used to con-trol the on/off operation of light ing systems for entrances,yards, and parking areas in many plants. Photocells havealso been used as power sources in space vehicles.

Figure 13D shows a device called a thermocouple. In a ther-mocouple, two different types of metals are mechanicallyjoined. Heating the junction of the two metals produces a DCvoltage across the output terminals of the thermocouple.

Now, take a few moments to review what you’ve learned bycomplet ing Power Check 3.


Power Check 3 1. What kind of cells can’t be recharged?

2. If a lamp is connected across the two electrodes of a cell, the electrons flow from the electrode to the electrode.

3. What kind of cells does a common 12 V automobile storage battery contain?

4. A simple dry cell contains a can and a rod.

5. To increase the current in a battery, connect the cells in a arrangement.

6. A battery containing three 1.5 V cells connected in series produces a voltage of .

7. A battery containing four 1 A cells connected in parallel produces a current of .

8. A cell generates a voltage when exposed to light.



STORAGE BATTERIES

Storage Battery Construction

A storage battery contains several wet storage cells placed ina plastic case. The storage battery shown in Figure 14 con-tains six cells, each capable of producing 2 V. The totalbattery voltage is 12 V.

The individual cells of a storage battery are housed in anacid-proof plastic or hard-rubber case to prevent electrolytefrom leaking through the case. The case contains separatemolded sections that hold the cells. Each cell section isribbed at the bottom so that the plates rest evenly. A covermade of acid-proof material is sealed in place at the top ofthe battery.

Figure 15 shows the parts of a storage battery in more detail.Figure 15A shows one cell; Figure 15B shows an entire bat-tery containing six cells. The individual cell sections, calledsediment chambers, provide space for the collection of mate-rial shed or dropped from the plates. Without the sedimentchambers, the material would collect between the cell plates,short the plates together, and destroy the chemical ac tion ofthe battery.

FIGURE 14—This figureshows a storage batterycontaining six 2-volt cells.The total voltage of thebattery is 12 V.


Once the cells are connected in series, the battery containsone posi tive (�) terminal at one end of the case and one neg-ative (�) terminal at the other end. To identify the batteryterminals, the positive termi nal is marked with a P or a plussign (�), or it may be painted red. The negative terminal ismarked with an N or a minus sign (�), or it may be paintedgreen (or some other color). If painted at the factory, the posi-tive terminal is always red.

After the cell connectors are welded in, the cell covers aresealed around the case with an acid-proof compound. Thismakes the joint leak-proof. The cell covers are fitted with

After the cells are placed inside the battery case, the manu-facturer connects the cells in series. The negative plate groupof one cell is connected to the positive plate group of the nextcell. The connections are made by welding heavy lead barscalled cell connectors to the cell terminals.

FIGURE 15—This figure shows the parts of a storage battery.

WARNING: Escaping gases from a charging or dischargingbattery is highly explosive hydrogen gas. Never operate astorage battery with out adequate ventilation.


caps or plugs. The caps allow an electrician or electronicstechnician to add water to the electrolyte as needed duringthe service life of the battery. Additional water may be neededin a battery because water evaporates from the electrolyte asa storage battery charges and discharges. Vent holes are alsopro vided in the caps to allow gases produced during thecharging and discharging of the battery to escape.

Cells in a typical storage battery are connected in series, asshown in Figure 16. The series connection produces a highervoltage output than a single cell can give. As with dry cells,storage cells can be con nected in parallel for a greater cur-rent output. They can also be con nected in series-parallel toprovide both high voltage and current output.

When cells are connected in series, the total battery voltage isequal to the sum of the voltages of the individual cells. Thebattery shown in Figure 16 contains six cells, and each cell iscapable of producing two volts. Therefore, the total battery volt-age is 12 volts (2 V � 2 V � 2 V � 2 V � 2 V � 2 V � 12 V).

Batteries made up of series-connected cells are common inplants. These batteries are used in forklifts, material-han-dling equipment, and lifting equipment. Typical combinationsof cells are 6 cells (12 V total); 12 cells (24 V total); 24 cells(48 V total); and 60 cells (120 V total).

