How To Measure the Specific Gravity of a Battery?

A fully charged battery is a helpful and prominent helper for your car. In this article, I will discuss how to Measure the Specific Gravity of a Battery?

Measure the Specific Gravity of a Battery
Measuring the Specific Gravity with Hydrometer


Measure the Specific Gravity of a Battery

The specific gravity is the most critical parameter, that needs to be checked and maintained for the good health of the battery. The measuring unit is the density of electrolytes present in the battery compared to the density of water. 
When the battery is fully charged the specific gravity will be at a higher point and when the battery is discharged the specific gravity will fall down. To measure the specific gravity of a battery the Hydrometer is used. 
A Hydrometer is a simple device that works on the principle of buoyancy. As the density of the electrolyte changes with the state of charge, the hydrometer float will further sink deeper or rise higher. Thus it indicates the specific gravity. First, we need to know what is specific gravity and how the hydrometer measures it?

What is Specific Gravity?

The electrolyte in a fully charged battery is usually 1.23 times as heavy as an equal volume of pure water when both liquids are at the same temperature. The battery electrolyte would, therefore be described as having a specific gravity of 1.230 meaning that its weight is 1.230 times the weight of pure water. 
When the battery discharges the Sulphuric acid in the electrolyte combines with the plates and the remaining electrolyte becomes lighter in weight. By determining the relative weight of the electrolyte we can tell how much acid has combined with the plates and, therefore, estimate how much electrical energy is still left in the battery. However the actual weighting of the electrolyte would be inconvenient, so an instrument called the Hydrometer is used. 

Temperature Effect on Specific Gravity

The specific gravity of the electrolytes carries with temperature. It decreases with the increase in temperature. Thus, the specific gravity is always mentioned in the specific standard temperature, which is usually 15 degrees Celsius. If the reading is taken at a different temperature, it is correct for 15 degrees Celsius. Usually, 18 points are added for every 25^0 C excesses of temperature above 15^0 C, and the same number is subtracted for every 25^0C reduction of temperature below 15^0C.
For example, if the specific gravity at 40^0C is 1.230, the corrected reading at 15^0C would be-
Temperatures effect on Specific Gravity

The freezing point of the electrolyte is affected by its specific gravity. It decreases with the increase of specific gravity. For a fully charged battery, the freezing point of electrolyte is much lower as compared with a completely discharged battery. 

How the Hydrometer Measure the Specific Gravity?

It consists of a glass barrel and bulb syringe for sucking up a sample of the electrolyte to float an enclosed glass hydrometer calibrated to read in terms of specific gravity. The depth to which the float sinks in the liquid indicates the relative weight of the liquid compared to water and gives us a measure of the specific gravity of the liquid. 
The hydrometer floats low in the liquid if the specific gravity is low and it floats high in the liquid if the specific gravity is high. The float is made of glass and is equipped with a paper scale built inside the hydrometer with marks on it which must be read on a level even with the liquid surface and this reading indicates the specific gravity of the liquid. 

Hydrometer Readings

Hydrometer Readings


The correct method for reading the hydrometer is shown in Fig. The eye should be on a level with the surface of the liquid in the hydrometer barrel. She discharged the curvature of the liquid where the surface rises against the float stem and the barrel due to the surface tension. Keep the float vertical.
Hydrometer readings should never be taken immediately after water has been added. The water should be thoroughly mixed with the underlying electrolyte by charging before hydrometer values are reliable. In reading a hydrometer, the barrel must be held vertically, and just the right amount of acid be drawn up into the barrel with the bulb fully expanded to lift the float freely so that it touches neither the side nor the top-notch bottom stoppers of the barrel.

Ending This Up With

The hydrometer barrel and float systems must be kept clean with soap and water, so that the float will not stick to the sides. The float must be inspected occasionally for cracks that would allow acid to enter the airtight float and make any reading with it unreliable. If the paper scale inside the float is wet, it is an indication that the float leaks, and should not be used. 
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