Automotive mechanics (volume i)(part 6, chapter38) the battery

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Chapter 38

The battery

Materials in the battery

Use of jumper leads

Battery construction

Technical terms

Chemical actions in the battery

Review questions

Low-maintenance and maintenance-free
batteries
Battery voltage
Battery specifications
Battery testing

Battery charging
Care of batteries in stock
Battery maintenance
Battery faults
Battery safety


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668 part six basics of the electrical system
Automotive batteries consist of a number of lead-acid
cells that are used to store electrical energy by
chemical means.
The battery supplies current for the operation of the
starter motor when the engine is being cranked for
starting, and also supplies current for the lights, radio,
instruments and other accessories that are used when
the engine is not running.
Once the engine is started, the alternator supplies
all the power requirements for the various electrical
systems, including recharging the battery, although
the battery continues to have a stabilising effect on the
electrical system.
Automotive light vehicles are almost universally

fitted with 12 volt electrical systems. Commercial,
motorcycle, recreational and older vehicles may use 6,
12 or 24 volt systems or a combination of two systems
(for example, 6/12V, 12/24V). The trend is to higher
voltages to increase efficiency and reduce size of
wiring and components. Manufacturers are now
producing 42V systems. Safety precautions for these
systems must be followed.

Materials in the battery
The main materials used within the battery are spongy
lead (a solid), lead oxide (a solid), and sulphuric acid
(a liquid). These three substances are brought together
in such a way that they can react chemically to produce
a flow of current.
The lead oxide and spongy lead are held in plate
grids to form positive and negative plates. The sulphuric
acid is diluted with water to become the electrolyte.
The plate grid (Figure 38.1) consists of a framework of lead alloy with horizontal and vertical bars.
The plate grids are made into plates (Figure 38.2) by
the application of lead oxide pastes, which harden. The
horizontal and vertical bars serve to hold the hardened
pastes in the plates.

figure 38.2

After the plates are assembled into the battery, the
battery is given an initial forming charge. This changes
the lead oxide in the negative plate to spongy lead, and
the lead oxide in the positive plate to lead peroxide.

Battery construction
When the battery is being made, several plates are spaced
and welded to a strap to form a plate group (Figure 38.3).
Plates of two different types are used, one for the
positive plate group, and the other for the negative
plate group. A positive plate group is nested with a
negative plate group, with separators placed between
the plates to form an element (Figure 38.4).
Separators are designed to hold the plates apart, but
at the same time they must be porous enough to allow
the electrolyte to circulate between the plates. Separators are made from various materials, including
plastic, rubber and fibreglass.
During manufacture, the elements are placed in
compartments in the battery case, which is made of
either hard rubber or polypropylene. Each of the
compartments forms a cell. The top of the case is
enclosed by a cover which is sealed to the case.

figure 38.3
figure 38.1

A battery plate grid

A battery plate – lead oxide paste has been
applied to the plate grid

A battery plate group – the plates are connected together by the plate strap

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chapter thirty-eight the battery

figure 38.5

7figure 38.4

The groups of positive and negative plates
are assembled with separators to form an
element – this becomes one cell when assembled into the
battery case

669

Battery cell connections – 2-volt cells
connected in series can provide 6 volts or
12 volts

terminals or battery cover may be identified (+) or (–).
Where the battery has round terminal posts, the
positive post is larger than the negative post.
The battery cover of a standard battery has a filler
hole for each cell, which enables the cells to be filled

with electrolyte during manufacture or topped up with
distilled water during service. The holes are fitted with
filler caps or plugs that have small vent holes.
A complete battery is shown in Figure 38.6, but
with one end cut away so that the internal construction
can be seen.
■ Low maintenance and maintenance free batteries
may not have filler plugs. These are covered later.