Sealed Batteries

Sealed or maintenance-free batteries are commonly seen inindustrial settings. These batteries have no filler caps orplugs—they’re completely sealed except for a small vent hole.

A sealed battery is a lead-acid storage battery. A specialchemical so lution inside the battery produces only a verysmall amount of gas at normal charging voltages. Also,

FIGURE 16—Connectingsix cells in series makesa 12 V battery. Notethat the positive termi-nal of one cell isconnected to the nega-tive terminal of the nextcell, and soon.


special materials in the plates give the battery exceptionalability to withstand overcharging. Overcharg ing causes lossof water, but because the battery is sealed, the water doesn’tevaporate. The only maintenance a sealed battery requires iscleaning and recharging.

Battery Straps and Cables

Electrical equipment is usually connected to a battery bystraps or cables. A strap is made of wires braided or woventogether; a cable is made of many strands of wire twistedtogether. Cables are usually covered with insulation.Generally, straps and cables are covered by natural or syn-thetic rubber. Sometimes the rubber is wrapped in one ortwo layers of braided cotton or silk.

To connect a standard battery to a piece of equipment, aninsulated cable is connected from the positive battery termi-nal to the machine circuit. A ground strap is connected fromthe negative battery terminal to the ground or frame of themachine. A ground strap is usually not insulated.

A ground strap and two types of insulated battery cables areshown in Figure 17. All of the straps and cables are fittedwith clamps. The clamps are used to attach the cables to thebattery terminals. The clamps may be either the nut-and-bolttype or the spring-ring type. These clamps must be tightlyconnected to the battery terminals and free from corrosion inorder to prevent problems.

FIGURE 17—Shown here isa ground strap and twotypes of insulated batterycables.


Some car models contain side-terminal batteries. In these batteries, the terminals are located on the side of the case toprotect them from damage and also to help the battery fitinto the car assembly more easily. The cables attached tosuch a battery are insulated and are at tached to the batteryterminals by cap screws. These cap screws are permanentlysecured in the cable ends.

Equipment manufacturers provide cables and straps of theproper size to carry the required current for their batteries.When replacing a defective battery cable or strap, be sure toreplace it with a cable or strap of the same size and current-carrying capacity.

Storage Battery Capacity

The capacity of a storage battery is the amount of currentthe battery can produce. Capacity is usually stated inampere-hours. An ampere-hour is a current of one amperemaintained for one hour. To calculate the capacity of abattery in ampere-hours, multiply the number of amperesthe battery can carry by the number of hours the batterycan produce the current. For example, a storage batterythat’s capable of discharging 5 A of current continuouslyfor a period of 8 hours has a capacity of 40 ampere-hours(5 � 8 � 40). Similarly, a battery that can deliver a cur-rent of 10 A for 12 hours has a capacity of 120ampere-hours (10 � 12 � 120).

Maintaining Storage Batteries

Storage batteries require maintenance to keep them in goodcondi tion. Battery terminals, cables, and cable clamps mustbe kept as clean as possible. Water must sometimes beadded to the electrolyte. Also, batteries must be inspectedfor damage. Not only will a damaged bat tery fail to work, butit could also leak electrolyte or even explode un der certainconditions. Figure 18 shows some of the areas to be checkedon a storage battery.


Inspecting a Battery

Inspect battery cables for worn spots or fraying. The cableclamps must make good, tight contact with the battery termi-nals, and all con tacting surfaces should be free of dirt andcorrosion. Badly corroded cables should be replaced.

When tightening or loosening cable clamps, use a box-endwrench or a special battery-cable pliers. If a clamp sticks onthe terminal, don’t try to pry it off. Instead, use a clamppuller to remove the clamp (Figure 19). Clamp pullers come inmany varieties. They’re useful for removing corroded clamps,both the nut-and-bolt type and the spring-ring type. To usethe tool, insert the jaws of the tool under the clamp, and turnthe handle so that the center screw rests on the bat tery ter-minal. Further turning of the handle pries the clamp upwardwithout disturbing the battery terminal. Never hammer oruse extra force on a battery terminal or clamp. You mightbreak the battery cover and spill the electrolyte.

