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PROJECT REPORT SHEET
PROJECT DESCRIPTION
NAME
STUDENT ID
: BATTERIES
: Atul Anuragi
: C61588
Electronic battery is a cell which is a combination of two or more than electrochemical
galvanic which has an ability to store energy inside it.
GENERAL FEATURES OF BATTERIES:
The battery has become a common power source for many household and industrial
applications, and is now a multi-billion industry. A battery is a cell which converts
chemical energy in to electrical energy source, it consist of one or more voltaic cells.
One half-cell includes electrolyte and the electrode to which anions which are negativelycharged ions migrate, like the anode or negative electrode; the other half-cell includes
electrolyte and the electrode to which positively-charged ions migrate, i.e. the cathode or
positive electrode.
The battery has always been important as a dc voltage source for the operation of
electronic equipments.
Primary Cell:
A primary cell is any kind of battery in which the electrochemical reaction is not
reversible. A common example of a primary cell is the disposable battery. Unlike a
secondary cell, the reaction cannot be reversed by running a current into the cell; the
chemical reactants cannot be restored to their initial position and capacity. Primary
batteries use up the materials in one or both of their electrodes.
Secondary Cell:
A rechargeable battery (also known as a storage battery) is a group of one or more
electrochemical cells. They are known as secondary cells because their electrochemical
reactions are electrically reversible. Rechargeable batteries come in many different
shapes and sizes, ranging anything from a cell phone battery to megawatt-scale systems
connected to stabilize an electrical distribution network. Several different combinations
of chemicals are commonly used, including: lead acid, nickel cadmium (NiCd), nickel
metal hydride (NiMH), lithium ion (Li-ion), and lithium ion polymer (Li-ion polymer).
VOLTAIC CELL:
A Voltaic cell, named after Alessandro Volta, inventor of the voltaic pile, the first
electrical battery, is an electrochemical cell that derives electrical energy from chemical
reactions taking place within the cell. It generally consists of two different metals
connected by a salt bridge, or individual half-cells separated by a porous membrane. It is
sometimes called a “Galvanic cell", after Luigi Galvani.
A voltaic cell uses a chemical reaction to current. It consist of
1: positive anode
2: negative cathode
3: electrolyte
In electric cell pieces of zinc and Cu dipped in ZnSO4 and CUSO4 respectively. And
these are connected through a voltmeter, the negative cathode releases the electrons and
they flow to positive electrons.
ZINC-CARBON DRY CELL:
Zinc-carbon dry cell is a type of primary cell because as the cell is discharged, it is not
intended to be recharged and must be discarded as well.
A zinc-carbon dry cell is packaged in a zinc can that serves as both a container and
negative terminal. It was developed from the wet Leclanché cell. The positive terminal is
usually a carbon rod surrounded by a mixture of manganese dioxide and carbon powder.
The electrolyte used is a paste of zinc chloride and ammonium chloride dissolved in
water. Zinc chloride cells are an improved version from the original ammonium chloride
variety. Zinc-carbon batteries are the least expensive primary batteries and thus a popular
choice by manufacturers when devices are sold with batteries included. They are
commonly labeled as "General Purpose" batteries. They can be used in remote controls,
flashlights, clocks, or transistor radios, since the power drain is not too heavy.
Construction:
The container of the zinc-carbon dry cell is zinc can. This contains a layer of NH4Cl with
ZnCl2 aqueous paste separated by a paper layer from a mixture of powdered carbon &
manganese (IV) oxide (MnO2) which is packed around a carbon rod.
Figure: Cross-section of a zinc-carbon battery.
Figure: Disassembled zinc chloride cell (similar to zinc carbon cell).
1: entire cell,
2: steel casing,
3: zinc negative electrode,
4: carbon rod,
5: positive electrode (Manganese dioxide mixed with carbon powder and electrolyte), 6:
paper separator,
7: polyethylene leak proof isolation,
8: sealing rings,
9: negative terminal,
10: positive terminal (originally connected to carbon rod)
Early types, and low-cost cells, use a separator consisting of a layer of starch or flour. A
layer of starch-coated paper is used in modern cells, which is thinner and allows more
manganese dioxide to be used. Originally cells were sealed with a layer of asphalt to
prevent drying out of the electrolyte; more recently a thermoplastic washer sealant is
used. The carbon rod is slightly porous, which allows accumulated gas to escape while
retaining the water for the electrolyte. The ratio of manganese dioxide and carbon powder
in the cathode paste affects the characteristics of the cell; more carbon powder lowers the
internal resistance, but more manganese dioxide improves capacity.