Each cell is a separate part of the battery, with
its own electrolyte, but the cells are electrically
connected. They are connected in series inside the
battery, with the positive terminal of a cell connected
to the negative terminal of its adjacent cell. The end
cells carry the main battery terminals, or posts.
With series connections, the voltages of the cells
are added. There are six cells in a 12 volt battery, the
cell connections for 12 volts are shown in Figure 38.5.

Chemical actions in the battery

Battery terminals

Action on discharge

The battery terminals, or posts, extend through the
cover, with the positive terminal located at one end of
the battery and the negative terminal at the other.
The terminals can be either in the form of posts or
lugs, and it is important to know the polarity of these

so that the battery can be correctly installed in the
vehicle. For this reason, the terminals are identified in
some way. This can be by means of a paint mark (red
for positive and black or green for negative) or the

When a circuit with a load is connected to the battery,
the acid in the electrolyte commences to combine with
the active material on the plates to form lead sulphate.
This process continues until the active material on both
the positive and negative plates has been converted to
lead sulphate.
This reduces the quantity of sulphuric acid in the
electrolyte, so that its density is reduced. When this
stage is reached, the battery is discharged.

The electrolyte in the battery is made up of about 40%
sulphuric acid and 60% water (Figure 38.7).
When the plates are given a charge, chemical
actions remove electrons from one group of plates and
mass them at the other. This transfer of electrons is
carried on until there is a nominal 2 volt potential
between the two groups of plates.


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670 part six basics of the electrical system

figure 38.6

Construction of a 12 volt battery

figure 38.7

Electrolyte is approximately 40% sulphuric
acid and 60% water

Action on charge
The battery can be recharged by current from the
alternator or a battery charger, which passes current
through the battery in a reverse direction. This reverses
the chemical activity.
The plates now are converted back to lead peroxide
(positive plates) and spongy lead (negative plates). The
sulphuric acid is removed from the plates during
charging to combine with the electrolyte and so
increase its density.
Therefore, the density of the electrolyte is related to
the state of charge of the battery – high when the battery
is charged, and low when the battery is discharged.
These conditions are illustrated in Figure 38.8,
where the electrolyte is being checked with a hydro-

figure 38.8

Battery action
(a) when discharged, the density of the
electrolyte is reduced and the float in the hydrometer does
not rise (b) when charged, the density of the electrolyte is
increased and the float rises

meter. The float does not rise when the battery is flat,
but floats quite high when the battery is charged.


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■ The method of testing is covered later in the section
‘Hydrometer test’.

Low-maintenance and
maintenance-free batteries
Some batteries are classed as low-maintenance
batteries and others as maintenance-free batteries.

Low-maintenance batteries are designed so that
they need minimum topping up. The plates are made
with special material, and the venting system is usually
modified to reduce the loss of vapour from the electrolyte. Low maintenance batteries usually have covers
that fit over the filler holes in the main battery cover.
These can be removed if it is considered necessary to
check the electrolyte level.
With maintenance-free batteries, sealing is further
developed so that the original electrolyte lasts the life
of the battery. They are easily identified because they
do not have the usual type of vent plugs. Instead, the
cells are sealed in some other way, sometimes with a
flat cover, which does not normally have to be
removed.

figure 38.9

The voltmeter connected to the battery
shows the voltage readings for different
conditions

Battery voltage
Passenger cars and light commercial vehicles have
12 volt batteries. This is the combined voltage of all
the cells of the battery. Heavier vehicles, particularly
those with diesel engines, have 24 volt electrical
systems, and these use two 12 volt batteries, or in some
instances, four 6 volt batteries.
Where more than one battery is used, they are
connected in series to provide the higher voltage.

Variations in battery voltage
While the battery cell has a nominal voltage of 2 volts,
a fully charged battery will give a reading of 2.1 volts
per cell when checked without any load. This is
referred to as the open-circuit voltage. A 12 volt
battery will therefore have an open-circuit voltage of
12.6 volts.
Figure 38.9 shows how battery voltage can vary
under different conditions of operation:
1. Open circuit. When tested without any load, a fully
charged battery should have a voltmeter reading of
12.6 volts.
2. Starter operating. When under a heavy load, when
the starter motor is operating, the battery voltage
could drop to 9.5 volts.

