Test an automobile battery for its serviceability.

Test an automobile battery for its serviceability.

OBJECTIVE

The objective of this practical is to perform different test on battery. By this practical student get knowledge about testing, maintenance & its service.

INTRODUCTION

The battery is the heart of the automobile electrical system. The battery serve to store electric energy when there is little or no damage for current acts a secondary source of a electrical when the engine is hot running alternatively to meet full requirement of the load on the system. A wide range of batteries varying from 5 to 200hr. Capacity is necessary to meet the requirement of motor industries.

The import of the battery to the electrical system cannot be over emphasized. You must first determine the condition of the battery when solving any electrical problems. Therefore battery tests almost precede all electrical diagnosis.

An Automobile Battery is a Device for storing chemical energy and converting the same into electricity. Storage battery is perhaps the most important component of the entire electrical system of a vehicle.

The primary purpose of an automotive battery is to provide a source of electrical power for starting and for electrical demands that exceed generator output. The battery also acts as a stabilizer to the voltage for the entire electrical system. The battery is a voltage stabilizer because it acts as a reservoir where large amounts of current (amperes) can be removed quickly during starting and replaced gradually by the generator during charging. The battery must be in good (serviceable) condition before the charging system and the cranking system can be tested.

There are mainly four testing methods:

• Specific Gravity Test OR Hydrometer Test

• Open Volt Test

• High Rate Discharge Test

• Cadmium Test

SPECIFIC GRAVITY TEST OR HYDROMETER TEST

While the chemical reactions are taking place in the battery during discharge, the electrolyte becomes dilute to formation of water. The proportion of water goes on increasing as the discharging continues. The relative amounts of water and acid is determined by the specific gravity test. This is done by the use of a hydrometer contained in a syringe. The electrolyte is drawn up into the syringe by the bulb and the hydrometer float sinks to a greater or lesser extent according to the amount of sulphuric acid in the electrolyte. If the hydrometer reads 1.280, it indicates that the liquid is 1.280 times as heavy as water, and at the reading the battery is fully charged. 

The scale of the hydrometer is read on the stem at the surface of the electrolyte when the hydrometer is floating in it. The specific gravity of the electrolyte is affected by its temperature. Thus, the temperature, of the electrolyte is also recorded by a thermometer and then the specific gravity reading is connected accordingly.

The specific gravity test should not be made while the battery is gassing. The test should also not be made immediately after the water has been added to adjust the electrolyte level; otherwise this would affect the reading.

More and more vehicles are now fitted with maintenance-free (sealed) batteries, where the hydrometer cannot be used to determine the state of charge. In such a case, the same can be determined from the open 'volt test of the battery. The open circuit voltage of the battery cell is measured with the help of an ordinary voltmeter. The open circuit voltage of a fully charged battery cell is about 2.1 volts. If the car has been recently driven or the battery has been just charged, the ‘surface charge’ must be removed from the battery by running on the headlights for one minute, turning off and waiting for about two minutes. Then with the engine and all electrical accessories shut off, connect the voltmeter to battery terminals. 

In case of a 12 -V Battery, if the voltmeter reads 12.6 V, it is fully charged, if it reads 12.2 V it is half-full charged while below 11.9 V it may be taken as fully discharged. It has been observed that a change of 0.01 volt of open-circuit voltage is equivalent to a change of 0.010 in the specific gravity of the electrolyte. Thus the gravity measurement can be indirectly made with an accurate voltmeter also. There is another way of expressing relation between the open circuit voltage of a battery cell and its specific gravity. This is Voltage of the cell = Specific gravity + 0.840 Open circuit voltage is not representative of the cell voltage under actual operating conditions. The cranking motor at the time of starting draws a very heavy current which causes the cell voltage to fall. 

To simulate this condition a high discharge test is made with the help of a carbon pile or a cell voltage tester. A cell voltage tester consists of a voltmeter connected to two legs with a high resistance placed across, these. Pressing the two legs of the cell tester on the cell terminals causes heavy current (of the order of 150 to 200 amperes) to flow. The test is made for 5-10 seconds at the end of which the voltage should not fall below 1.5 volts and the difference of cell voltages of various cells should not exceed 0.2 volts; otherwise the battery may be taken to be defective.

This is a very severe test and should be made only on the fully charged or at least on half - fully charged batteries. Even then it should not be made continuously for more than 15 seconds; otherwise the battery plates will get permanently damaged. If the cell tester is not available, the high discharge test may also be made with the help of the cranking motor when the ignition switch is off. The voltage readings are taken with the help of an accurate voltmeter.

This test is conducted to know the chemical conditions of the positive and negative plates and is performed only when the battery is either on charge or discharge. A cadmium rod is enclosed in a perforated ebonite tube and is immersed in the battery electrolyte. The rod is then connected to the negative terminal of a high resistance voltmeter whose positive terminal is connected alternately to the positive and negative terminals of the battery cell. In case of positive terminal, the voltage reading should not be less than 2.5 volts if the battery is nearing completion of its charge or 2.0 volts if the battery is being discharged at normal rate.

A lower reading indicates defective positive plates. On the other hand, reading in case of connection with negative battery terminal should not be more than 0.2 volt when the battery is being discharged at normal rate. A higher reading in this case will indicate defect in negative plates.