FIGURE 18—A poorly main-tained battery like this onewill soon fail completely.All the problems indicatedhere could have beenavoided by proper mainte-nance.


Cleaning Batteries

Batteries and battery cables should be kept as clean as pos-sible at all times. Use a dilute solution of water and ammoniaor water and bak ing soda to clean the top of the battery, thenflush the top with clean water. Cleaning removes dirt and thecorrosion caused by spilled electrolyte, and thus preventsleakage that can cause a battery to slowly discharge. Thistype of cleaning is especially important for bat teries that havetheir terminals on top.

When cleaning a battery, keep the cap on tight and plug thevents to prevent the cleaning solution from entering the bat-tery. Special wire brushes are available for cleaning batteryterminals and cable clamps.

Adding Water to a Battery

Keep the electrolyte in a battery at the manufacturer’s recom-mended level. If the electrolyte level is low, add just enoughdistilled water to bring the electrolyte up to the proper level.Never add too much wa ter to the battery. Figure 20 showshow an automatic cell filler can be used to add water to a cellin an industrial truck battery. Note that the worker in the fig-ure has neglected to wear rubber gloves, which shouldalways be worn when working with batteries.

FIGURE 19—A clamp pulleris a useful tool for remov-ing corroded cable clampsfrom battery terminals.


Figure 21 shows a method used in some plants to measurethe height of the electrolyte in a cell. Insert a glass tube likethe one shown into the cell until it touches the top of a plate.Then, close the upper end of the tube with a finger, and with-draw the tube. The height of the col umn of liquid in the tubeindicates the height of the electrolyte above the plate tops.

FIGURE 20—The electrolyte inindustrial truck batteries is usuallybrought up to the proper levelabout once a week. Seldom-usedbatteries may need replenishingonly once every 3 or 4 months.Water can be added to a batterywith an automatic cell filler shownhere. Rubber gloves, which thisworker has overlooked, should beworn when working with batteries.(Courtesy of Exide Power Sys temsDivision, ESB Inc.)

FIGURE 21—The method shown hereis used in some plants to measurethe height of the electrolyte in a cell.


Working Safely with Batteries

Before working with any battery, be sure to follow all safetyprecau tions. Batteries are dangerous. It’s very important toprevent the electro lyte in a battery from escaping. The elec-trolyte in a storage battery is sulfuric acid. This powerful acidcan eat through skin, cloth, metal, leather, and almost anythingelse. When handling batteries, be careful not to spill any elec-trolyte; it’s very dangerous to the eyes and can se riously burnthe skin.

To prevent electrolyte from coming in contact with your skin,always wear protective clothing when handling batteries(Figure 22). This special clothing protects your face, hands,and body from contact with acid.

If you should get electrolyte on your skin, go to your first-aidstation for immediate attention. If that isn’t possible, immedi-ately wash your skin with baking soda and rinse with a largequantity of cold water. If you get acid in your eyes, flush themwith plenty of clean water and get to a doctor as quickly asyou can. NEVER USE BAKING SODA IN YOUR EYES.

In addition to the safety precautions listed above, follow therules listed here to prevent serious accidents and injury.

1. Never smoke near a battery.

2. Never light matches or use lighters near batteries.

3. Never allow sparks from electrical devices or tools to getnear a battery.

4. Never use power tools near a battery. The brushes in thetools can create sparks inside the tool.

FIGURE 22—Wear this protective clothing whenhandling storage batteries.


5. Keep vent caps and plugs tightly in place at all times,except when adding water or when taking hydrometer orthermometer readings.

6. Avoid overcharging. Overcharging can result in an exces-sive formation of gas or a high electrolyte temperature.

7. Avoid overdischarging (discharging more than the ratedampere-hours).

8. Never add acid unless recommended by the manufac-turer.

9. Maintain good ventilation of the battery room, area, orcompart ment to prevent explosive gas mixtures frombuilding up and to keep down the electrolyte temperature.