Flat cells are also made for assembly into batteries with higher voltages, up to about 450
volts. A number of flat cells are stacked up, and the whole assembly is coated in wax to
prevent evaporation of water from the electrolyte.
ALKALINE CELL:
Alkaline batteries and alkaline cells (a battery being a collection of multiple cells) are a
type of disposable battery or rechargeable battery dependent upon the reaction between
zinc and manganese dioxide (Zn/MnO2).
Compared with zinc-carbon batteries of the Leclanché or zinc chloride types, while all
produce approximately 1.5 volts per cell, alkaline batteries have a higher energy density
and longer shelf-life. Compared with silver-oxide batteries, which alkalines commonly
compete against in button cells, they have lower energy density and shorter lifetimes but
lower cost.
The alkaline battery gets its name because it has an alkaline electrolyte of potassium
hydroxide, instead of the acidic ammonium chloride or zinc chloride electrolyte of the
zinc-carbon batteries which are offered in the same nominal voltages and physical size.
Other battery systems also use alkaline electrolytes, but they use different active
materials for the electrodes.
Construction:
Alkaline batteries are manufactured in standardized cylindrical forms interchangeable
with zinc-carbon batteries, and in button forms. Several individual cells may be
interconnected to form a true "battery", such as those sold for use with flashlights and the
9 volt transistor-radio battery.
A cylindrical cell is contained in a drawn steel can, which is the cathode current collector.
The cathode mixture is a compressed paste of manganese dioxide with carbon powder
added for increased conductivity. The paste may be pressed into the can or deposited as
pre-molded rings. The hollow center of the cathode is lined with a separator, which
prevents mixing of the anode and cathode materials and short-circuiting of the cell. The
separator is made of a non-woven layer of cellulose or a synthetic polymer. The separator
must conduct ions and remain stable in the highly alkaline electrolyte solution.
The anode is composed of a dispersion of zinc powder in a gel containing the potassium
hydroxide electrolyte. To prevent gassing of the cell at the end of its life, more
manganese dioxide is used than required to react with all the zinc.
ADDITIONAL TYPES OF PRIMARY CELLS:
Aluminium battery:
Aluminium batteries or aluminum batteries are commonly known as aluminium-air
batteries or Al-air batteries, since they produce electricity from the reaction of oxygen in
the air with aluminium. They have one of the highest energy densities of all batteries, but
they are not widely used because of previous problems with cost, shelf-life, start-up time
and byproduct removal, which have restricted their use to mainly military applications.
An electric vehicle with aluminium batteries could have potentially ten to fifteen times
the range of lead-acid batteries with a far smaller total weight, at the cost of substantially
increased system complexity.
Al-air is primary batteries, i.e. non-rechargeable. Once the aluminium anode is consumed
by its reaction with atmospheric oxygen at a cathode immersed in a water-based
electrolyte to form hydrated aluminium oxide, the battery will no longer produce
electricity. However, it may be possible to mechanically recharge the battery with new
aluminium anodes made from recycling the hydrated aluminium oxide. Such recycling
will be essential if aluminium-air batteries are to be widely adopted.
Chromic acid cell:
The Chromic acid cell was a type of primary cell which used chromic acid as a
depolarizer. The chromic acid was usually made by acidifying (with sulfuric acid) a
solution of potassium dichromate. The old name for potassium dichromate was potassium
bichromate and the cell was often called a Bichromate cell. This type of cell is now only
of historical interest.
Mercury battery:
A mercury battery (also called mercuric oxide battery or mercury cell) is a nonrechargeable electrochemical battery, a primary cell. Due to the content of mercury, and
the resulting environmental concerns, the sale of mercury batteries is banned in many
countries. Both ANSI and IEC have withdrawn standards for mercury batteries. Mercury
batteries were made in button types for watches and calculators, and in larger forms for
other applications.