3. Headlights on. When under a lighter load, as when
the headlamps are switched on, battery voltage
could be 11 volts.
4. Alternator operating. With the engine running and
the alternator operating, the battery voltage could
rise as high as almost 15 volts.
■ The alternator regulator limits the voltage during
charging so that the system voltage does not
become too high.

Battery specifications
Apart from the voltage and the actual dimensions of
the battery case, batteries can be specified by the
manufacturer in a number of different ways: the

number of plates, the capacity, the cranking current,
and the reserve capacity. This enables the correct
replacement battery to be obtained.
While all these specifications are used, the trend is
for battery manufacturers to quote cranking current
and reserve capacity because these are the most
important specifications when it comes to starting the
engine.


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672 part six basics of the electrical system
Number of plates
This is the number of plates in each cell of the battery.
A battery is commonly referred to by both its voltage
and the number of plates per cell.
For example, a 12 volt, 7 plate battery indicates that
the battery has 6 cells, each of 2 volts, with 7 plates in
each cell. In some cases, the total number of plates is
stated instead of the plates per cell. The battery
referred to would then be specified as a 12 volt,
42 plate battery.
Battery capacity

The capacity of a battery is stated in ampere hours,
and this relates to its electrical size. This depends on
the total area and the volume of the active plate
material. The capacity of a battery can be rated in
several ways, the most common being the twenty hour
rate.
The twenty hour rate represents the current a
battery can deliver for twenty hours without cell
voltage dropping below 1.75 volts, starting with a
temperature of 25°C. A battery capable of delivering
5 amperes for twenty hours would be rated as a
100 ampere hour battery.
■ An ampere hour is 1 amp delivered from the battery
for a period of one hour.
Cold-cranking amps
The cold-cranking amps (CCA) rating of a battery is
the current that a fully charged battery can supply for a
short period during a cold start. This would be around
200 amps for a small battery and 400 amps for a large
battery.
The most severe load imposed on an automotive
battery occurs immediately after the starter is engaged
and when it is just starting to turn (crank) the engine.
This is greatest when the engine is cold. The cranking
current that can be provided by a battery is therefore
important.
The actual test, which is an SAE performance test,
measures the load in amps which a new, fully-charged
battery can deliver for thirty seconds, while
maintaining a voltage of 1.2 volts per cell or higher.

The battery is then given a cold-cranking amps rating
based on its performance.

its voltage dropping below 10.5 volts for a 12 volt
battery.

Battery testing
There are two methods commonly used to test
batteries:
1. The hydrometer test. The hydrometer test measures
the density of the electrolyte in each cell to determine the state of charge of the battery.
2. The high-rate discharge test. The high-rate
discharge test indicates the condition of the battery
by checking its ability to maintain its voltage under
load.
Hydrometer test
As previously stated, the electrolyte loses sulphuric
acid to the plates as it is discharged, and so the
electrolyte becomes less dense and more like water.
Measuring the density of the electrolyte will therefore
show the state of charge of the battery.
A hydrometer is used to measure the electrolyte
density, as shown in Figure 38.10. The bulb on the top
of the hydrometer is used to draw electrolyte from a
battery cell up into the clear tube. The float inside the
tube will float high in the electrolyte when it is dense
(battery charged) and low in the electrolyte when it is
less dense (battery discharged). The stem of the float
is graduated to show the electrolyte density and the
state of battery charge.

Reserve capacity
This is the time, in minutes, that a new fully charged
battery will supply a constant load of 25 amps without

figure 38.10

Checking a battery with a hydrometer
MITSUBISHI


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To check a battery
Measure the density of the electrolyte in each cell with
a hydrometer. All the readings should be approximately the same.
1. If one cell gives a reading much lower than the
others, this indicates a faulty cell.
2. If all cells are low, then the battery is discharged. It
should be recharged and then tested.

