SIMPLE
Page 70
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MECHAN I
IN A PREVIOUS article in this series it was stated that the specific gravity of the electrolyte of a battery is raised as the battery is charged. This varying of the specific gravity with the state of charge provides a convenient way of checking the condition of the battery.
For the purpose of measuring specific gravity a hydrometer illustrated in Fig. 1 is used. The hydrometer consists of a plastic or glass syringe which contains 'a float. A dismantled hydrometer is shown in Fig. 2 and it will be seen that it consists of a rubber bulb at one end of a transparent tube and a rubber .tubing at the other.
The float shown in the centre will stand higher in a dense liquid than in a liquid which is not so dense. It is calibrated with a scale on its side so that a number, level with the liquid in which it floats, indicates the specific gravity of the liquid.
• In use, a sample of electrolyte is sucked from each cell in turn and the readings noted. The readings from each cell should be about the same. if, however, the reading from one cell is much lower than the others, this indicates that something is wrong with that particular cell. At 15.5'C (61c F) the readings should be as follows: Fully charged battery: 1.270-1.290 Half charged: about 1.210 Fully discharged: below 1.130 Often the decimal point is omitted when talking of specific gravity readings. For example it would be said that the reading for a fully charged battery if "twelve ninety".
When using a hydrometer naked flames must be kept away from the battery; the instrument must be kept vertical and it must be ensured that the float is free to move in the tube. The check' should not be made just after the battery has been topped up.
Allowance should be made. for temperature variations. These will be very slight except in very extreme weather conditions. The reading will be slightly higher than correct at low temperatures and lower than correct at higher temperatures.
Although a battery may pass the hydrometer test this does not mean that it is capable of dealing with the very heavy load imposed upon it when the starter motor is being used. To test its ability to meet this load a high-rate discharge tester, illustrated in Fig. 3, is used. This instrument consists of two prongs across which is connected a bar or rod to form a low resistance.
A centre zero voltmeter is connected across the resistance bar. The prongs of the tester are pressed firmly into the bridge connecting bar of each cell in turn; there will be a heavy discharge and the voltmeter will show how the cell stands up to the heavy load placed upon it.
A sound cell will give a steady reading of about 1.5 volts for about 10 seconds. If the reading falls rapidly a weak cell is indicated. This severe test must not be prolonged longer than is necessary.
It is not possible on modern batteries to test each cell individually as the connecting bridges are completely concealed. In the instrument illustrated an additional heavy resistance can be connected to one of the prongs of the tester. The prong on the end of the resistance and the free prong on the instrument are then pressed firmly on to the positive and negative terminals of the battery so that a nine-volt reading is obtained. A rapid fall in the reading will show the battery is faulty but it will not be possible, of course, to single out any one particular cell as being defective.
More about batteries in the next article.