Silver oxide battery:
A silver oxide battery, also known as a silver–zinc battery, is a primary cell (although
it may be used as a secondary cell with an open circuit potential of 1.86 volts). Silver
oxide batteries have a long life and very high energy/weight ratio, but a prohibitive cost
for most applications due to the high price of silver. They are available in either very
small sizes as button cells where the amount of silver used is small and not a significant
contributor to the overall product costs, or in large custom design batteries where the
superior performance characteristics of the silver oxide chemistry outweigh cost
considerations. The large cells found some applications with the military, for example in
Mark 37 torpedoes or on Alfa class submarines.
Zinc-air batteries:
Zinc-air batteries (non-rechargeable) and zinc-air fuel cells, (mechanicallyrechargeable) are electro-chemical batteries powered by oxidizing zinc with oxygen from
the air. These batteries have high energy densities and are relatively inexpensive to
produce. Sizes range from very small button cells for hearing aids, larger batteries used in
film cameras that previously used mercury batteries, to very large batteries used for
electric vehicle propulsion.
In operation, a mass of zinc particles forms a porous anode, which is saturated with an
electrolyte. Oxygen from the air reacts at the cathode and form hydroxyl ions which
migrate into the zinc paste and form zincate (Zn(OH)2−4), releasing electrons to travel to
the cathode. The zincate decays into zinc oxide and water returns to the electrolyte. The
water and hydroxyls from the anode are recycled at the cathode, so the water is not
consumed. The reactions produce a theoretical 1.65 volts, but this is reduced to 1.4–1.35
V in available cells.
Zinc-air batteries have some properties of fuel cells as well as batteries: the zinc is the
fuel, the reaction rate can be controlled by varying the air flow, and oxidized
zinc/electrolyte paste can be replaced with fresh paste. A future possibility is to power
electric vehicles.
SUMMARY OF DRY CELLS:
The types of dry cells include carbon-zinc, zinc chloride, and manganese-zinc (alkaline).
Actually, the alkaline cell is better for heavy-duty use than the zinc-chloride type. They
are commonly used in the round, cylinder types. The small button cells generally use
either mercury or silver oxide. All these dry cells are the primary type that cannot be
recharged. Each has output of 1.5 V except for the 1.35 V mercury cell.
Any dry cell loses its ability to produce output voltage even when it is not being used.
The shelf life is about 2 years for the alkaline type, but much less with the carbon-zinc
cell, especially for small sizes and partially used cells. The reasons are self-discharge
within the cell and loss of moisture in the electrolyte. Therefore, dry cells should be used
fresh from the manufacturer.
LITHIUM CELL:
Lithium batteries are disposable (primary) batteries that have lithium metal or lithium
compounds as an anode. Depending on the design and chemical compounds used, lithium
cells can produce voltages from 1.5 V to about 3.7 V, over twice the voltage of an
ordinary zinc-carbon battery or alkaline cell battery. Lithium batteries are widely used in
products such as portable consumer electronic devices.
CR2032
Lithium
Button Cell
Battery
Lithium 9V,
AA & AAA
sizes
Description:
The term "lithium battery" refers to a family of different chemistries, comprising many
types of cathodes and electrolytes.
The most common type of lithium cell used in consumer applications uses metallic
lithium as anode and manganese dioxide as cathode, with a salt of lithium dissolved in an
organic solvent.
Another type of lithium cell having a large energy density is the lithium-thionyl chloride
cell. Lithium-thionyl chloride batteries are generally not sold to the consumer market, and
find more use in commercial/industrial applications, or are installed into devices where
no consumer replacement is performed. In this cell, a liquid mixture of thionyl
chloride (SOCl2)
and lithium
tetrachloroaluminate (LiAlCl4)
acts
as
the electrolyte and cathode respectively. A porous carbon material serves as a cathode
current collector which receives electrons from the external circuit. Lithium-thionyl
chloride batteries are well suited to extremely low-current applications where long life is
necessary, e.g. wireless alarm systems.
Applications:
Lithium batteries find application in many long-life, critical devices, such as artificial
pacemakers and other implantable electronic medical devices. These devices use
specialized lithium-iodide batteries designed to last 15 or more years. But for other, less
critical applications such as in toys, the lithium battery may actually outlast the device. In
such cases, an expensive lithium battery may not be cost-effective.
