Page 295 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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278 Automobile mechanical and electrical systems

Figure 3.44 Reading a hydrometer: fully Figure 3.45 Heavy-duty discharge test
charged equipment

a specialist item of equipment ( Fig. 3.45 ). The test consists of connecting a
Safety fi rst low-value resistor in circuit with the battery for a few seconds and measuring
the voltage drop. This test applies a load similar to that of the starter circuit. In
A discharge test applies a load general, the voltage should not fall below 10 V. Figure 3.45 shows test equipment
similar to that of the starter circuit,
which results in a high current and for this purpose; as always with this type of thing, refer to the manufacturer’s
heat. instructions before use.
Battery performance can be determined by its ampere-hour (Ah) rating, but this
is also affected by the ambient temperature. A cold battery will have a poor
performance in comparison to a warm battery. A more useful measure of battery
performance is its ‘reserve capacity’ rating. This refers to the time taken in
minutes for a battery to discharge to a cell voltage of 1.75 V, when supplying a
constant current of 25 A. This test refl ects a typical current draw for a vehicle that
would be needed if the charging system failed during night driving. The reserve

capacity for a 40 Ah battery will be approximately 60 minutes.
Another measure is the ‘cold cranking amps’ rating, which is a measure of the
maximum current that can be supplied for a period of 30 seconds before the
battery voltage falls below 7.2 V. This test is carried out at minus 18°C (0°F) so
that it represents the most severe conditions of cold engine starting. The cold-
cranking amps (CCA) rating of a battery is an important measure in regions that
suffer very cold winter temperatures.
Some maintenance-free batteries incorporate a built-in hydrometer to indicate
the state of charge and condition of the battery. The hydrometer is colour coded
( Fig. 3.46 ). A green colour indicates that the battery is charged and serviceable.
A green–black or black colour indicates that the battery requires recharging. A
yellow colour indicates that the battery is faulty. Where a yellow hydrometer is
showing, the battery should not be recharged or tested, and the use of jump
leads for starting should not be carried out. A new battery should be fi tted and
the alternator checked for correct operation.
Battery charging can be described as slow or fast. Slow charging is best but in
an emergency a fast charge is acceptable. When recharging, a battery should
ideally be disconnected from the vehicle electrical systems.
There are two types of battery charger: the bench charger, which has a current
output of up to about 10 A ( Fig. 3.47 ), and the fast charger, which can recharge a
battery in about 30 minutes, with a current of up to 50 A ( Fig. 3.48 ).