3. If the level of the electrolyte is so low that a
hydrometer reading cannot be taken, the cell should
be topped up with water and charged for at least
thirty minutes before an accurate reading will be
obtained.
■ Electrolyte should not be transferred from one cell
to another.
Variations of electrolyte density
The density of the battery electrolyte can vary as
a result of several conditions: state of charge,
temperature, age of battery, and self-discharge.
Variation with state of charge
As the state of charge of the battery changes, more or
less sulphuric acid will be present in the electrolyte,
causing a higher or lower density reading when the
battery is tested with a hydrometer.
The following density readings are an approximate
guide to battery condition:
1. 1240–1260 – fully charged battery
2. 1210–1240 – 75% charged
3. 1180–1210 – 50% charged
4. 1150–1180 – 25% charged
5. 1130–1150 – barely operative
6. 1110–1130 – completely discharged.
A reading of 1.250 shows that the electrolyte is 1.25
times the density of water (Figure 38.11). The decimal
point is usually omitted and the reading stated as a
whole number, in this example, 1250.
■ The term relative density is also used, indicting the
relationship with the density of water.

Variation of density with temperature
Temperature also changes the density. This is due to
the fact that when a liquid cools, it contracts, and when
a liquid becomes heated, it expands. Therefore,
temperature should be considered when a density

figure 38.11

Electrolyte for a fully charged battery is
1.25 times heavier than water – it has a
density of 1.25

reading is taken and a correction made if the
temperature varies greatly from standard.
The correction involves the addition or subtraction
of points, according to whether the electrolyte
temperature is above or below the 25°C standard. The
density of electrolyte changes about five points for
every 5° in temperature. To make a temperature
correction, five points must be added for every 5°
above 25°C, and five points subtracted for every 5°
below 25°C.
Loss of density with age
As the battery ages, the electrolyte gradually loses
density. This is because of the loss of active material
from the plates. Material is gradually shed from the
plates and drops into the bottom of the cells as
sediment.
The electrolyte can also suffer a gradual loss of acid
due to gassing.

■ Overcharging can cause excessive gassing and this
will result in loss of acid from the electrolyte.
Loss of density with self-discharge
If a battery is allowed to stand idle for a long period of
time, it will slowly self-discharge. This is caused by
internal chemical reactions between the battery
materials, even though there is no flow of current at
the time. The higher the battery temperature, the more
rapidly self-discharge will take place.
The lead sulphate that forms on the battery plates as
a result of self-discharge is difficult to reconvert into
active material. Therefore, a battery that has badly selfdischarged may be ruined.


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674 part six basics of the electrical system
■ This is what would occur if a battery was allowed
to remain in a vehicle that was not used for a long
period.
High-rate discharge test
A high-rate discharge tester consists of a voltmeter
for reading the battery voltage, an ammeter capable
of reading high current (over 200 amps), and a

carbon-pile rheostat for applying a load to the battery
(Figure 38.12).

battery is in good condition – a reading lower than this
would indicate a flat or faulty battery.
During the test, the ignition system must be isolated
to prevent the engine from starting and the fuel
injection system isolated to prevent injection.
■ The starter and cables are also part of the test and
a faulty starter or cable connections would affect
the voltmeter reading.