Lithium batteries can be used in place of ordinary alkaline cells in many devices, such
as clocks and cameras. Although they are more costly, lithium cells will provide much
longer life, thereby minimizing battery replacement. However, attention must be given to
the higher voltage developed by the lithium cells before using them as a drop-in
replacement in devices that normally use ordinary cells.
Small lithium batteries are very commonly used in small, portable electronic devices,
such as PDAs, watches, camcorders, digital cameras, thermometers, calculators, laptop
BIOS, communication equipment and remote car locks. They are available in many
shapes and sizes, with a common variety being the 3 volt "coin" type manganese variety,
typically 20 mm in diameter and 1.6–4 mm thick. The heavy electrical demands of many
of these devices make lithium batteries a particularly attractive option. In particular,
lithium batteries can easily support the brief, heavy current demands of devices such
as digital cameras, and they maintain a higher voltage for a longer period than alkaline
cells.
Some other lithium batteries use a platinum-iridium alloy instead of more usual
compounds. These batteries are generally not preferred, as their cost is high and they tend
to be fragile.
LEAD-ACID WET CELL:
Lead-acid batteries, invented in 1859 by French physicist Gaston Planté, are the oldest
type of rechargeable battery. Despite having a very low energy-to-weight ratio and a low
energy-to-volume ratio, their ability to supply high surge currents means that the cells
maintain a relatively large power-to-weight ratio. These features, along with their low
cost, make them attractive for use in motor vehicles to provide the high current required
by automobile starter motors.
Construction:
Plates
The lead acid cell can be demonstrated using sheet lead plates for the two electrodes.
However such a construction produces only around one ampere for roughly postcard
sized plates, and for only a few minutes.
Gaston Planté found a way to provide a much larger effective surface area. In Planté's
design, the positive and negative plates were formed of two spirals of lead foil, separated
with a sheet of cloth and coiled up. The cells initially had low capacity, so a slow process
of "forming" was required to corrode the lead foils, creating lead dioxide on the plates
and roughen them to increase surface area. Initially this process used electricity from
primary batteries; when generators became available after 1870, the cost of production of
batteries greatly declined. Planté plates are still used in some stationary applications,
where the plates are mechanically grooved to increase their surface area.
Faure pasted-plate construction is typical of automotive batteries. Each plate consists of a
rectangular lead grid alloyed with antimony or calcium to improve the mechanical
characteristics. The holes of the grid are filled with a paste of red lead and 33% dilute
sulfuric acid. (Different manufacturers vary the mixture). The paste is pressed into the
holes in the grid which are slightly tapered on both sides to better retain the paste. This
porous paste allows the acid to react with the lead inside the plate, increasing the surface
area many fold. At this stage the positive and negative plates are similar; however
expanders and additives vary their internal chemistry to assist in operation. Once dry, the
plates are stacked with suitable separators and inserted in the battery container. An odd
number of plates are usually used, with one more positive plate than negative. Each
alternate plate is connected.
The positive plates are the chocolate brown color of Lead (IV) Oxide, and the negative
are the slate gray of 'spongy' lead at the time of manufacture. In this charged state the
plates are called 'formed'.
One of the problems with the plates is that the plates increase in size as the active
material absorbs sulfate from the acid during discharge and decrease as they give up the
sulfate during charging. This causes the plates to gradually shed the paste. It is important
that there is room underneath the plates to catch this shed material. If it reaches the plates,
the cell short-circuits.
The paste contains carbon black, blanc fixe (barium sulfate) and lignosulfonate. The
blanc fixe acts as a seed crystal for the lead–to–lead sulfate reaction. The blanc fixe must
be fully dispersed in the paste in order for it to be effective. The lignosulfonate prevents
the negative plate from forming a solid mass during the discharge cycle, instead enabling
the formation of long needle–like crystals. The long crystals have more surface area and
are easily converted back to the original state on charging. Carbon black counteracts the
effect
of
inhibiting
formation
caused
by
the
lignosulfonates.
Sulfonated naphthalene condensate dispersant is a more effective expander than
lignosulfonate and speeds up formation. This dispersant improves dispersion of barium
sulfate in the paste, reduces hydroset time, produces a more breakage-resistant plate,
reduces fine lead particles and thereby improves handling and pasting characteristics. It
extends battery life by increasing end–of–charge voltage. Sulfonated naphthalene
requires about one-third to one-half the amount of lignosulfonate and is stable to higher
temperatures.
