Lead-Acid Batteries
One of the most common batteries used today is the lead-acid battery. They are the work horse of the rechargeable battery world. They are economical, reliable and have a high tolerance for abuse. In a vehicle, for example, they are used for starting/ignition as well as to operate the lighting system. Lead-acid batteries can also provide the current to operate the vehicle's stereo, power door-locks and electrical windows.

	Car battery. From Battery Council International
1. Resilient plastic container. 2. Positive and negative internal plates made of lead. 3. Plate separators made of porous synthetic material. 4. Electrolyte, a dilute solution of sulfuric acid and water better known as battery acid. 5. Lead terminals, the connection point between the battery and whatever it powers.

In a lead-acid battery, the chemical reaction is produced when two unlike metals are immersed in an electrolyte. The battery must have plates made from lead or an alloy of lead and other metals. A battery must have a positive and a negative plate in order to conduct electricity. Lead (Pb) is used at one terminal and the other is lead oxide (PbO2). The lead terminal holds a negative electrical charge. The lead oxide plate is positively charged. The metals are separated by an insulating material to prevent a short circuit. The battery is filled with an electrolyte, usually sulfuric acid, that provides the sulfate ion for the discharge (load) reactions.

When a fully charged battery is placed under a load, such as starting a car, a chemical reaction occurs between the positive and negative terminals. This completes the circuit that allows charge (electrons) to flow through the circuit. In the external part of the circuit, the flow of charge results in the electric current needed to "turn the car's starter".

Within the cell, the charge flows in the form of ions that are transported from one terminal to the other. The positive electrode receives electrons from the external circuit when the battery is discharging. These electrons then react with the active metal of the positive electrode in "reduction" reactions that continue the flow of charge through the electrolyte to the negative electrode.

At the negative electrode, "oxidation" reactions between the active metal of the negative terminal and the charge flowing the electrolyte results in a surplus of electrons that can be donated to the cars electrical system.

The chemical reactions that take place are summarized as follows:

The general reaction is shown below:

Pb + PbO2 + 2H(+) + 2 HS04 -----> 2PbS04 + 2H20 + 2e(-)

Inside the battery, a chemical reaction produces the electrons. The speed of electron production controls how many electrons can flow between the positive and negative terminals. Electrons flow from the battery into wire (conductor) and must travel from the negative to the positive terminal for the chemical reaction to take place. This property explains why a battery can sit on a store shelf for an extended period of time without having to be recharged.

The chemical reaction in the battery only occurs if the circuit is closed. For every electron generated in an oxidation-reaction at the negative terminal, an electron is consumed in a reduction reaction at the positive terminal.

Trying to start a car on a very cold day can be very frustrating. The key is turned in the ignition switch and the battery provides the electrical energy to turn the starter. The engine, however, will not start. The process of trying to start the car continues for a few minutes. However, the car simply will not start. It doesn't take long before the battery can not produce enough electrical energy and the driver must try to get a "boost" or will call a tow truck for a ride to the nearest mechanic.

In this situation, the sulfuric acid is the catalyst in this electro-chemical reaction. As the cells discharge in a lead-acid battery, the H2S04 content of the electrolyte changes. In a fully charged battery, the electrolyte is approximately 25% H2S04 - the remainder is water. As the battery is discharged, the active materials are used up and the reactions slow down until the battery is no longer capable of supplying electrons. At this point the battery is discharged. In the discharged state, the concentration of H2S04 is less than five percent.

Each cell in a lead-acid battery develops a voltage of approximately 2 volts. By linking these cells in series (six cells for a 12 volt battery), the voltage will increase from 2 volts to approximately 12 volts.

Although this diagram configuration shows dry cell batteries, it clearly demonstrates cells linked in series. Copyright 1991, The National Academy of Sciences. Reproduced with permission from Electric Circuits, National Sciences Research Center.

In this diagram, the dry cells are linked in parallel. Copyright 1991, The National Academy of Sciences. Reproduced with permission from Electric Circuits, National Sciences Research Center.

To increase the capacity of the battery, the cells would be linked in parallel. This would provide greater amperage but less voltage. The larger the cell, the more materials it contains and the large the electrical capacity.

As the lead-acid battery discharges, the lead plates become more chemically alike; the acid becomes weaker; and the voltage drops. Eventually the battery is so discharged that it can no longer deliver electrons at a useful voltage.

Recharging

There are two types of batteries: primary and secondary.

1. Primary batteries, such as alkaline batteries, are used once and then replaced. The chemical reactions that take place inside these batteries is irreversible.