10. Keep proper records of maintenance.

11. Keep all the external parts of a battery as clean and dry aspossi ble, and keep battery connections clean and tight.

12. Keep tools off the top of a battery.

Now, take a few moments to review what you’ve learned bycomplet ing Power Check 4.


Power Check 4 1. In a typical storage battery, the cells are connected in a arrangement to increase the

voltage output of the battery.

2. In a storage battery, the collects material shed from the cell plates.

3. The vent in a storage battery cap allows the escape of as the battery charges anddischarges.

4. What color is the positive terminal of a storage battery?

5. A storage battery that has no cap is called a battery.

6. Battery cables or straps must be replaced with cables of the same andcapacity as the old cables.

7. Overcharging a storage battery will cause the battery to lose .

8. A storage battery contains 24 2-volt cells. What is the total voltage output of the battery?



BATTERY TESTING AND CHARGING

Why Batteries Need Testing

Batteries need testing for three reasons: (1) to make sure thebattery is stable; (2) to see if the battery requires recharging;and (3) to see if the battery should be replaced. If a batteryfails to provide the needed voltage to run a piece of equip-ment, you can check the battery’s con dition by performingvarious tests. These tests include the specific gravity test,light-load test, high-rate discharge test, and full-chargehydrometer test.

Specific Gravity Test

Specific gravity is a measure of how much a given volume of asub stance weighs in comparison to the same volume ofwater. The spe cific gravity of water is 1.000. In comparison,concentrated sulfuric acid weighs 1.835 times as much asthe same volume of water. Thus, the specific gravity of sulfuricacid is 1.835.

A hydrometer is an instrument used to measure and comparethe rela tive weights of liquids (Figure 23). A hydrometermeasures the spe cific gravity of a liquid directly. It consists ofa glass tube with a weighted float inside. When liquid isdrawn up into the glass tube, the float rises or sinks accord-ing to the specific gravity of the liquid. The float is marked sothat the specific gravity can be read.

As a battery discharges, the sulfuric acid in the electrolytebegins to break down. When this happens, the electrolyte inthe battery becomes more watery, and the specific gravity ofthe electrolyte goes down.

A specific-gravity test can show how much a battery ischarged or discharged. In the test, a sample of electrolyte istaken from each bat tery cell with a hydrometer. The hydrom-eter will contain markings to indicate a “good” or “poor” rangefor the electrolyte. The hydrometer readings tell you whetherthe electrolyte has broken down and be come too watery.


In general, when the electrolyte in a battery is too weak tomaintain proper voltage after the battery has been charged,the battery must be replaced. Never attempt to add acid to acell unless the manufactur er’s directions indicate that this ispossible.

Remember, battery electrolyte contains dangerous sulfuricacid. Keep it away from your eyes, your skin, and yourclothes. Wear a safety shield or goggles when working aroundbatteries. Also, be very care ful to avoid dropping electrolyteon surfaces where it could cause damage to equipment.

Electrical Tests for Batteries

Several different electrical tests can be performed on a bat-tery to determine its condition and capacity. The first testyou should try is the open-circuit voltage test. In this test, avoltmeter is connected to a disconnected battery to test thevoltage. If the battery fails to produce any voltage in this test,it’s discharged and will need to be either recharged orreplaced.

A battery can pass the open-circuit voltage test but still failto produce the needed voltage when connected to a circuit. Inthis situation, the battery will need to be tested while con-nected to a load to determine the problem.

FIGURE 23—This figureshows a hydrometer, aninstrument used to deter-mine the specific gravity ofa liquid.


A light-load test can be performed next to determine a cell’sability to produce voltage under a light load. The instrumentused in this test is the cell analyzer or the cadmium-tip tester.

In a light-load test, the battery caps are removed. Next, thetips of the cell analyzer are used to probe inside the batteryand make contact with each of the cell connectors. Each ofthe battery’s cells are tested individually.

