mod 3 rg 3.5 DC sources of Electricity

Primary cells can immediately supply voltage without

charging beforehand. The electrochemical processes taking place are not reversible.

primary cells cannot be re-charged and, therefore, cannot

be reused although they can and should be recycled

Galvanic cells can only supply a

certain current, i.e., a certain charge, over a certain period of time.

the capacity of galvanic cells depends on their

construction and size

nearly all primary cells are

dry cells

Nearly all primary cells are dry cells. These are a type of galvanic cell in which the electrolyte is a

paste rather than a liquid.

The zinc container acts as both

a package to hold the other active chemicals and as the negative electrode.

The positive electrode is a carbon rod in the centre of the cell. It is surrounded by

a paste containing manganese (IV) oxide, zinc chloride, ammonium chloride, carbon powder, and a small amount of water.

All zinc-carbon dry cells have a nominal initial voltage of about

1.5 V, and it decreases as the battery is discharged.

Generally, larger batteries can store

more electrical energy.

One disadvantage of zinc-carbon dry cells is that as the

zinc container oxidises, its contents can eventually leak

zinc-carbon batteries should only be installed in devices that

are being used and not stored in any device for an extended period.

Alkaline primary batteries are an improvement over

zinc-carbon dry cells.

An alkaline can deliver up to

five times the energy of a similarly-sized, zinc-carbon battery.

Like zinc-carbons, alkaline batteries are prone to

leaking, in this case, potassium hydroxide, so these should also be removed from devices for long-term storage.

Unlike zinc-carbons, which are never rechargeable, some alkaline batteries

can be recharged

attempting to recharge an alkaline battery that is not designed to be recharged, can

rupture the battery, causing leakage of the potassium hydroxide electrolyte.

While rechargeable alkaline batteries are relatively inexpensive, they can only be recharged

relatively few times compared with other chemicals like nickel cadmium or lithium-ion.

The electrochemical reactions in secondary (rechargeable) cells are reversible, and the chemical reactants can be restored (recharged) by

running a charging current into the cell. They are used in aircraft, cars, laptops, and many other devices.

In aircraft storage batteries, secondary cells are made of lead acid or nickel cadmium and are connected

in series within the battery case A group of cells connected together in this way is called a “battery of cells”, which is abbreviated to 'battery'.

Batteries are capacity-rated in ampere hours (Ah). For example, a battery with a capacity of 50 Ah supplies a current of

5 A for 10 hours.

when charged Lead acid cells have a voltage of

2.2V

when discharged Lead acid cells have a final voltage of

1.83

A lead acid cell consists of positive plates filled with

lead peroxide (PbO₂)

A lead acid cell consists of negative plates filled with

pure spongy lead (Pb)

A lead acid cell consists of a liquid electrolyte made up of

sulphuric acid (30%) and distilled water (70%).

During discharge, the sulphuric acid in the electrolyte breaks up into

hydrogen ions which are carrying a positive charge and sulphate ions which are carrying a negative charge.

The sulphate ions combine with the negative lead plate and form

lead sulphate, At the same time, they give up their negative charge, creating an excess of electrons on the negative plate.

The hydrogen ions go to the positive plate and combine with the

oxygen of the lead peroxide to form water. During this process, they also take electrons from the positive plate.

This action gives the positive plate a deficiency of electrons and the negative plate an excess of electrons.

When the plates are connected externally by a conductor, conventional current flows from the positive plate to the negative plate until

both plates are coated with lead sulphate and no further chemical action can take place. The battery is then said to be discharged.

During the charging process current is passed through the storage battery in a

reverse direction.

The sulphate ions are driven back into the solution in the electrolyte, where they combine with

the hydrogen ions of the water, thereby forming sulphuric acid.

The plates then return to their original composition of lead peroxide and spongy lead, and the battery is said to be charged.