2. Secondary batteries, such as lead-acid batteries, can be recharged and reused. They use reversible chemical reactions. By reversing the flow of electrons, the chemical reactions are reversed to restore active material that had been depleted.

A full charge restores the chemical difference between the metal plates and the battery is ready to provide electrons when required. This unique process of discharge and charge in the lead-acid battery makes is a very versatile and important technology.

Rechargeable batteries
Lead-acid batteries are usually large and heavy. It would be difficult to carry one around to power your portable CD player. However, technology has created a number of new, rechargeable batteries. These include Nickel-Cadmium Batteries and Lithium-ion Batteries.

Nickel-Cadmium Battery

Lithium-ion Battery

Alkaline Batteries (Dry Cells)

Alkaline dry cell (voltaic) From "Ultimate Visual Dictionary of Science," Stoddart 1998.

The cathode in an alkaline battery is composed of manganese dioxide - a compound with a tendency to gain electrons. Any time a material gains electrons a reduction reaction has occurred.

Zinc is the metal most commonly found at the anode of the battery. When the battery is under a load, the zinc gives up electrons in a process called oxidation. The electrons leave the zinc atoms and flow up the battery's anode collector to the external circuit. Any time a material loses electrons, an oxidation reaction has occurred.

The electrolyte is a water solution containing potassium hydroxide - a nonmetallic material that conducts electricity in the form of ions (not electrons). The alkaline battery gets its name from this reactant which is an alkaline (basic) substance. The electrolyte completes the internal circuit allowing the process of oxidation-reduction to occur.

b. Generators

Generator
A generator is an electromechanical device that converts kinetic energy into an electrical current. Michael Faraday, in 1831, demonstrated that a current is produced when a wire is moved through a magnetic field. Faraday is credited with the development of the electric generator or dynamo. A dynamo works by turning a copper wheel across magnetic lines of force to produce electricity. Faraday found that by pushing the magnet very quickly through a coil of wire he could generate a current. He also determined that a stronger electric current can be generated by adding more "turns" of conducting wire in a coil.

The mechanical energy supplied to the generator is often some form of turbine. A turbine provides rotational mechanical motion as some type of fluid pressure. The pressure of running water represents mechanical (kinetic) energy. The turbine blades transmit the energy to a shaft which, in turn, rotates. This rotational kinetic energy is used to turn a coil of wire through a magnetic field.

Parts of a Generator

Permanent Magnets
Moving magnets create an electric current in a closed coil of wire. A moving magnet causes a pumping action and, If the circuit is closed, the electrons in the coil of wire start to flow.

Coil of Wire
All metals contain a moveable substance called electric charge. Wires contain an equal number of protons (+) and electrons (-). When a coil of wire surrounds a magnetic field, and the magnetic field changes (moves), a circular pressure called voltage appears. This circular voltage forces electrons in the coil of wire to begin flowing.

Armature
In some generators, it is the coil of conducting wire and not the magnets that move. An armature is the part of a generator that turns, converting mechanical energy of motion into electric energy. The electric current is generated in the wires of the armature. The electric current can only be produced in a closed circuit.

Induction of a Current

Generators produce voltages by magneto-electric induction. When a wire conductor cuts through the lines of force of the magnetic field, a voltage is induced in the wire.

The magnitude of the output voltage of a generator is determined by:

• the speed of the coil of wire passing through the magnetic field
• the strength of the magnetic field
• the number of coils of wire moving through the magnetic field

Once again, the voltage is only generated by induction when the wire conductor and the magnetic field move relative to each other. Induction of an electric current will occur if the magnetic field remains stationary while the wire conductors (coil) moves. Induction can also occur if the wire conductors remain stationary and the magnetic field moves.

c. Wind Power

Humans have been using wind energy for centuries. Windmills have been used to grind grain and pump seawater. Today, wind power is used to generate electricity by having the wind drive fan blades, which turn an electric generator.

The movement of air is another form of kinetic energy - the energy of motion. Winds are an indirect form of solar heating. This is due to the fact that the sun's energy does not heat the Earth's surface evenly. The temperature differences on the surface of the earth create high and low pressure areas. The movement of air from an area of higher pressure to an area of lower pressure, along with the spinning of the earth, is the primary cause of wind.

There are two laws of physics concerning wind power. The first law states that the power available is proportional to the "cube" of the wind speed (velocity). This means that if the wind speed doubles, the power available changes by 2³, that is 2 to the power of 3, or eight times. Even small increases in the velocity of the wind can dramatically change the power output.

The second law has to deal with the length of the blades on the turbine. The power available to the blades is directly proportional to the square of the diameter of the rotor. In other words, if you double the diameter of the rotor by making the blades twice as long, you increase the power by 2², that is 2 to the power of 2, or by a factor of four.