About 60% of the weight of an automotive-type lead-acid battery rated around 60 Ah
(8.7 kg of a 14.5 kg battery) is lead or internal parts made of lead; the balance is
electrolyte, separators, and the case.
Separators
Separators between the positive and negative plates prevent short-circuit through physical
contact, mostly through dendrites (‘treeing’), but also through shedding of the active
material. Separators obstruct the flow of ions between the plates and increase the internal
resistance
of
the
cell. Wood, rubber,
glass
fiber
mat, cellulose,
and PVC or polyethylene plastic have been used to make separators. Wood was the
original choice, but deteriorated in the acid electrolyte. Rubber separators were stable in
the battery acid.
An effective separator must possess a number of mechanical properties; such
as permeability, porosity, pore size distribution, specific surface area, mechanical design
and strength, electrical resistance, ionic conductivity, and chemical compatibility with the
electrolyte. In service, the separator must have good resistance to acid and oxidation. The
area of the separator must be a little larger than the area of the plates to prevent material
shorting between the plates. The separators must remain stable over the
battery's operating temperature range.
Applications:
Wet cell stand-by (stationary) batteries designed for deep discharge are commonly used
in large backup power supplies for telephone and computer centers, grid energy storage,
and off-grid household electric power systems [5]. Lead-acid batteries are used in
emergency lighting in case of power failure.
Traction (propulsion) batteries are used for in golf carts and other battery electric
vehicles. Large lead-acid batteries are also used to power the electric motors in dieselelectric (conventional) submarines and are used on nuclear submarines as well. Motor
vehicle starting, lighting and ignition (SLI) batteries (car batteries) provides current for
starting internal combustion engines.
Valve-regulated lead acid batteries cannot spill their electrolyte. They are used in back-up
power supplies for alarm and smaller computer systems (particularly in uninterruptible
power supplies) and for electric scooters, electrified bicycles, marine applications, battery
electric vehicles or micro hybrid vehicles, and motorcycles.
Lead-acid batteries were used to supply the filament (heater) voltage (usually between 2
and 12 volts with 2 V being most common) in early vacuum tube (valve) radio receivers.
ADDITIONAL TYPES OF SECONDARY CELLS:
Alkaline Battery:
Alkaline battery (also known as Alkaline Rechargeable or Rechargeable Alkaline
Manganese (RAM)) is a type of alkaline battery that is rechargeable. The first generation
rechargeable alkaline technology was developed by Battery Technologies Inc in Canada
and licensed to Pure Energy, EnviroCell, Rayovac and Grandcell. Subsequent patent and
advancements
in
technology
have
been
introduced.
The
shapes
include AAA, AA, C, D and snap-on 9-volt batteries. Rechargeable alkaline batteries
have the ability to carry their charge for years, unlike most NiCd and NiMH batteries
which self-discharge in 90 days (see below **), however new low-discharge NiMH cells,
such as Sanyo "Eneloop", claim to retain 85% charge after 1 year. If produced properly,
rechargeable alkaline batteries are an environmentally friendly form of energy storage.
Rechargeable Alkaline AA Battery
Nickel-cadmium battery (NiCd):
Created by Waldemar Jungner of Sweden in 1899, based on Thomas Edison's first
alkaline battery. Using nickel oxide hydroxide and metallic cadmium as electrodes.
Cadmium is a toxic element, and was banned for most uses by the European Union in
2004. Nickel-cadmium batteries have been almost completely superseded by nickel-metal
hydride (NiMH) batteries.
Nickel-metal hydride battery (NiMH):
First commercial types were available in 1989.]These are now a common consumer and
industrial type. The battery has a hydrogen-absorbing alloy for the negative electrode
instead of cadmium.
Lithium-ion battery:
The technology behind lithium-ion battery has not yet fully reached maturity. However,
the batteries are the type of choice in many consumer electronics and have one of the best
energy-to-mass ratios and a very slow loss of charge when not in use.
Lithium-ion polymer battery:
These batteries are light in weight and can be made in any shape desired.