The cell analyzer places a voltmeter in parallel with the cellunder test. The cell analyzer also places a large-value resistorand a series-connected ammeter in parallel with the cell. Inthe light-load test, only one cell is tested at a time at a lowcurrent.

All the cells tested should display a steady reading of 2.0VDC, and each should display the same current. If one celldisplays a lower voltage or current than the others, the bat-tery should be replaced.

Another electrical test that can be performed is the high-ratedischarge test. In this test, a variable resistor (a rheostat) isconnected into the circuit along with a voltmeter and anammeter (Figure 24). The rheostat places a heavy load on thebattery, and the voltmeter and ammeter measure the batteryterminal voltage after 15 seconds of discharging. Note thatthe open-circuit voltage test and the high-rate discharge testare the only tests you can perform on a sealed or mainte-nance-free battery.

FIGURE 24—In the high-rate discharge test, avariable resistor isplaced across the bat-tery to provide a heavyload. The voltmeter andammeter measure thebattery voltage after 15seconds of discharging.


If a battery passes the specific gravity test but later fails inservice, you the full-charge hydrometer test to determine thebattery problem. Perform this test as follows:

1. Remove the battery from service. Adjust the electrolytelevel as necessary by adding distilled water.

2. Fully charge the battery at a slow rate.

3. Use the hydrometer to measure the specific gravity of theelectrolyte in each cell. Full-charge hydrometer readingsless than 1.230 indicate that the battery is defective andshould be replaced.

Remember that the specific gravity reading you get from thehydrometer will vary with the temperature of the electrolyte.A cold electrolyte has a higher specific gravity than warm orhot electrolyte. Therefore, always use the manufacturer’stemperature correction charts and measure the temperatureof the electrolyte when taking hydrometer readings.

Recharging a Storage Battery

When a battery delivers current to its load, the voltage at thebattery decreases. This is due to the chemical changesoccurring in the mate rial inside the battery. If a battery con-tinues in use, after a while it won’t be able to producecurrent anymore. At that point, the battery is said to be dis-charged. Restoring the battery to its original (maxi mum)current-producing ability is called recharging the battery.

When a battery is being recharged, it will at first accept a highrate of charge. This tapers off as the battery reaches full charge.In a car, re charging is done automatically by the alternatorwhen the engine is running. Emergency lights that switch onautomatically during a power failure contain built-in chargersto keep their batteries fully charged at all times. However, inmost industrial equipment and de vices that contain batteries, acharger must be connected to a battery to recharge it.

The two methods of recharging batteries are the slow-chargeand fast-charge methods. The slow-charge method requiresless current than the fast-charge method. The main differ-ence between the two meth ods, however, is the amount oftime needed for recharging.


Methods for handling large batteries for recharging are shownin Figure 25. Large industrial batteries (such as those in aforklift or other equipment) are usually lifted into place with ahoist, as shown in Figure 25A. These batteries are oftenremoved daily for charging. When a battery is removed forcharging, it may be replaced by an other battery so that thetruck can continue to be used. An industrial battery can alsobe charged while in place, as shown in Figure 25B. Thechargers are stacked with a space between them to permit aircirculation.

Note that when a storage battery is being charged, hydrogenand oxy gen gases form an explosive mixture under the cellcovers. The gase ous mixture then escapes through the bat-tery vents. However, if the battery is being heavily charged,the gas may accumulate near the top of the battery. This gascan present an explosion hazard if air circula tion is poor or ifsparks come in contact with the gas.

For this reason, great care must be taken to keep sparks andflames away from the battery. Make sure proper air circula-tion and ventila tion is provided in the area near the battery.

EMERGENCYLIGHT

CABLESCONNECTOR

CHARGER

CHARGER INDUSTRIALTRUCK

FIGURE 25—In 25A, a forklift battery is being removed for charging. (Courtesy of Exide PowerSystems Division, esb inc.) In 25B, the battery is being charged while still in the forklift. (Courtesy ofLead Industries Associates, Inc.)