Wind Power

The electrical output of a wind generator can be calculated using the following formula:

• wind power = volts x amps and is measured in joules/second or Watts
• p is the density of air (1.29 kg/m³)
• A is the area swept by the blades and is calculated using the formula A= pir² (metres squared)
• V is the velocity of the wind (metres/sec)

Example problem 1:
Calculate the amount of wind power generated when the velocity of the wind is 10 m/s and the length of the blades is 1 metre.

A= pir²
A= pi(1)² A= 3.14 m²

wind power = 1/2pAV³

= ½ (1.29 kg/m³)( 3.14 m²)(10m/s)³
= ½ (1.29 kg/m³)( 3.14 m²)(1000m³/s³)
= 2025.3 joules/s or 2025.3 watts
= 2.03 kilowatts

Example problem 2:
Calculate the amount of wind power generated when the velocity of the wind is 10 m/s and the length of the blades is 2 metres.

A= pir²
A= pi(2)²
A=12.6 m²

wind power = 1/2pAV³

= ½ (1.29 kg/m³)( 12.6 m²)(10m/s)³
= ½ (1.29 kg/m³)( 12.6 m²)(1 000m³/s³)
= 8,127 joules/s or 8,127 watts
= 8.1 kilowatts

As you can see, by doubling the length of the turbine blades, the wind power has increased by a factor of four.

Locating Wind Generators

To produce the maximum amount of electricity, wind generators must be placed in areas that are usually windy. Large groups of wind turbines, called wind farms, are connected to electric utility power lines and provide electricity to many people.

An advantage of wind turbines over some other forms of renewable energy is that they can produce electric current whenever the wind blows. In theory, this type of system can produce electric current 24 hours a day. However, the wind does not always blow. Wind generators are usually connected to batteries that can store the electric energy until needed.

d. Solar Energy

The sun's energy can be used to warm and light homes, heat water, and provide electrical current to power light bulbs. This energy comes from process called solar heating, solar water heating, and photovoltaic energy (converting sunlight directly into electricity).

Solar power (sunlight) is converted to electrical current using photovoltaic cells (PV). The solar cells are made with a wafer of silicon that is treated with specific chemicals. Electrons are excited by the light and move through the silicon. The combination of these chemicals generates the electricity without moving, making noise, or directly polluting the environment.

Diagram of a photovoltaic cell as part of an array, from Solar Energy Society of Canada. Visit the site for more on photovotaic solar energy, including its use in developing countries.

Photovoltaic cells produce Direct Current (DC). Photovoltaic cells come in many sizes, but most are 10 cm by 10 cm and generate about half a volt of electricity. The photovoltaic cells are bundled together in modules or panels to produce higher voltages and increased power. For example, a system of 30 or 40 photovoltaic cells can produce 12 volts. This current can be used to power electrical equipment and/or to recharge batteries. Due to their expense, they are practical for small amounts of electricity in areas that get a great deal of sunshine. PV systems only produce electric current when the sun is shining and need batteries to store the electricity.

e. Electric Current and Magnetism

Diagram of the deflection of a compass needle when current flows through a wire from "Safe and Simple Electrical Experiments," R.F. Graf, 1964

In 1820, Hans Christian Oerstedd, a Danish scientist, made an important discovery that there is a direct link between magnetic force and electric force.

This discovery was made quite accidentally as he was actually trying to prove that there wasn't a relationship between magnetism and electricity.

During a university lecture, Oerstead placed a current-carrying conductor parallel on the needle of a compass. The magnetic needle immediately rotated and came to rest in a position at right angles to the current-carrying conductor. He moved the wire around and noticed that as long as the current flowed through the wire the compass needle would move.

When the current is reversed, the needle flips the other way Diagram from "Safe and Simple Electrical Experiments," R.F. Graf, 1964

Oerstedd continued his investigations. He found that if the current is reversed, the magnetic needle in the compass swings around in the opposite direction.

He also discovered that when he held the current-carrying conductor below the compass, its effects on the magnetic needle were exactly opposite to those he obtained when he held the wire above it.

Diagram of the "Right hand rule" from "Safe and Simple Electrical Experiments," R.F. Graf, 1964

Electric current flowing in a conductor can generate a magnetic field. The Right-Hand-Rule, as it is known commonly, is an easy way to remember which way the magnetic field flows around a straight, current-carrying wire.

Imagine the right hand holding the wire with the forefinger pointed in the direction of the current in the wire. Remember, electric current moves from negative (-) to positive (+). Your middle finger should point down towards the compass. Your thumb will indicate the direction in which the North pole of the compass will point.