Battery charging
If the battery is to be charged while in the vehicle, it
should be isolated by removing both battery cables
before connecting the battery charger.
The charger has two leads with battery clips for
attaching the charger to the battery (Figure 38.13).
These are marked positive (+) and negative (–). The
(+) clip is connected to the (+) battery terminal, and
the (–) clip is connected to the (–) battery terminal.
(Positive-to-positive and negative-to-negative connections.)
The charger provides a slightly higher voltage than
the battery, so that current flows from the charger to
the battery. For normal slow charging, this is adjusted
to provide a charging rate of 4 to 6 amps.

figure 38.12

Battery capacity test – the battery is tested

under load MITSUBISHI

The connections are made to the battery and the
load adjusted by turning the knob of the rheostat. The
load is adjusted until the current shown on the ammeter
is three times the ampere hour capacity of the battery.
Therefore, for a 50 ampere hour battery, a current of
150 amps is required.
After fifteen seconds, the voltmeter should show a
battery terminal voltage of 9.25 volts or more for a
12 volt battery. By using a current of three times the
battery capacity, the size of the battery is taken into
account during the test.
The cranking-current rating of the battery can be
used instead of ampere hours. In this case, half the
specified cranking current is used for the test.
Starter load test
A similar test can be made with a voltmeter connected
across the battery, while cranking the engine with the
starter motor to provide a load.
The voltmeter should read 9.5 volts or more if the

figure 38.13

A battery charger is connected to the battery
with positive-to-positive and negative-tonegative


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The charging is continued until the battery is
gassing freely and there is no further rise in density for
two hours. The charging voltage is held at a constant
value. The battery, as it approaches a charged condition, increases in resistance to the charging current.
This causes the current input to taper off gradually
until, when the battery is fully charged, the current
input will be reduced to a few amps.
Fast chargers
These operate by charging the battery at a high rate for
a short period (up to thirty minutes) so that the battery
is brought up to a reasonable state of charge before its
temperature increases excessively.
Provided that temperatures can be controlled, fast
charging does not appear to be damaging to the
battery. To bring a battery up to full charge, it is
necessary to complete charging by a slow-charging
method.
■ Batteries in doubtful condition should not, in
general, be fast-charged as complete failure could
result.

Care of batteries in stock

Wet batteries
Wet batteries, that is, batteries with electrolyte, are
subject to self-discharge, which, if allowed to occur for
a period, will cause sulphation of the plates. Batteries
should be recharged at regular intervals. Alternatively,
a special slow-rate charger, known as a trickle charger,
can be used to keep batteries in stock in a fully charged
condition.

675

1. Remove the vent plugs and take out any seals.
2. Fill each battery cell in turn. Wait a few minutes
and then add more electrolyte if necessary. Do not
overfill the battery.
3. Before discarding the container, for safety reasons,
empty it and rinse it thoroughly with water to
remove any remaining acid.
4. After filling the battery, it should be allowed to
stand for a short time to allow it to become activated before being used. If the battery is not being
used at once in a vehicle, then it should be given a
short slow-charge.

Battery maintenance
Battery maintenance consists of regular inspection,
cleaning, testing, and charging when necessary.
The battery terminals, top of the battery and battery
carrier should be kept clean. Corroded parts can be
cleaned with a solution of ordinary baking soda and
water, but the solution must be kept out of the battery

cells.
Figure 38.14 illustrates a battery in its mounting
and indicates various parts that should be checked
during battery service. These are as follows:
1. terminals clean and
cables in good condition
2. battery posts
clean

3. electrolyte
level correct

Dry-charged batteries
Some replacement batteries that are designed to be
held in stock are dry charged. Dry-charged batteries
contain fully charged positive and negative plates but
no electrolyte.
The batteries are sealed with rubber or plastic seals
placed in the vent plugs. Because the batteries contain
no moisture, practically no chemical action can take
place. This means that they will remain in good
condition for many months, provided they are properly
stored.
Dry-charged batteries are supplied with readymixed electrolyte in a special acid-proof plastic
container. To activate a battery, the following is
necessary:

6. hold-down
clamp secure

4. case and
cover clean
5. battery tray clean

figure 38.14

Battery and mounting – points to be checked
during service HOLDEN LTD


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676 part six basics of the electrical system
1. Cables in good condition and terminals clean.

Installing a battery

2. Battery posts clean.

■ A ‘battery saver’ should be installed before removing
the battery to save computer codes and personal
settings on audio and climate control systems etc.