(A) (B)


Also, never remove or connect battery cables while a batteryis charging or discharging—this could cause a dangerousspark.

When charging a battery, observe the following safety precau-tions both before and during charging to prevent batterydamage or injury to workers.

1. Check the cells and add distilled water, if necessary, tobring the electrolyte up to the proper level.

2. During charging, the temperature of the electrolyteshould never go beyond 52°C (125°F). The temperaturecan be moni tored by inserting a thermometer into thebattery through the cap hole.

3. Make sure proper air circulation and ventilation is pro-vided in the area where the battery is charging.

4. Keep all sparks or flames away from the battery.

5. Never connect or disconnect battery cables when a bat-tery is charging or discharging.

Now, stop for a moment and check what you’ve learned bycomplet ing Power Check 5.


Power Check 5 1. The test and the test are the only tests that can be made on a maintenance-

free battery.

2. If the specific gravity of a liquid is 1.28, is the liquid lighter than or heavier than water?

3. What instrument is used to measure the specific gravity of liquids?

4. When a battery has been in use for a long period of time and it can no longer de liver the electric current required, would you say that the battery is charged or discharged?

5. Before recharging any battery, the cells should be checked and should be added ifneeded to bring the electrolyte up to the proper level.

6. When charging a battery, good air circulation and ventilation must be provided to prevent agas .



SPECIAL BATTERY TYPES

Nickel Cadmium (NiCad) Batteries

The nickel cadmium battery or NiCad is a very popular type ofrechargeable battery used on many different types of indus-trial tools and systems. Almost all cordless orbattery-powered tools use NiCad battery packs containingtwo or more cells. Also, most of the com puter memory boardsin industrial controllers and personal computers use NiCadbatteries to maintain DC power to the memory circuits.

A single NiCad cell is shown in Figure 26. A NiCad cell ismade much like a capacitor. A positive plate and a negativeplate are kept apart by a separator that electrically isolatesthe plates and absorbs an alka line electrolyte. The positiveplate is made of nickel hydroxide, and the negative plate ismade of cadmium hydroxide.

The cell in Figure 26 is a size “D” cell that produces 1.2 VDC.It works by means of an internal chemical reaction. On dis-charge, the internal chemicals combine to create electricity.Once the chemicals are com bined completely, the reactionstops and the battery is dead. No fur ther voltage is produced.To recharge the battery, the charging system produces a cur-rent that flows in the opposite direction from normal currentflow. This causes the chemicals to separate back to theirorigi nal compounds.

More than 100 charging and discharging cycles can occurbefore a NiCad battery can no longer produce electricity.However, ultimately the internal chemicals and plates willdegrade, and small short cir cuits will occur between the plates.

FIGURE 26—Shown here isa simple NiCad cell.


NiCad cells are often connected together to make a batterypack. The battery pack is placed in a plastic container orhoused in a shrink wrap package. Two wire leads exit thepackage to be connected to the circuit. Or, the plastic con-tainer may have terminals that auto matically connect thebattery pack to the device to be powered.

Table 6 lists some of the types of NiCad batteries available,their cur rent output ranges in milliamp-hours (mAH), andtheir recharging times.

The current used to recharge a NiCad battery is very impor-tant. Too much current will heat up the battery or cell anddestroy it. In con trast, if not enough current is applied, thebattery will require an ex cessive amount of time to charge.

To determine the amount of time and current required tocharge a NiCad battery, it’s necessary to know the type ofbattery (standard or rapid charge) and the size of the batteryin AH (ampere-hours). From the size of the battery, you candetermine the battery’s C rate. The C rate of a battery is theamount of current the battery can deliver in one hour. Forexample, the C rate of a 1 AH battery is 1 A, and the C rateof a 5 AH battery is 5 A. Standard charge batteries are usually charged with a constant current of 1⁄10 of the C rate(0.1 � C). If the C rate is 1 A, then the charging current is0.1 A (0.1 � 1 A � 0.1 A). If the C rate is 5 A, the chargingcurrent is 0.5 A (0.1 � 5 A � 0.5 A).

