Battery faults
7.25.4 Battery faults
The above suggested charge rates are to be rec- Any electrical device can suffer from two main ommended as the best way to prolong battery life. faults; open circuit or short circuit. A battery is no They do all, however, imply a constant current exception but it can also suffer from other prob- charging source. A constant voltage charging sys- lems such as low charge or low capacity. Often a tem is often the best way to charge a battery. This problem which seems to be with a vehicle battery implies that the charger, an alternator on a car for can be traced to another part of the vehicle such example, is held at a constant level and the state as the charging system. The following table lists of charge in the battery will determine how much all of the common problems encountered with current will flow. This is often the fastest way to lead-acid batteries, together with typical causes. recharge a flat battery. If a constant voltage of less
than 14.4 V is used then it is not possible to cause excessive gassing and this method is particularly
Symptom or fault Likely causes
appropriate for sealed batteries.
Low state of
Charging system fault
Boost charging is a popular technique often
charge
Unwanted drain on battery
applied in many workshops. It is not recommended
Electrolyte diluted
as the best method but, if correctly administered and
Incorrect battery for application
not repeated too often, it is suitable for most bat-
Low capacity
Low state of charge
teries. The key to fast or boost charging is that the
Corroded terminals
battery temperature should not exceed 43°C. With Impurities in the electrolyte
Sulphated
sealed batteries it is particularly important not to
Old age – active material fallen from the
let the battery gas excessively in order to prevent
plates
Figure 7.55 Battery charger
158 Advanced automotive fault diagnosis
Excessive gassing Overcharging
Repairing modern batteries is not possible.
and temperatures Positioned too near exhaust component
Most of the problems listed will require the bat-
Short circuit cell Damaged plates and insulators
tery to be replaced. In the case of sulphation it is
Buildup of active material in sediment trap
sometimes possible to bring the battery back to life with a very long low current charge. A forti-
Open circuit cell Broken connecting strap Excessive sulphation
eth of the Ah capacity or about a two hundredth
Very low electrolyte
of the cold start performance for about 50 hours
Service life shorter Excessive temperature
is an appropriate rate.
than expected Battery has too low a capacity Vibration excessive Contaminated electrolyte
7.25.5 Testing batteries
Long periods of not being used Overcharging
For testing the state of charge of a non-sealed type of battery, it was traditional to use a hydrometer. The Hydrometer is a syringe which draws elec- trolyte from a cell and a float which will float at a particular depth in the electrolyte according to its density. The relative density or specific gravity is then read from the graduated scale on the float. A fully charged cell should show 1.280, when half charged 1.200 and if discharged 1.120.
Most vehicles are now fitted with maintenance free batteries and a hydrometer cannot be used to find the state of charge.
This can, however, be determined from the voltage of the battery, as given in the following table. An accurate voltmeter is required for this test (Figure 7.57) – note the misleading surface charge shown here.
Battery Volts at 20°C
State of Charge
12.0 V
Discharged (20% or less)
12.3 V
Half charged (50%)
Charged (100%) Figure 7.56 Battery charger and engine starter
12.7 V
Figure 7.57 Battery voltage testing
Engine systems 159
Starting test data ● Average cranking current
● Maximum cranking current ● Pre-set voltage ● Pre-set load voltage ● Average cranking voltage ● Minimum cranking voltage.
Battery test data ● Diagnosis
● Actual CCA ● Percentage capacity ● Open circuit voltage ● Impedance (often described as internal
resistance).
Figure 7.58 ‘MicroVAT’, charging system, starter and battery tester (Source: Snap-on).
Alternator test data ● Diagnosis
● Failure mode
To test a battery more thoroughly, it is now pre- ● Charging at idle ferred to use a volt, amp tester (VAT). There are ● Charging volts under load many variations on the market; however, this ● Average current at idle section will outline just one type. Snap-on pro- ● Peak current duce a compact and very useful tester called the ● Peak to peak ripple at idle MicroVAT (Figure 7.58). This equipment will ● Peak to peak ripple under load.
carry out a range of diagnostic tests. The device, as with many similar types, will do not only battery condition tests, but also tests
7.26 Starting
on the charging and starting system. This VAT takes advantage of new impedance/