3. Electrolyte level correct.

4. Case and cover clean.
5. Battery tray clean and free of corrosion.
6. Hold-down clamp clean and holding the battery
securely.
Electrolyte level
The electrolyte should be 6 to 8 mm above the top of
the separators. The level of the electrolyte should be
checked regularly, except for maintenance-free
batteries, which are usually sealed.
Water should be added to each cell, if necessary, to
bring the electrolyte to the correct level. Distilled or
deionised water should be used, but any water that is
fit to drink can be used in an emergency. Clean rainwater is also suitable.
The battery should not be overfilled because this
will cause electrolyte to be forced from the vent hole
during charging. This can cause corrosion at the
terminals, and corrosion of the battery mounting and
adjacent parts.
Checking the level
There are different ways of checking the electrolyte
level. The electrolyte level can be seen by removing
the vent plugs and looking through the vent-plug holes.
Most batteries have a vent well at the plug hole, and
the electrolyte is topped up to the bottom of the well
(Figure 38.15).
Batteries with a transparent case have high-level
and low-level marks on the case. The height of the
electrolyte in the cells can be seen through the case and
checked against the level marks. This can be done
without removing the vent plugs.

figure 38.15

Electrolyte level
(a) level to bottom of filler well (b) level marks
on transparent case

Apart from being clean and secure in its mounting, the
battery must be installed the right way, that is, with its
negative terminal connected to earth. The alternator or
other electrical equipment could be damaged if the
battery is installed with its polarity reversed.
The procedure for installing a battery is as follows:
1. First identify the positive and negative cables.
2. Then identify the positive and negative posts of the
battery and install the battery in its carrier so that
the posts are in the right position for the cables.
3. Fit the terminal of the positive cable to the positive
battery post first.
4. Then fit the terminal of the negative cable to the
negative battery post.
5. The battery terminals should be tightened firmly
and given a light coating of grease to protect them
against corrosion.
■ To prevent the possibility of short-circuiting the
battery, the negative (earth) terminal should be
removed first and replaced last.
Battery shroud
Some vehicles provide a shroud into which the battery
is fitted. Figure 38.16 shows the arrangement of a

shroud that has an air scoop. This directs cooler outside air through the shroud and around the battery.
Batteries are mounted in the engine compartment
and the shroud provides some protection from the heat
of the engine.
Replacement battery
If a new battery is to be installed, it must have a rating
which is the same or greater than the original battery.
The cold-cranking amps rating is the easiest way to
compare battery ratings.
The battery must be fit for the purpose it is going to
be used for. Off road and commercial vehicles require
stronger internal construction because of vibration and
jolting on rough roads. Recreational vehicles, such as
mobile homes, use deep cycle batteries that are
designed to be completely discharged and recharged.
Car batteries are not suitable for this type of use or for
marine applications.


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Undercharging

battery

If the battery is discharged, it should be recharged as outlined in previous sections. In addition, an attempt should
be made to determine the cause of the discharged battery.
It could be caused by the alternator or regulator
malfunctioning, by defective connections in the
charging circuit between the alternator and the battery,
or by excessive load demands on the battery.
A battery that has been allowed to stand idle for long
periods could be flat from self-discharge. If a battery
will not recharge, it could be defective due to its age.

shroud

Alternator check
air scoop

figure 38.16

Where overcharging or undercharging is suspected,
a quick check of the alternator can be made by
connecting a voltmeter to the terminals of the battery
as shown in Figure 38.17. Note that the connections
are made positive-to-positive and negative-to-negative.

A battery shroud with an air scoop

FORD

Other things to consider are the external dimensions
of the battery and the type of posts. While most
vehicles will take a fairly standard type of battery,
others may need a squat (lower height) battery, or one
which has lugs or smaller posts.
The method of holding the battery in its mounting
can vary and some batteries have a bottom or side
ledge so that they can be clamped in the carrier.

