assembly of plates being immersed in dilute sulphuric acid, having a specific gravity of about
1
.300 (i.e. being about 1.3 times as heavy as water).
W
hen current is drawn from a battery (due, for instance, to the lights being switched on), the
lead peroxide and spongy lead both begin to change into lead sulphate, and the electrolyte
becomes more dilute. When the battery is fully discharged (i.e., is "flat"),both plates largely
c
onsist of lead sulphate, and the liquid is very dilute, having a specific gravity of about 1.100.
A
s a battery is recharged, the positive plate changes back to lead peroxide, the negative plate
to spongy lead, the sulphuric acid gets stronger, and eventually the state is reached where the
battery is fully charged.
Also as the battery is recharged, the voltage at the terminals of the battery increases. When
the voltage reaches around 14.1 volts for a 12 volt battery (7.05 volts for a 6 volt battery),
f
urther charging then causes the effect known as "gassing", which is where the water in the
e
lectrolyte is broken down into the gasses hydrogen and oxygen, which then bubble off, quite
v
igorously if the charge rate is high.
T
his gas mixtur
e is explosive, which is why batteries should not be recharged in a confined
space or in the presence of naked lights, and why the mains should always be switched off
before connecting or disconnecting a battery charger to the mains so as to avoid the
production of sparks.
Virtually no gassing occurs at charging voltages below 14.1 volts, which is one reason why
alternator regulators are usually set at around this value. A battery can remain continuously
connected to a v
oltage of up to 14.1 volts without being overcharged in any way.
With many cycles of discharging and recharging, the plates are continually changing into lead
sulphate and back again, and occasionally particles will fall off and sink to the bottom of the
battery to form sludge. As the sludge builds up, there is less lead available for the chemical
processes, and the battery loses its capacity to carry charge. Batteries also deteriorate due
to an effect kno
wn as "sulphation",
which is wher
e a permanent form of lead sulphate forms
on the plates, shielding the plates from contact with the acid. Eventually these natural ageing
processes cause the battery to deteriorate to the stage where it is incapable of starting the
engine
, and a new battery is required. The deterioration of a battery is often noticed in the
first cold snap of winter,because not only is the engine oil thick and the engine more difficult
to turn,
but batteries having lower performance at low temperature.
To mak
e batteries last a long time the
y must be cared for
. It is important that the alternator
voltage regulator is correctly set so that the battery is not overcharged. Vibration is bad for
batteries, and so are very "deep" discharges,and very high charging rates. Also, it is important
that batteries are kept fully charged,
and on no account must a battery be left for an
extended period in a discharged state, otherwise the dreaded "sulphation" will set in.
2. TESTING BATTERY CHARGE
The traditional way is to use a hydrometer
, but these can be a bit messy, and not everyone
has one. Of course with sealed batteries hydrometers are no use anyway. There is another
way,which is to make a very accurate measurement of the voltage at the battery terminals.
Gunson's TESTUNE,
or a g
ood quality digital voltmeter (DVM) is needed. For 12 volt
batteries, Gunson's START-CHECK can also be used.
There is one slight complication, which is that it takes batteries a long time to settle down to
a stable voltage when they have been on charge,or when they have been used to provide
current.
In fact, it takes longer for a battery to settle down when it has been on charge,than
when it has been used to provide current. To get an accurate measurement, it is best to wait
several hours, say overnight, with the battery totally disconnected. However, a reasonably
good indication of the state of charge of a battery can be obtained by carrying out the
following procedure: With the battery in the vehicle, and irrespective of whether the battery
has been on charge (or the engine running and the battery being charged by the alternator),
switch the headlamps on for 1 minute, then disconnect the battery and wait a further 15
minutes at least before making the measurement.
A fully charged battery should measure 12.65 to 12.9 volts or thereabouts. Part charged is
typically 12.15 to 12.65, and a flat battery is below 12.15. TESTUNE has coloured bands to
indicate these ranges. START-CHECK does the same thing by means of a LED (light emitting
diode) displa
y
,
wher
e individual LEDs ar
e accuratel
y set to be illuminated at these v
oltages.
F
or a 6 volt battery,the voltages are exactly half those given above, i.e.: Fully charged 6.32 to
6.45 volts, part charged 6.07 volts to 6.32 volts,and flat below 6.07 volts.
Remember that as a battery ages it ceases to be able to carry a full charge, and this will be
reflected in the voltage measurement. Also remember that this test measures battery charge,
i
t does not say whether the battery is faulty: some faulty batteries can in fact carry a full
charge; you just can not get at it.
3
. TESTING FOR FAULTY BATTERIES
U
ndoubtedly the best test of a faulty battery, a test so conclusive that you can be sure that
the battery is worn out, and that you are not wasting money by buying a new one, is to use
a
"high current discharge meter". Unfortunately these are so expensive that only the trade
can afford them, although you often find one in a good quality car accessory shop for use by
the counter staff. To give your battery a fair test,make sure that it is fully charged before
taking it along to the professionals.
A
nother good way to test a battery,which gives a fairly good indication, is to test a battery
at what it is supposed to do, that is to crank the engine. Before carrying out this test it is
absolutely essential to test for voltage drops in the starter circuit (see section 4 below:
TESTING BATTERY TERMINALS). It is also essential that the batter y is as fully charged
as it can be.
Having satisfied yourself that there are no other faults,the method then is to simply disable
the engine so that it can not star
t (for instance, by disconnecting the electrical supply from
the ignition coil, which is either + or - depending on the particular system),and crank the
engine on the starter,carefully measuring the time for which the starter is cranking briskly.