Nickel cadmium cells have a voltage of

1.2V

Nickel cadmium cells have a discharge voltage of

1.0V

The external physical structure of a nickel cadmium cell is similar to that

of a lead acid cell. The active material however is significantly different.

The positive plate on nickel cadmium is impregnated with

nickel oxyhydroxide

the negative plates in nickel cadmium plates are

metallic cadmium

the electrolyte in nickel cadmium is

potassium hydroxide

As the nickel cadmium battery discharges, hydrogen ions from the electrolyte combine with the negative plates and electrons are released to the plates. The cadmium is converted to

cadmium hydroxide in the process.

hydroxide ions from the positive nickel oxyhydroxide plate move into the electrolyte, carrying extra electrons with them. Thus,

electrons are removed from the positive plate and delivered to the negative plate during discharge.

in a nickel cadmium battery The electrolyte remains

a solution of potassium hydroxide throughout the process, as electrons are being added as quickly as they are being removed.

When a nickel cadmium cell is being charged, the hydroxide ions are forced from

the negative plate and into the electrolyte. Thus, the cadmium hydroxide of the negative plate reverts to metallic cadmium.

Hydroxide ions from the electrolyte recombine with the nickel hydroxide of the positive plates, bringing them back to

their original higher level of oxidisation as nickel oxyhydroxide.

The electrolyte level in a nickel cadmium battery is at its lowest after

discharge

The level of electrolyte in a nickel cadmium will rise during charging, so it must only be refilled when

fully charged

advantages of a nickel cadmium vs a lead acid battery are

• Tolerates higher discharge currents

• Closed circuit voltage remains almost constant throughout discharge

• Higher energy density

• Greater number of charge/discharge cycles

disadvantages of a nickel cadmium vs a lead acid battery are

• More costly

• More complicated charging requirements

• Risk of thermal runaway. As the battery temperature rises, the internal resistance decreases.

• High toxicity of cadmium

galvanic cells are

single cells, designed as round cells and flat cells (coin cells).

both round cells and flat cells are manufactured in different sizes and, therefore, different capacities but with the

same voltage

Mercury Oxide Coin Cells: These consist of

pressed zinc powder as the negative pole and mercury-oxide (chemical formula HgO) as the positive pole.

the electrolyte in mercury oxide coin cells is

potassium hydroxide

mercury oxide coin cells voltage rating of

1.35V

the key characteristics of mercury oxide coin cells is

constant internal resistance during the whole discharging process and a constant voltage, even at continuous operation.

Mercury oxide coin cells are used in

hearing aids, watches, meters, exposure meters, and cameras.

silver oxide coin cells contain

silver oxide, potassium hydroxide, and zinc

silver oxide coin cells have a high energy density and a low

voltage depression

silver oxide coin cells ratED voltage is

1.55V

silver oxide coin cells are used for

watches, calculators, photo electric exposure devices and hearing aids for example.

Lithium Coin Cells: These come in multiple combinations and designs. They have high-rated voltages of up to

3.5V

lithium coin cells have the characteristics of

high energy density, and a high lifespan

Lithium silver chromate cells are used for

pacemakers, cameras, video cameras, and many other devices.

Sealed nickel cadmium (Ni-Cad) batteries were developed for

portable electronic devices. These take the form of round or coin cells.

in Ni-CadThe gases generated during the charging process are bound again. Consequently, the electrolyte

does not change

Ni-Cad batteries were once widely used in

portable power tools, photography equipment, torches, emergency lighting, and portable electronic devices.

Voltage measurements can easily show that the total voltage is the sum of the voltages of the individual cells, when

connected in series.

The series connection of power sources increases the

voltage

The current through a series circuit is

constant, i.e., the same current flows through each cell.

When the cells within a battery are connected in series, the internal resistance of the battery is equal to the

sum of the internal resistance of the cells. Therefore, a series connection increases the total internal resistance.

