Gibilisco S. Teach Yourself Electricity and Electronics

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19. The actual resistance of the component described in the previous question can be expected to

vary above or below the specified ohmic value by up to what amount?

(a) 11 Ω

(b) 110 Ω

(d) 220 Ω

20. Suppose a carbon-composition resistor has the following colored bands on it: gray, red,

yellow. This unit can be expected to have a value within approximately what range?

(a) 660 kΩ to 980 kΩ

(b) 740 kΩ to 900 kΩ

(d) The manufacturer does not make any claim.

Quiz 101

IN ELECTRICITY AND ELECTRONICS, A CELL IS A UNIT SOURCE OF DC ENERGY. WHEN TWO OR MORE

cells are connected in series, the result is known as a battery. There are many types of cells and bat-

teries, and new types are constantly being invented.

Electrochemical Energy

Early in the history of electrical science, laboratory physicists found that when metals came into

contact with certain chemical solutions, voltages appeared between the pieces of metal. These were

the first electrochemical cells.

A piece of lead and a piece of lead dioxide immersed in an acid solution (Fig. 7-1) acquire a per-

sistent potential difference. This can be detected by connecting a galvanometer between the pieces

of metal. A resistor of about 1000 Ω must be used in series with the galvanometer in experiments

of this kind, because connecting the galvanometer directly will cause too much current to flow, pos-

sibly damaging the galvanometer and causing the acid to boil.

The chemicals and the metal have an inherent ability to produce a constant exchange of charge

carriers. If the galvanometer and resistor are left hooked up between the two pieces of metal for a

long time, the current will gradually decrease, and the electrodes will become coated. All the chem-

ical energy in the acid will have been turned into electrical energy as current in the wire and gal-

vanometer. In turn, this current will have heated the resistor (another form of kinetic energy), and

escaped into the air and into space.

Primary and Secondary Cells

Some electrical cells, once their chemical energy has all been changed to electricity and used up,

must be thrown away. These are called primary cells. Other kinds of cells, such as the lead-and-acid

type, can get their chemical energy back again by means of recharging. Such a cell is a secondary cell.

Primary cells include the ones you usually put in a flashlight, in a transistor radio, and in vari-

ous other consumer devices. They use dry electrolyte pastes along with metal electrodes. They go by

names such as dry cell, zinc-carbon cell, or alkaline cell. Go into a department store and find a rack

of batteries, and you’ll see various sizes and types of primary cells, such as AAA batteries, D batteries,

102

CHAPTER

Cells and Batteries

camera batteries, and watch batteries. (These are actually cells, not true batteries.) You’ll also see 9-V

transistor batteries and large 6-V lantern batteries.

Secondary cells can also be found in consumer stores. Nickel-based cells are common. The most

common sizes are AA, C, and D. These cost several times as much as ordinary dry cells, and a charg-

ing unit also costs a few dollars. But if you take care of them, these rechargeable cells can be used hun-

dreds of times and will pay for themselves several times over if you use a lot of batteries in everyday life.

The battery in your car is made from secondary cells connected in series. These cells recharge

from the alternator or from an outside charging unit. This battery has cells like the one in Fig. 7-1.

It is dangerous to short-circuit the terminals of such a battery, because the acid (sulfuric acid) can

bubble up and erupt out of the battery casing. Serious skin and eye injuries can result. In fact, it’s a

bad idea to short-circuit any cell or battery, because it can get extremely hot and cause a fire, or rup-

ture and damage surrounding materials, wiring, and components.

The Weston Standard Cell

Most electrochemical cells produce 1.2 to 1.8 V. Different types vary slightly. A mercury cell has a

voltage that is a little less than that of a zinc-carbon or alkaline cell. The voltage of a cell can also be

affected by variables in the manufacturing process. Most consumer-type dry cells can be assumed to

produce 1.5 V.

There are certain cells whose voltages are predictable and exact. These are called standard cells.

A good example is the Weston cell, which produces 1.018 V at room temperature. It has a solution

of cadmium sulfate, a positive electrode made from mercury sulfate, and a negative electrode made

from mercury and cadmium. The device is set up in a container, as shown in Fig. 7-2.

Storage Capacity

Recall that the common electrical units of energy are the watt-hour (Wh) and the kilowatt-hour

(kWh). Any electrochemical cell or battery has a certain amount of electrical energy that can be ob-

tained from it, and this can be specified in watt-hours or kilowatt-hours. More often, though, it’s

given in ampere-hours (Ah).

A battery with a rating of 2 Ah can provide 2 A for 1 h, or 1 A for 2 h, or 100 mA for 20 h.