Eventually the battery will "die" and the cranking will become very laboured and might even
stop altogether. The change in cranking speed from brisk to laboured is usually quite sudden
and easy to identify. Both TESTUNE and START-CHECK have tests which give a visual
indication of the moment when a 12 volt battery is deemed to be flat. A good battery will
briskl
y crank the engine f
or a minute and more, even in the coldest weather. If your battery
dies after a few seconds, it is a sure sign that it needs replacing.
One word of warning: starter motors are not designed for continuous cranking, so do not
car
ry on f
or more than a minute or so without waiting for the motor to cool.
Another fault that batteries can have is that they can "self discharge" - a battery that appears
to be fully charged today can be flat tomorrow or the day after. It is easy to confuse this fault
with a fault in the circuits of the car - a glove compartment light that stays on all the time for
instance. To check whether a battery can hold charge it is necessary to test its charge over
a period of days using a hydrometer or good quality voltmeter. To check whether current
is leaking from the battery to the car when ever
ything is supposed to be switched off, it is
necessary to use an ammeter,or the "leakage current test" of TESTUNE, which accurately
measures leakage whether it is a few milliamps or a few amps. The TESTUNE scale has
coloured bands to show acceptable levels of "leakage" (e.g. due to a car clock, security system
or computer that are on all the time), and unacceptable levels (e.g. the light on in the boot).
4. TESTING BATTERY TERMINALS
When a vehicle will not start because the starter will not turn the engine quickly enough,
the problem may be dirty or corroded battery terminals. The same effect is caused by
faults such as a bad solenoid, loose or frayed earth straps etc, which may be tested in a
similar way.
The problem with battery terminals (and solenoid, earth straps etc) is that they have to
car
ry such a high current that even the minutest resistance, due to dirty deposits or
cor
rosion, can impede the flow of the current and prevent the car from starting.
The way to test for this fault is to measure the voltage drop across the component while
the engine is being cranked on the starter. For example, to check battery terminals hold
one meter pr
obe on the terminal that fits on to the battery post and hold the other
probe right on the terminal post itself. Then the engine should then be cranked on the
star
ter
, having first prevented the engine from starting by disconnecting the + or - lead
fr
om the coil. If the terminal is making good contact then the voltage drop will be zero
even measured on a 0 - 1 volt scale. A reading of 0.25 volts is a bit worrying, and 0.5 volts
is an indication of a definite fault (halv
e the se voltages for a 6 volt vehicle). Gunson's
TESTUNE has a suitable voltage scale with coloured bands to indicate good and bad
battery terminals, solenoid, earth straps etc.
5.
TESTING ALTERNATORS
A
faulty alternator will eventually result in damage to the battery,either through
undercharging or overcharging.
The way to test an alternator is to use a voltmeter connected directly across the terminals of
the battery while the engine is running, but be sure to start with the battery in a reasonable
s
tate of charge.
If the alternator is working correctly,the voltage across the terminals of the battery will
n
ormally be within the range 13.5 - 14.1 volts (half this for a 6 volt battery). This is the
voltage that the alternator provides to keep the battery fully charged but not overcharged.
H
owever, it should be noted that some vehicle manufacturers specify a rather higher voltage
for the alternator regulator,and reference should be made to the workshop manual for the
exact recommended voltage for a particular vehicle.
A
n alternator should be able to hold this voltage at the battery terminals at all speeds above
idle (as soon as the ignition warning light goes out), and the reading should be approximately
t
he same irrespective of any electrical equipment that is switched on,such as headlamps,
windscreen wiper motor etc.
If the voltage reading is too low,or falls too low when equipment is switched on (e.g.
headlamps), then the alternator is undercharging.
If the voltage is too high, or rises too high when the engine speed is increased (with or
without the headlamps on), then the alternator is overcharging.
Although any good quality voltmeter should be capable of carrying out this test, the Gunson
TESTUNE has been designed particularly to make this test easy. It has an offset scale (one
that does not start at zero, in fact it starts at 12v, and only measures voltages in the range 12v
to 17v) to give a scale that is greatly expanded to be accurate and easy to read at the
appropriate voltages for alternator tests. It also has coloured bands to show when an
alternator is OK, or when it is undercharging or overcharging.
TECHNICAL SPECIFICATIONS
INPUT
MODEL 1 230V AC 50Hz 15 W
OUTPUT
6V 1.5A RMS(EFF) 1.0A DC(ARITH) CONTINUOUS
12V 1.2A RMS(EFF) 0.8A DC(ARITH) CONTINUOUS
RECOMMENDED MINIMUM BATTERY CAPACITY: 6Ah
FULL OVERLOAD PROTECTION
REVERSE POLARITY PROTECTION
THERMAL CUTOUT PROTECTION
CONFORMS TO BS 3456
DOUBLE INSULATED
GUARANTEE
This guarantee is additional to the purchaser's statutory rights.
The Tool Connection has made every effort to ensure that this BATTERY CHARGER
is of the highest quality
.
If this BATTERY CHARGER should need service or repair, it should be returned direct to
The Tool Connection Limited,
Kineton Road, Southam,
Warwickshire CV47 0DR
T
el:
T
echnical Ser
vice Depar
tment ++44 (0)1926 818181
When g
oods ar
e returned for service or repair,full details of faults requiring attention
should be giv
en,
and also date of pur
chase
.