Lithium sulfur battery:
A new battery chemistry developed by Sion Power since 1994. Claims superior energy to
weight than current lithium technologies on the market. Also lower material cost may
help this product reach the mass market. Not to be confused with lithium sulfur dioxide
(Li-SO2) batteries which explode when recharged.
Thin film battery (TFB):
An emerging refinement of the lithium ion technology by Excellatron. The developers
claim a very large increase in recharge cycles, around 40,000 cycles. Higher charge and
discharge rates. At least 5C charge rate. Sustained 60C discharge, and 1000C peak
discharge rate. And also a significant increase in specific energy, and energy density.
Also Infinite Power Solutions makes thin film batteries (TFB) for micro-electronic
applications that are flexible, rechargeable, solid-state lithium batteries.
Potassium-ion battery:
This type of rechargeable battery can deliver the best known cycleability, in order of a
million cycles, due to the extraordinary electrochemical stability of potassium
insertion/extraction materials such as Prussian blue.
SERIES AND PARALLEL CELLS:
It is possible to vary total voltage and current from a number of batteries by connecting
them in different ways in the circuit. It does not matter where in the circuit the batteries
are placed; it is how they are placed with respect to each other that are important.
Basically, they can be connected in series or in parallel. The resultant voltage and current
can be calculated by using a few simple rules.
Connecting batteries in series:
The word series means "following on from the
previous one", like a TV series for example.
It's important to connect the batteries with their
terminals in the correct order. Batteries in series need
to be connected with the positive end of one battery
to the negative end of the next battery.
If they are incorrectly connected, the batteries will
cancel out each other's energy and quickly flatten
each other.
Batteries correctly placed in series, positive to
negative, will add their output voltages, producing a greater voltage.
Voltage and current produced by batteries in series:
If two 1.5 volt batteries are connected head to tail, the total voltage is 3.0 volt. This is
because batteries in series produce a voltage equal to the number of batteries multiplied
by the voltage of each individual battery.
Batteries with voltages greater than 1.5 volts are made up of
cells connected in series inside a single case. In the 9 volt
battery above, there are six cells connected in series. The
calculation is 6 × 1.5 Volt = 9 Volt.
When batteries are connected in series the flow of electrons,
as measured by the current, is the same anywhere in the
circuit.
A 9 Volt battery will produce a voltage 6 times larger than a
single 1.5 Volt battery in the same circuit, but the current in
each circuit will be the same no matter where the current is
measured.
This happens because the batteries are arranged in a line, and like water flowing through
different hoses connected in a line, what goes in one end must come out the other. The
same electrons must flow through all the batteries at the same rate, so the current must be
the same in each battery and in each part of the circuit.
Batteries in parallel:
The word parallel means "alongside each other". When batteries are placed in parallel all
the positive terminals are joined together with a single wire to one part of the circuit, and
all the negative terminals are joined with a single wire to the rest of the circuit.
Remember the voltage increases when batteries are in
series, but with batteries in parallel this is not the
case. When two or more batteries are placed in
parallel, the voltage in the circuit is the same as each
individual battery. That is two; three, four or more 1.5
volt batteries in parallel will produce a voltage of 1.5
Volts!
In a parallel circuit, individual electrons can only pass
through one of the alternative paths and batteries at a
time, thus each electron can only gain energy from one of the batteries in the circuit. As
voltage is a measure of the energy carried by the electrons in the circuit, the increase in
voltage for each electron in the circuit is the same as if they had passed through only one
battery.
Purpose of batteries in parallel:
When batteries are connected in parallel, the current flowing through the circuit increases
with the number of batteries in the circuit. Each battery can pump a set number of
electrons per second, for a given circuit, so if two or more batteries are connected in
parallel the number of electrons they push out each second and energy supplied is added,
hence the total current in the circuit is increased.
Summary of batteries in series:
When batteries are connected in series, the voltage increases.
Summary of batteries in parallel:
When batteries are connected in parallel, the voltage remains the same, but the current
that can flow in the circuit increases.
CURRENT DRAIN DEPENDS ON LOAD RESISTANCE:
It is important to note that the current rating of batteries, or any voltage source, is only a
guide to typical values permissible for normal service life. The actual amount of current
produced when the battery is connected to a load resistance is equal to I=V/R, by Ohm’s
law
The two principles here are;
 The cell delivers less current for a higher resistance in the load circuit.