Electrochemical Energy 103

7-1 Simplified drawing

of the construction

of a lead-acid

electrochemical cell.

There are infinitely many possibilities here, as long as the product of the current in amperes and the

use time in hours is equal to 2. The limitations are the shelf life at one extreme, and the maximum

deliverable current at the other. Shelf life is the length of time the battery will last if it is never used;

this can be years. The maximum deliverable current is the highest amount of current that the bat-

tery can provide before its voltage drops because of its own internal resistance.

Small cells have storage capacity of a few milliampere-hours (mAh) up to 100 or 200 mAh.

Medium-sized cells can supply 500 mAh to 1 Ah. Large automotive or truck batteries can provide

upward of 50 Ah. The energy capacity in watt-hours is the ampere-hour capacity multiplied by the

battery voltage.

An ideal cell or ideal battery (a theoretically perfect cell or battery) delivers a constant current for

a while, and then the current starts to drop (Fig. 7-3). Some types of cells and batteries approach

this level of perfection, which is represented by a flat discharge curve. But many cells and batteries

are far from perfect; they deliver current that declines gradually, almost right from the start. When

the current that a battery can provide has tailed off to about half of its initial value, the cell or bat-

tery is said to be weak. At this time, it should be replaced. If it’s allowed to run all the way out, until

the current actually goes to zero, the cell or battery is dead. The area under the curve in Fig. 7-3 is a

graphical representation the total capacity of the cell or battery in ampere-hours.

104 Cells and Batteries

7-2 Simplified drawing of the construction of a Weston standard cell.

Grocery Store Cells and Batteries

The cells you see in grocery stores, department stores, drugstores, and hardware stores provide

1.5 V, and are available in sizes known as AAA (very small), AA (small), C (medium large), and

D (large). Batteries are widely available that deliver 6 or 9 V.

Zinc-Carbon Cells

Figure 7-4 is a translucent drawing of a zinc-carbon cell. The zinc forms the case and is the negative

electrode. A carbon rod serves as the positive electrode. The electrolyte is a paste of manganese diox-

ide and carbon. Zinc-carbon cells are inexpensive and are good at moderate temperatures and in ap-

plications where the current drain is moderate to high. They are not very good in extreme cold.

Alkaline Cells

The alkaline cell has granular zinc as the negative electrode, potassium hydroxide as the electrolyte,

and a device called a polarizer as the positive electrode. The construction is similar to that of the

zinc-carbon cell. An alkaline cell can work at lower temperatures than a zinc-carbon cell. It lasts

longer in most electronic devices, and is therefore preferred for use in transistor radios, calculators,

Grocery Store Cells and Batteries 105

7-3 A flat discharge curve.

This is considered ideal.

7-4 Simplified drawing of

the construction of

a zinc-carbon

electrochemical cell.

and portable cassette players. Its shelf life is much longer than that of a zinc-carbon cell. As you

might expect, it costs more.

Transistor Batteries

A transistor battery consists of six tiny zinc-carbon or alkaline cells in series. Each of the six cells sup-

plies 1.5 V. Thus, the battery supplies 9 V. Even though these batteries have more voltage than in-

dividual cells, the total energy available from them is less than that from a C cell or D cell. This is

because the electrical energy that can be obtained from a cell or battery is directly proportional to

the amount of chemical energy stored in it, and this, in turn, is a direct function of the volume

(physical size) of the cell or the mass (quantity of chemical matter) of the cell. Cells of size C or D

have more volume and mass than a transistor battery, and therefore contain more stored energy for

the same chemical composition.

Transistor batteries are used in low-current electronic devices such as remote-control garage-

door openers, television (TV) and hi-fi remote controls, and electronic calculators.

Lantern Batteries

The lantern battery has much greater mass than a common dry cell or transistor battery, and conse-

quently it lasts much longer and can deliver more current. Lantern batteries are usually rated at 6 V,

and consist of four good-size zinc-carbon or alkaline cells. Two lantern batteries connected in series

make a 12-V battery that can power a 5-W citizens band (CB) or ham radio transceiver for a while.

They’re also good for scanner radio receivers in portable locations, for camping lamps, and for other

medium-power needs.

Miniature Cells and Batteries

In recent years, cells and batteries—especially cells—have become available in many different sizes

and shapes besides the old cylindrical cells, transistor batteries, and lantern batteries. These are used

in wristwatches, small cameras, and various microminiature electronic devices.