Table 6

NICAD BATTERY TYPES, OUTPUT, AND CHARGING TIMES

Type Output Capacity Charging Time

Standard 10–5,000 mAH 15 hours

Rapid charge 300–2,000 mAH 1.5 hours

High temperature 4–130 mAH 48 hours

High capacity/rapid charge 600–2,000 mAH 1.5 hours

High capacity/ultrahigh charge 700–2,800 mAH 1.5 hours

High-rate discharge/rapid charge 1,400 mAH 1.5 hours


Since the charging current is 1⁄10 of the discharge current thebattery can deliver in one hour, it can be seen that the totaltime required to recharge the battery is ten hours. For exam-ple, a 1 AH battery charging at 0.1 C or 0.1 A requires tenhours to charge (0.1 A � 10 H � 1AH). In actual practice, thecharging time is usually increased by 50 percent becausesome of the energy that enters a battery during the chargingcycle is lost as heat. Therefore, standard charge batteriesneed to be charged at a current of 0.1 C for 15 hours.

A rapid charge battery is usually charged with a constantcurrent of 1.0 times the C rate. Therefore, a 1 AH battery ischarged with a current of 1 A in one hour, and a 5 AH bat-tery is charged with a current of 5 A in one hour. Again, theactual charging time must be increased by 50 percent toallow for heat loss. Therefore, rapid charge batteries arecharged at a current of 1.0 C for 1.5 hours.

WARNING: NiCad cells can be dangerous. A cell canexplode if it’s charged incorrectly or if you attempt to sol-der something to the termi nals. Therefore, always use theproper charging equipment, and never attempt to solder awire or other device to a NiCad cell. In addition, someindustrial NiCad battery packs contain internal tempera-ture sen sors that open the circuit to the battery pack ifexcessive tempera tures are detected in the cells. Be sureto keep NiCad cells away from excessive heat to preventexplosion.

The high-temperature NiCad battery has the slowest rechargerate. These batteries are used as back-up power supplies incomputer memory board circuits. A high-temperature batteryis charged when the computer system is on, and only pro-duces electricity when the system is turned off. Thesebatteries are disk-shaped and are mounted in a circularsocket on the printed circuit board.

Special battery-charging systems are available that can moni-tor the voltage of cells or a battery pack, shutting off when thecell or battery reaches a certain voltage. Other chargers canmonitor cell tempera ture, shutting off when a certain cell


temperature has been reached. Standard chargers, also calledtrickle chargers, use very low current to recharge a cell or bat-tery pack and may be left on the cell or pack indefinitely.

It’s very important not to reverse the polarity on a NiCad cellor pack when you’re charging it. Some battery packs containa diode connected in series with the battery pack to preventpolarity reversal. This diode allows the charger to send powerto the cell or pack only if the charger is correctly connectedto the battery pack.

If you attempt to recharge a NiCad battery before it’s beenfully discharged, the battery may only recharge to a smallfraction of its possible charge level. This is known as a “memory” problem. The rem edy for this problem is to fullydischarge the cell before recharging. A fully depleted batterywill be able to accept a full charge.

New types of NiCad cells are being developed and used inindustrial and commercial applications. For example, newchemical additives are being used to increase the cells’ dischargerate and shorten the re charge time.

Lithium Cells and Batteries

Lithium cells, like NiCad cells, are used as backup power sup-plies for memory circuit boards, alarm systems, and smokealarms. Lithium batteries are also used in watches and pace-makers.

Lithium batteries have a long life (up to 10 years) before theydis charge due to internal resistance. Also, lithium batterieshave a high discharge rate per unit volume and weight. Thisis known as a high energy density.

