OPTICAL BATTERY/ANTIFREEZE TESTER
Since the amount of sulfuric acid in the electrolyte decreases as the battery discharges, the gravity of the battery also decreases as the battery's state of charge decreases. By using a tester, known as an optical battery/antifreeze tester, to measure the gravity of the electrolyte, we can tell the battery's state of charge.
The gravity of the battery is affected by temperature. When heated, the electrolyte expands so it occupies more space. When the temperature drops, the electrolyte contracts and takes up less space. Therefore, warm electrolyte weighs less than the same volume of cold electrolyte, so warm electrolyte has a lower specific gravity than cold electrolyte.
Electrolyte mixed for normal use will test 1.280 at 80°F temperature. (This is the temperature of the electrolyte and not the surrounding air.) At ordinary temperatures, it is not necessary to consider any variations when testing the gravity of a battery. However, any large variation above or below 80° is very important when deciding the true state of battery charge.
The state of the battery charge indicated by gravity readings is shown below.
Other factors to consider when you test the gravity of a battery are the level of the electrolyte and how well the electrolyte is mixed.
The electrolyte should be at the correct level in the cell. Water in the electrolyte evaporates; the acid does not. If the water has evaporated enough so the electrolyte level is low, the mixture will be strong and the gravity reading will be high. On the other hand, if the battery has been overfilled with water, the electrolyte will be weakened and the reading will be low.
When water is added, it will tend to remain at the top of the cell, and you should not test immediately after adding water. If water must be added before a reading, the battery should be charged for one to two hours to mix the electrolyte before the optical battery/antifreeze tester is used.
Gravity readings will not tell the true state of the electrolyte just after a battery has been discharged at a high rate (such as just after the engine has been cranked for a long time). In this case, the acid has been used up next to the plates but the electrolyte near the top of the plates is still strong. The optical battery/antifreeze tester will read a higher state of charge than actually exists. The electrolyte will show a true reading if the battery is allowed to stand unused for several hours or if it is charged for one to two hours. |