 The cell can deliver a smaller load current for a longer time.
The amount of current is determined by the circuit, not by the current rating of the
battery.
INTERNAL RESISTANCE OF A GENERATOR:
The internal resistance ri is important when a generator suppplies load current because its
internal voltage drop subtracts from the generated emf, resulting in lower voltage across
the output terminals. Physically, ri may be the resistance of the wire in a rotary generator,
or in a chemical cell ri is the resistance of the electrolyte between electrodes. More
generally, the internal resistance is the opposition to load cruurent inside the generator.
Since any current in the generator must flow through the internal resistance, ri is in sseries
with the generator must flow through the internal resistance, ri is in series with the
generated voltage. It may be of interest to note that, with just one load resistance
connected across a generator, they are in series with each other because RL is in series
with ri.
The internal resistance of any generator can be measured indirectly by determining how
much the output voltage drops for a specified amount of load current. The difference
between the no-load voltage and the load voltage is the amount of internal voltage drop
ILri. dividing by IL gives the value of ri. As a formula:
ri = VNL – VL / IL
WHY THE TERMINAL VOLTAGE DROPS WITH MORE LOAD CURRENT:
The below figure illustrates how the output of a 100V source can drop to 90V because of
the internal 10 V drop across ri. In the first figure the voltage across the output terminals
is equal to the 100V of VG because there is no load current on an open circuit. With no
current, the voltage drop across ri is zero. Then the full generated voltage is available
across the output terminals. This value is the generated emf, open-circuit voltage, or noload voltage.
In the second figure with a load, however, current of 0.1A flows, to produce a drop of
10V across the 100ohm of ri. Note that RT is 900+100=1000ohm.Then IL equals 100/1000
which is 0.1A
As a result, the voltage output VL equals 100-10 =90V. This terminal voltage or load
voltage is available across the output terminals when the generator is in a closed circuit
with load current. The 10V internal drop is subtracted from VG because they are seriesopposing voltages.
The graph shows how the terminal voltage VL drops with increasing load current IL.
CONSTANT-VOLTAGE & CONSTANT-CURRENT SOURCES:
Constant – Voltage Generator:
A generator with very low internal resistance is considered to be a constant voltage
source. Then the output voltage remains essentially the same when the load current
changes. For a 6V lead – acid battery with an ri of 0.005ohm. If the load current varies
over the wide range of 1 to 100A, for any of these values, the internal Iri drop across
0.005ohm is less than 0.5V.
Constant – Current Generator:
It has very high resistance, compared with the external load resistance, resulting in
constant current, although the output voltage varies
The constant-current generator shown below has such high resistance, with a ri of
0.9Mohm that it is the main factor determining how much current can be produced by VG.
Here R L varies in a 3:1 range from 50 to 150kohm. Since the current is determined by
the total resistance of RL and ri in series, however, I is essentially constant at 1.05 to 0.95
mA, or approximately 1 mA.
Note that the terminal voltage VL varies in approximately the same 3:1 range as RL. Also,
the output voltage is much less than the generator voltage because of the high internal
resistance compared with R L. This is a necessary condition, however, in a circuit with a
constant-current generator
MATCHING A LOAD RESISTANCE TO THE GENERATOR:
When RL equals ri, the load and generator are matched. The matching is significant
because the generator then produces maximum power in RL .
Maximum Power in RL :
When RL is 100ohm to match the 100ohm of ri, maximum power is transferred from the
generator to the load. With higher resistance for RL, the output voltage VL is higher, but
the current is reduced. Lower resistance for RL allows more current, but VL , is less. When
ri and RL both equal 100ohm, this combination of current and voltage produces the
maximum power of 100W across RL.
Maximum Voltage Across RL :
If maximum voltage, rather than power, is desired, the load should have as high a
resistance as possible.
Maximum Efficiency:
The efficiency ijncreases as RL invreases because there is less current, resulting in less
power lost in ri. When RL equals ri , the efficiency is only 50 percent, since one half the
total generated power is dissipated in ri, the internal resistance of the generator. In the
conclusion, matching the load and generator resistances is desirable when the load
required maximum power rather than maximum voltage or deeiciency, assuming that the
match does not result in excessive current.