Silver-Oxide Cells and Batteries

A silver-oxide cell is usually found in a buttonlike shape, and can fit inside a small wristwatch. These

types of cells come in various sizes and thicknesses, all with similar appearances. They supply 1.5 V,

and offer excellent energy storage for the weight. They also have a nearly flat discharge curve, like

the one shown in the graph of Fig. 7-3. Zinc-carbon and alkaline cells and batteries, in contrast,

have current output that declines more steadily with time, as shown in Fig. 7-5. This is known as a

declining discharge curve.

Silver-oxide cells can be stacked to make batteries. Several of these miniature cells, one on top

of the other, can provide 6, 9, or even 12 V for a transistor radio or other light-duty electronic de-

vice. The resulting battery is about the size of an AAA cylindrical cell.

Mercury Cells and Batteries

A mercury cell, also called a mercuric-oxide cell, has properties similar to those of silver-oxide cells.

They are manufactured in the same general form. The main difference, often not of significance, is

a somewhat lower voltage per cell: 1.35 V. If six of these cells are stacked to make a battery, the re-

106 Cells and Batteries

sulting voltage will be about 8.1 V rather than 9 V. One additional cell can be added to the stack,

yielding about 9.45 V.

There has been a decline in the popularity of mercury cells and batteries in recent years, because

of the fact that mercury is toxic to humans and animals, even in trace amounts. When mercury cells

and batteries are dead, they must be discarded. Eventually the mercury or mercuric oxide leaks into

the soil and groundwater. Mercury pollution has become a significant concern throughout the

world.

Lithium Cells and Batteries

Lithium cells gained popularity in the early 1980s. There are several variations in the chemical

makeup of these cells; they all contain lithium, a light, highly reactive metal. Lithium cells can be

made to supply 1.5 to 3.5 V, depending on the particular chemistry used. These cells, like silver-

oxide and mercury cells, can be stacked to make batteries.

The first application of lithium batteries was in memory backup for electronic microcomput-

ers. Lithium cells and batteries have superior shelf life, and they can last for years in very-low-

current applications such as memory backup or the powering of a digital liquid crystal display

(LCD) watch or clock. These cells also provide high energy capacity per unit volume or mass.

Lead-Acid Batteries

You’ve seen the basic configuration for a lead-acid cell. This has a solution of sulfuric acid, along

with a lead electrode (negative) and a lead-dioxide electrode (positive). These cells are rechargeable.

Automotive batteries are made from sets of lead-acid cells having a free-flowing liquid acid. You

cannot tip such a battery on its side, or turn it upside-down, without running the risk of having

some of the acid electrolyte spill out. Lead-acid batteries are also available in a construction that

uses a semisolid electrolyte. These batteries are sometimes used in consumer electronic devices that

require a moderate amount of current. The most common example is an uninterruptible power

supply (UPS) that can keep a desktop personal computer running for a few minutes if the utility

power fails.

Lead-Acid Batteries 107

7-5 A declining discharge

curve.

A large lead-acid battery, such as the kind in your car or truck, can store several tens of ampere-

hours. The smaller ones, like those in a UPS, have less capacity but more versatility. Their main at-

tributes are that they can be charged and recharged many times, and they are not particularly

expensive.

Nickel-Based Cells and Batteries

Nickel-based cells include the nickel-cadmium (NICAD or NiCd) type and the nickel-metal-hydride

(NiMH) type. Nickel-based batteries are available in packs of cells. These packs can be plugged into

equipment, and sometimes form part of the case for a device such as a portable radio transceiver.

All nickel-based cells are rechargeable, and can be put through hundreds or even thousands of

charge/discharge cycles if they are properly cared for.

Configurations and Applications

Nickel-based cells are found in various sizes and shapes. Cylindrical cells look like ordinary dry cells.

Button cells are those little things you find in cameras, watches, memory backup applications, and

other places where miniaturization is important. Flooded cells are used in heavy-duty applications,

and can have storage capacity in excess of 1000 Ah. Spacecraft cells are made in packages that can

withstand the rigors of a deep-space environment.

Most orbiting satellites are in darkness half the time and in sunlight half the time. Solar panels

can be used while the satellite is in sunlight, but during the times that the earth eclipses the sun, bat-

teries are needed to power the electronic equipment on board the satellite. The solar panels can

charge a nickel-based battery, in addition to powering the satellite, for the daylight half of each

orbit. The nickel-based battery can provide the power during the dark half of each orbit.

Cautions

Never discharge nickel-based cells all the way until they totally die. This can cause the polarity of a

cell, or of one or more cells in a battery, to reverse. Once this happens, the cell or battery is ruined.

A phenomenon peculiar to nickel-based cells and batteries is known as memory or memory

drain. If a nickel-based unit is used over and over, and is discharged to the same extent every time,

it might begin to die at that point in its discharge cycle. Memory problems can usually be solved.