Battery faults
The following sections list faults related to batteries
and discuss their causes.

figure 38.17

Overcharging

With the engine stopped, the voltmeter should read
approximately 12 volts. With the engine running at a
reasonable speed, the voltmeter reading should be
higher, but should not be more than about 14.5 to
15 volts. The actual reading will depend on the state of
charge of the battery.

A battery that requires frequent topping-up is probably
being overcharged. This overstresses the active
material in the plates. It causes deterioration of the
plates and shortens the life of the battery.

Overcharging literally boils the water away from the
electrolyte, and the electrolyte level is likely to fall below
the tops of the plates, exposing them to the air. This will
ruin the plates and could also affect the separators.
■ Where overcharging is suspected, the alternator
output should be checked and the voltage regulator
replaced or adjusted as necessary.

A voltmeter connected to the battery terminals checks that the alternator is working

■ This is a quick method of checking whether the
alternator is working.
Poor topping up
If the electrolyte level is too high due to overfilling,
then electrolyte can be forced through the hole in the


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678 part six basics of the electrical system
vent caps and cause corrosion of the terminals and
other parts. If the level is allowed to get too low, then
the battery will become damaged internally.
Sulphation

Normally, the active materials in the plates are
converted into lead sulphate while the battery is being
discharged, and reconverted into active material while
the battery is being recharged.
However, if the battery is allowed to stand for long
periods in a discharged condition, the lead sulphate
becomes converted into a hard, crystalline substance
that is difficult to reconvert into active materials by
normal charging processes. A battery in this condition
would be replaced with a new one.
Damaged case
A cracked or damaged case could result from
excessively loose or tight hold-down clamps.
A bulged case could be caused by overtight holddown clamps or from the effects of high temperatures.
This could also be the effect of age. The plates warp as
the battery ages and this can distort the case.
Corroded terminals
Corrosion of terminals is a condition that occurs on
battery terminals (Figure 38.18). It can occur more
on some batteries than others and is due to acid in
electrolyte vapour.
Any corrosion should be removed from the battery
post and terminals. Cable terminals should be
disconnected from the battery posts and the posts and
terminals cleaned.
After the clamps are replaced, they should be coated
with a light grease or petroleum jelly to retard corrosion.

Corroded battery carrier
Some vapour from the battery electrolyte escapes

through the vents in the filler plugs as the battery is
being charged, and the battery mounting may
become corroded from the effects of the acid.
Corrosion can be cleaned off, when the battery is
removed, by the use of a wire brush and also with a
common baking-soda solution. The soda solution is
applied and after the foaming stops, flushed off with
clean water.
■ Dirty battery connections are the most common
cause of a starter failing to operate.
Dirty battery case
If not cleaned periodically, the top of the battery can
collect a coating of grime with traces of electrolyte.
This should be cleaned off periodically with a wet rag.
If a baking-soda solution is used on the battery, care
should be taken that the solution does not get into the
cells and ruin the battery.
Battery failure
Batteries deteriorate over time and with use. The
active material of the plates is gradually reduced by
the actions of charging and discharging and by
vibration. Eventually, the plates lose so much of
their capacity that the battery is no longer of any
use.
What appears to be a sudden failure of a battery is
usually due to the slow corrosion of grids and electrical connections until the electrical circuit through
one of the cells is finally broken. The battery will then
have an open circuit.

Battery safety

Warnings similar to the following appear on many
batteries and in workshop and owner’s manuals.
Lead-acid batteries generate explosive gases.
Keep sparks, flames and lighted cigarettes away.
Provide ventilation when charging or using a
battery in a confined space.

figure 38.18

Corroded battery terminals are the most
common of all battery problems

The battery contains sulphuric acid. If acid comes
into contact with eyes, skin or clothing, flush
immediately with large amounts of water.
Also, in the case of eye contact, seek immediate
medical attention.