There are two basic types of lithium cells, the poly carbonmonofloride cell and the manganese dioxide cell. The polycar-bon monofloride cell has a flat discharge rate, which meansthat the cell maintains the same voltage throughout its dis-charge cycle. The manganese dioxide cell has a highdischarge rate, meaning that it can maintain a high current(without voltage loss). The polycarbon cell has a current dis-charge time of between 25 and 500 mAH, while themanganese cell has a dis charge time of between 32 and


1,000 mAH. At cold temperatures (�20°C), the manganesecell can produce more then four times the en ergy of the poly-carbon cell. Each of these standard lithium cells pro duces 3.0V, more than double the voltage of a NiCad cell. Some non-standard lithium batteries produce between 2 V and 3.6 V.

When a lithium cell is placed in a computer’s memory circuit,a block ing diode and resistor are also placed in the circuit(Figure 27). The blocking diode prevents the main power sup-ply from attempting to charge the battery when the computeris drawing power from its own DC power supply.

Some modern lithium batteries use a solid electrolyte insteadof a liq uid electrolyte. This allows the battery to be muchsmaller, increasing its energy density. In addition, this type oflithium battery is recharge able. Solid-electrolyte lithium bat-teries are being considered for use in many types ofindustrial and consumer applications (even electric cars).

Other Types of Batteries

Many different chemical combinations can be used with met-als to create DC electricity. For example, the lead/acidbattery that’s used in forklifts is also available in small sizes.This smaller battery is used in special backup systems forcomputers called uninterrwptable power supplies or UPS sys-tems. These batteries are maintenance-free and don’t evencontain vent holes. Instead, all internal gases are chemi callyrecombined into their original compounds. Safety lightingsys tems also use these lead/acid batteries.

FIGURE 27—When a lithiumbattery is used in a com-puter, a blocking diode isrequired in the circuit toprevent the main powersupply from charging thebattery.


Two other types of storage batteries are nickel/iron andsodium/sulfur batteries. These batteries are currently beingtested for use in comput ers and electric vehicles.

One other type of rechargeable storage battery you should beaware of is the zinc/air battery. This type of battery is cur-rently being used in laptop and notebook computers.

Environmental Responsibility

Batteries and cells contain dangerous and poisonous chemi-cals, and thus must be disposed of carefully. Batteries areone of the major con tributors of poisonous heavy metals tolandfills. Metals such as cad mium, chromium, lithium, andlead can seep out of a landfill into nearby water supplies.

Most manufacturers have now developed recycling programsfor their batteries. These programs require that you mail oldcells or bat tery packs back to the manufacturer. The manu-facturer can then dis assemble the cells and recycle thematerials into new cells.

Lead/acid and other storage cells can also be recycled.Typically, stor age cell suppliers will reclaim cells for return tothe manufacturer.

Take a few moments to review what you’ve learned by com-pleting Power Check 6.


Power Check 6 1. The positive terminal of a NiCad battery is made from .

2. A NiCad battery can go through more than charging and discharging cycles before thebattery can no longer produce electricity.

3. NiCad cells are often connected together to make battery .

4. A standard lithium cell has an output voltage of .

5. A NiCad cell could if exposed to excessive heat.

6. cells are typically used in watches and pacemakers.



NOTES

57

Answers

Answers

Power Check Answers 1 1. Aluminum

2. superconductor

3. 2.575 �

4. #10 AWG

Power Check Answers 2 1. Nylon

2. 60°C

3. Moisture-resistant latex rubber

4. high-pot testers or megohm meters

Power Check Answers 3 1. Primary cells

2. negative, positive

3. Wet cells

4. zinc, carbon

5. parallel

6. 4.5 V

7. 4A

8. photovoltaic

Power Check Answers 4 1. series

2. sediment chambers

3. gases

4. Red

5. sealed or maintenance-free

Power Check Answers58

6. size, current-carrying

7. water

8. 48 V

Power Check Answers 5 1. high-rate discharge, open-circuit voltage test

2. Heavier

3. Hydrometer

4. Discharged

5. water

6. explosion

Power Check Answers 6 1. nickel hydroxide

2. 100

3. packs

4. 3 V

5. explode

6. Lithium

Self-Check Answers 59

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