Use the cell or battery almost all the way up, and then fully recharge it. Repeat the process several

times.

Nickel-based cells and batteries work best if used with charging units that take several hours to

fully replenish the charge. So-called high-rate or quick chargers are available, but these can some-

times force too much current through a cell or battery. It’s best if the charger is made especially for

the cell or battery type being charged. An electronics dealer, such as the manager at a RadioShack

store, should be able to tell you which chargers are best for which cells and batteries.

In recent years, concern has grown about the toxic environmental effects of discarded heavy

metals, including cadmium. For this reason, NiMH cells and batteries have replaced NICAD types

in many applications. In most practical scenarios, a NICAD battery can be directly replaced with a

NiMH battery of the same voltage and current-delivering capacity, and the powered-up device will

work satisfactorily.

Some vendors and dealers will call a nickel-based cell or battery a NICAD, even when it is ac-

tually a NiMH cell or battery.

108 Cells and Batteries

Photovoltaic Cells and Batteries

The photovoltaic (PV) cell is different from any electrochemical cell. It’s also known as a solar cell. This

device converts visible light, infrared (IR), and/or ultraviolet (UV) directly into electric current.

Solar Panels

Several, or many, photovoltaic cells can be combined in series-parallel to make a solar panel. An ex-

ample is shown in Fig. 7-6. Although this shows a 3 × 3 series-parallel array, the matrix does not have

to be symmetrical. And it’s often very large. It might consist of, say, 50 parallel sets of 20 series-

connected cells. The series scheme boosts the voltage to the desired level, and the parallel scheme in-

creases the current-delivering ability of the panel. It’s not unusual to see hundreds of solar cells

combined in this way to make a large panel.

Construction and Performance

The construction of a photovoltaic cell is shown in Fig. 7-7. The device is a flat semiconductor

P-N junction, and the assembly is made transparent so that light can fall directly on the P-type sili-

con. The metal ribbing, forming the positive electrode, is interconnected by means of tiny wires.

The negative electrode is a metal backing or substrate, placed in contact with the N-type silicon.

Most solar cells provide about 0.5 V. If there is very low current demand, dim light will result

in the full-output voltage from a solar cell. As the current demand increases, brighter light is needed

to produce the full-output voltage. There is a maximum limit to the current that can be provided

from a solar cell, no matter how bright the light. This limit is increased by connecting solar cells in

parallel.

Practical Applications

Solar cells have become cheaper and more efficient in recent years, as researchers have looked to

them as an alternative energy source. Solar panels are used in satellites. They can be used in conjunc-

tion with rechargeable batteries, such as the lead-acid or nickel-cadmium types, to provide power

independent of the commercial utilities.

A completely independent solar/battery power system is called a stand-alone system. It uses large

solar panels, large-capacity lead-acid batteries, power converters to convert the dc into ac, and a

sophisticated charging circuit. These systems are best suited to environments where there is sun-

shine a high percentage of the time.

Photovoltaic Cells and Batteries 109

7-6 Connection of cells in series-parallel.

Solar cells, either alone or supplemented with rechargeable batteries, can be connected into a home

electric system in an interactive arrangement with the electric utilities. When the solar power system

can’t provide for the needs of the household all by itself, the utility company can take up the slack.

Conversely, when the solar power system supplies more than enough for the needs of the home, the

utility company can buy the excess.

Fuel Cells

In the late 1900s, a new type of electrochemical power device emerged that is believed by some sci-

entists and engineers to hold promise as an alternative energy source: the fuel cell.

Hydrogen Fuel

The most talked-about fuel cell during the early years of research and development became known

as the hydrogen fuel cell. As its name implies, it derives electricity from hydrogen. The hydrogen

combines with oxygen (that is, it oxidizes) to form energy and water. There is no pollution, and there

are no toxic by-products. When a hydrogen fuel cell “runs out of juice,” all that is needed is a new

supply of hydrogen, because its oxygen is derived from the atmosphere.

Instead of combusting, the hydrogen in a fuel cell oxidizes in a more controlled fashion, and at

a much lower temperature. There are several schemes for making this happen. The proton exchange

membrane (PEM) fuel cell is one of the most widely used. A PEM hydrogen fuel cell generates ap-

proximately 0.7 V of dc. In order to obtain higher voltages, individual cells are connected in series.

A series-connected set of fuel cells is technically a battery, but the term used more often is stack.

Fuel-cell stacks are available in various sizes. A stack about the size and weight of an airline suit-

110 Cells and Batteries

7-7 Construction of a

silicon photovoltaic cell.