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chapter thirty-eight the battery

Use of jumper leads

The use of jumper leads for emergency starting is not
just a case of connecting two batteries together. It can
be dangerous if not done correctly.
Also, with the amount of electronic equipment that
is now fitted to vehicles, extra precautions must be
taken to prevent damage to electronic components
from the high voltages that can occur. Be guided by
information in the particular vehicle handbook or
service manual.
■ The safest way to rectify a flat battery problem is to
remove the flat battery and replace it with a
charged battery.
Starting procedure
The following points summarise the type of procedure
and the precautions that are generally recommended
when using jumper leads:
1. Both the booster battery and the discharged
battery must be of the same voltage.
2. If the booster battery is fitted in a vehicle, the
vehicles should not be touching. Both batteries
must have the same polarity in the vehicle
(negative to earth).
3. Switch off the ignition, the lights and all other
electrical equipment.

figure 38.19

679

4. Check that the vent caps of both the booster

battery and the discharged battery are tight and
place a cloth over the filler caps of both batteries.
5. With the red jumper lead, connect the positive
terminal of the discharged battery to the positive
terminal of the booster battery. (The sequence of
connecting the batteries is shown in Figure 38.19.)
6. Attach one end of the black jumper lead to the
negative terminal of the booster battery and the
other end to an earth on the engine away from
the battery (not to the negative battery terminal).
Both vehicles would normally have their batteries
installed with negative to earth.
7. Start the engine of the vehicle with the booster
battery and then the engine of the vehicle with the
discharged battery.
8. Allow both engines to run for a few minutes to
allow the batteries to stabilise.
9. Disconnect the jumper leads carefully in the
reverse order to that used when they were being
connected.
10. Dispose of the cloths used to cover the filler holes,
because they could contain acid.
■ The cloths are placed over the filler caps to absorb
the gas that comes from the cap vent holes. This is
an explosive gas that could be ignited by a spark.

Connecting a booster battery for emergency starting – the connecting sequence is shown as 1, 2, 3, 4


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680 part six basics of the electrical system
Related points
Recharging or replacing a flat battery is preferred to
using jumper leads. With jumper leads, voltage spikes
can be generated under some conditions. These are
sudden surges of high voltage that can damage electronic components.
Electronic components operate on very low
voltages and will not withstand high voltages, even for
a short time. To overcome the problem, special jumper
leads are available. These have light-emitting diodes to
show that the cables are connected with the correct
polarity, and they also have voltage-spike protection.

Technical terms
Spongy lead, lead oxide, sulphuric acid, plate grid,
separator, battery cell, electrolyte, positive,
negative, charge, discharge, lead sulphate,
sulphation, low-maintenance, maintenance-free,
open-circuit voltage, capacity (of battery), cranking
current, ampere hours, cold cranking, hydrometer,
density, specific gravity, self-discharge, high-rate
discharge test, battery charger, fast charger, drycharged battery, overcharging, topping-up, distilled
water, deionised water, jumper leads.

4.

Consider the actions that occur when a battery is
being charged.

5.

What is meant by the term ampere hours?

6.

What is density?

7.

By means of a sketch, show how the cells of a
battery are connected together to provide
12 volts. Show the polarity of each cell.

8.

Explain how the density of the electrolyte is
checked.

9.

A 12 volt battery in good condition is tested
with a voltmeter while under heavy load. What
is the voltmeter likely to show?

10.

Consider dry-charged batteries and how these
are activated before use.

11.

Describe the various ways in which specifications for batteries are given.

12.

State the items that would be checked when
servicing a battery.

13.

What are some of the possible causes of a
discharged battery?

14.

If an engine will not start because of a discharged battery, what can be done to rectify the
problem?

15.

Outline the procedure to be used when connecting batteries with jumper leads for emergency
starting.

16.

What particular safety precautions should be
observed when using jumper leads?

Review questions
1.

What is the purpose of the battery?

2.

Name the parts of a battery.

3.

Why is a section of the battery referred to as a
lead-acid cell?