The diagram below shows three cells connected in series, with voltages V₁, V₂ and V₃ and internal resistances R₁, R₂ and R₃. When a battery is connected to an external load, the internal resistance causes the terminal voltage of the battery to

drop

Connecting like terminals, (positive to positive, and negative to negative) of power sources to each other creates a

parallel connection.

Connecting like power sources in parallel provides the same voltage but lowers

the total internal resistance of the supply, because connecting resistors in parallel lowers the total resistance. A battery with cells in parallel can therefore supply a larger current.

in a parallel The total current is the sum of the

individual currents

in a parallel circuit The total resistance is lower than the

lowest individual resistance. The voltage across parallel branches is constant.

Parallel connection of power sources is applied if

the required current exceeds the permitted load of a single power source (overload)

For this reason, generators in power plants are connected in parallel.

These rules apply for like power sources with the same voltage. Connecting unlike power sources in parallel causes an unintended circulating current within the supply and is not recommended.

A power source (e.g., battery) generates voltage, known as the

“source voltage”.

Due to the internal resistance of the power source, the total source voltage is not available at the terminals, only the so-called

“terminal voltage”

All electrical supplies have internal resistance. In batteries, it is mainly due to

the resistance of the electrolyte and the plates. In electrical generators, it is mainly the resistance of the machine windings.

The voltage (V) across the open-circuited terminals of a supply is equal to the

Electromotive Force ‑ EMF (E)

When a load is connected to the supply, the load current flows through the internal resistance of the supply and causes a

potential difference or voltage drop across the internal resistance.

The on-load terminal voltage (V) is equal to

the Electromotive Force ‑ EMF minus the potential difference across the internal resistance. Thus, as the current drawn from the supply increases the terminal voltage falls.

Assuming that the internal resistance remains constant, the fall in terminal voltage is

directly proportional to the load current.

In a voltage generator operating off-load (open-circuit operation) no current flows and, therefore, the terminal voltage (V_T)

is the same as the source voltage (V_o).

In a voltage generator operating on-load, where there is a current flowing through the generator, the terminal voltage

decreases as the current increases. This is because the potential difference across the generator’s internal resistance increases as current increases.

To determine the behaviour of a generator in an electric circuit, the power supply is modelled as a

pure EMF in series with an internal resistor

If the terminals of the battery are short-circuited, i.e., they are directly connected to each other, a high current flows in the circuit, this is called a

“short-circuit current”.

The short circuit current is only limited by the

internal resistance of the battery.

(when shorted) The total source voltage drops at the

internal resistance.

The internal resistance of many power sources is very low, which can cause very high short circuit currents. These currents can quickly

destroy the power source.

To achieve a reasonably stable terminal voltage when there are load variations, the choice and combination of power sources is important For example

a power supply unit contains additional components, such as voltage regulators and load resistors. These components ensure a constant voltage and current, which is necessary for electrical equipment.

A battery’s internal resistance is equal to the difference between the

Open Circuit Voltage (OCV) and the Closed-Circuit Voltage (CCV), divided by the load current.

A battery’s internal resistance always becomes greater

as the battery discharges.

Applying heat to metals increases

their temperature, the vibration of the atoms in the metal lattice, and the proper motion of the electrons.

The electrons in a heated metal behave in the same way as the

particles in a gas, moving faster and faster when heated.

Different metals have different numbers of free electrons and, therefore, different

electron densities.

If metals with different electron densities touch each other, electrons move from

the region of higher electron density to the region of lower electron density. This effect is greater at higher temperatures.

If two different metals (e.g., copper and iron) are connected at one end, e.g., by soldering, and their junction is then heated, a voltage is generated between the two free ends. This voltage is referred to as

“thermoelectric voltage”.

the thermoelectric voltage depends on the temperature difference between the junction and the two free ends, as well as the

combination of metals

Chromel – Alumel thermoelectric voltage

4.2 mV per 100℃

Bismuth – Antimony thermoelectric voltage

10.0 mV per 100℃

Constantan – Copper thermoelectric voltage

4.2 mV per 100℃