The battery 667 Materials in the battery 668 Battery construction 668 Chemical actions in the battery 669 Lowmaintenance and maintenancefree batteries 671 Battery voltage 671 Battery specifications 671 Battery testing 672 Battery charging 674 Care of batteries in stock 675 Battery maintenance 675 Battery faults 677 Battery safety 678 Use of jumper leads 678 Technical terms 680 Review questions 680
Trang 1The battery
Chapter 38
Materials in the battery
Battery construction
Chemical actions in the battery
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
Use of jumper leads
Technical terms Review questions
Trang 2Automotive 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
frame-work 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.
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 Sepa-rators 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 38.1 A battery plate grid
figure 38.2 A battery plate – lead oxide paste has been
applied to the plate grid
figure 38.3 A battery plate group – the plates are
con-nected together by the plate strap
Trang 3Each 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.
Battery terminals
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
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.
Chemical actions in the battery
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.
Action on discharge
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.
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
figure 38.5 Battery cell connections – 2-volt cells
connected in series can provide 6 volts or
12 volts
Trang 4Action 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-meter The float does not rise when the battery is flat, but floats quite high when the battery is charged.
figure 38.6 Construction of a 12 volt battery
figure 38.7 Electrolyte is approximately 40% sulphuric
acid and 60% water
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
Trang 5■ 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
electro-lyte 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.
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.
figure 38.9 The voltmeter connected to the battery
shows the voltage readings for different conditions
Trang 6Number 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.
Reserve capacity
This is the time, in minutes, that a new fully charged
battery will supply a constant load of 25 amps without
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 deter-mine 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.
figure 38.10 Checking a battery with a hydrometer
MITSUBISHI
Trang 7To check a battery
Measure the density of the electrolyte in each cell with
a hydrometer All the readings should be
approxi-mately 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
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 self-discharged may be ruined.
figure 38.11 Electrolyte for a fully charged battery is
1.25 times heavier than water – it has a density of 1.25
Trang 8■ 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).
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
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 con-nections.)
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
figure 38.13 A battery charger is connected to the battery
with positive-to-positive and negative-to-negative
Trang 9The 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
condi-tion, 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.
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
ready-mixed electrolyte in a special acid-proof plastic
container To activate a battery, the following is
necessary:
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 acti-vated 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:
figure 38.14 Battery and mounting – points to be checked
during service HOLDEN LTD
1 terminals clean and cables in good condition
2 battery posts clean
4 case and cover clean
6 hold-down clamp secure
5 battery tray clean
3 electrolyte level correct
Trang 101 Cables in good condition and terminals clean.
2 Battery posts clean.
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
rain-water 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.
Installing a battery
■ A ‘battery saver’ should be installed before removing the battery to save computer codes and personal settings on audio and climate control systems etc 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 out-side 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.
figure 38.15 Electrolyte level
(a) level to bottom of filler well (b) level marks
on transparent case