All cells contain an internal resistance caused by factors such as the
plate material and size etc. In primary cells this resistance is quite
high, with secondary cells due to the plates having a much larger
cross sectional area compared to primary cells the internal
resistance is considerably lower.
When cells and batteries are connected together the internal
resistance has to be taken into account so as to determine the
output characteristics, this is achieved in the following ways:
In order to obtain a high voltage from any cell type it is necessary to
connect the cells in series. In this arrangement the voltage from
each cell is added for example 6, x 2volt cells connected in series
will give an open circuit terminal voltage (EMF) of 12volts. The
disadvantage of a series arrangement will be that the internal
resistance of each cell is also added, this results in a high EMF
voltage but low current output, therefore an increase in current
drawn from the cell will result in a drop in the output voltage due to
the high internal resistance.
In this arrangement all the cells will be required to be of the same
hour rating, as this type of connection will take the lowest
ampere/hour rated cell as its output.
If cells are connected in parallel this will have the effect of the individual
cell voltage being the EMF voltage, due to the large surface area of the
all the cathode plates being connected together, and also all the anode
plates being connected together, the internal resistance is greatly
reduced, in this arrangement the current from each cell is added
together, therefore the current capacity from a parallel connected group
of cells is greatly improved, and as the EMF voltage is the same as the
cell voltage it is considerable more stable when a load is connected to
In this arrangement the cells ampere/hour rating does not need to be
considered, however the voltage rating of each cell needs to be equal
as this type of connection will reflect the lowest voltage of cell as its
From the 2 methods of connection above, it would make sense to
connect all cells in a battery in a series/parallel configuration, this
would provide the battery with a high voltage and also a high current
capacity with an overall lower internal resistance, in this
arrangement the power obtained from the battery would be at its
EMF (Terminal Voltage) and internal
Internal resistance in batteries is mainly due to the resistance of the
electrolyte and the cross sectional area of the plates. The voltage
that is measured across the terminals of the battery when it is off
load is known as the EMF voltage. When a load is connected to a
battery the current flows through the internal resistance and causes
a voltage drop proportional to this resistance, if the internal
resistance remains constant then the fall in the terminal voltage will
be proportional to the load current.
With switch “s” open, the Voltmeter V reads EMF (off load voltage).
With switch “s” closed, the Voltmeter reads Pd (the volt drop across
the load or on-load voltage).
There are several types of alkaline batteries, so to differentiate the
types, the metal used in the manufacture of the plates usually gives
them their name, for example the most common in use on aircraft is
the Nickel/Cadmium battery as it is these 2 metals that are used for
the plates. However in all alkaline batteries the electrolyte solution
used is Potassium Hydroxide which has a specific gravity of 1.24 to
1.30, dependant on the size and material used for the metal plates
this will determine the value of EMF of the cell, with some cells being
as low as 1.2volts.
The cells in these batteries are arranged in steel containers and fitted
with safety valves, this is due to the fact that the cells can be charged
at a high rate – and so can be easily overcharged, this overcharge will
make the electrolyte “bubble” gas and create a large amount of heat,
which can if not dealt with cause the battery to go into a state of
thermal meltdown, Under normal conditions these batteries require
very little maintenance and only a periodic capacity check.
These type of batteries are constructed similar to the semi sealed type,
but when on charge they are deliberately allowed to “gas”, this gassing
is allowed to vent to atmosphere, this aids the temperature control of
the cells and indeed is used to monitor the battery charge by switching
the battery charger on or off dependant on the temperature reached by
Due to the cells being allowed to gas during charge the recharge time
is considerably shorter than the semi sealed type however this has the
disadvantage that the electrolyte requires “topping up” more often, the
frequency of this topping up will be determined by the aircraft manual.
The cells are fitted with the same type of vent as the lead acid battery
in that they allow the electrolyte gasses to escape but not the liquid.
Nickel Cadmium Alkaline
This is a common type of alkaline battery; the plates are formed on
a woven wire mesh by heating, the cathode plate is formed by using
Nickel salts and the anode by using Cadmium salts, a separator of
nylon cloth and a gas barrier of cellophane is used between them,
the electrolyte has a specific gravity of 1.3 and consists of
Potassium Hydroxide and distilled water.
The chemical reaction of alkaline batteries works on the same principle
as the lead acid battery:However as can be seen, the difference is that the electrolyte does not
change its specific gravity or chemical structure during
charge/discharge, it is used purely as a medium in which the electrons
can flow through between the plates. As the charge of the battery
cannot be determined from the electrolyte specific gravity as in the lead
acid type, the temperature of the cell has to be monitored to determine
the charge state, for this reason, switching on or off the charge supply
to maintain the correct ampere/hour rating
Electricity can be generated in 6 basic ways. These are by:
Magnetism and Motion.
We shall look at these methods briefly.
Chemical Action (Cell)
This is by use of 2 different metals placed in a liquid called an
electrolyte. We call this a cell and by grouping these cells we
produce a battery.
Friction (Static Electricity)
When a comb is passed through hair it acquires an electrical charge.
If a piece of tissue paper is then held close to the comb it is attracted
towards it. These effects are due to static electricity (electrostatics).
Aircraft in flight build up large amounts of static electricity and acquire
a charge potential much greater than that of the atmosphere. This
charging of the aircraft is undesirable but unavoidable, although the
effects can be minimized.
Pressure (Crystal Controlled Oscillators)
Certain crystalline substances, notably quartz, exhibit a piezoelectric
effect which results in PD appearing between the opposite faces of
the crystal when it is mechanically deformed and vice versa. The
Crystal can be shown to
Light (Photovoltaic Cells)
These devices utilize the energy from a light source to produce
These devices can be used power battery chargers when connected
as solar panels.
At the point of contact between 2 different metals there exists an
electrical potential difference, which depends on the temperature of
the junction. When we complete the circuit with a second junction at
a different temperature, a current flows in the circuit.
This thermoelectric effect is called the Seebeck effect after the man
who discovered it, and the junction is called a ‘thermocouple’. Either
the net EMF in the circuit or the resulting current may be used to
Primary cells are called so, as the chemical reaction used to
produce the output voltage is extremely difficult, or impossible to
reverse i.e. they cannot be recharged, an example of this is the dry
primary cell, a standard type of dry primary cell is the Zinc carbon
cell, these are constructed by having 2 poles, one of them being a
carbon rod which is the + ve (cathode) and the other pole being
made of zinc, this being the –ve (anode) connected to a steel disc at
the bottom of the cell to aid connection to a circuit. In a dry cell the
zinc pole also acts as the container and surrounds the carbon rod,
and also holds the electrolyte, which is constructed from ammonium
chloride (sal-ammoniac). With this basic arrangement there is a
very distinct disadvantage in that as the cell is being discharged the
chemical reaction that takes place produces hydrogen bubbles
which accumulate around the carbon rod and effectively insulate it
from the electrolyte. To overcome this reaction – known as
polarisation, manganese dioxide is added to the electrolyte during
construction, Another problem that occurs in a dry primary cell is
that because the case is made of zinc this has a tendency to
corrode rapidly, this leads to the case leaking the electrolyte that it is
holding, to prevent this the electrolyte is mixed with wheat flour so
that it forms a thick paste and hence the term “dry” cell. The EMF of
a dry cell is approximately 1.5 volts.