When it comes to automotive lead-acid batteries, one way of checking the amount of charge left is by measuring the specific gravity. It is defined as the ratio of the battery’s electrolyte weight against the weight of water with exact volume.
The higher the acid concentration within the cell, the higher the specific gravity it will have.
That means that the lower the strength of the acid within the battery, the lower the specific gravity it will have.
If the specific gravity of a battery is low, that means that something’s wrong with it, which can eventually lead to more severe issues. So be sure to keep it in check.
What should you do if specific gravity is low? When the specific gravity of a battery goes low, it may lead to severe issues and damage the battery. The only thing you have to do is increase it by raising its electrolytes’ acid concentration.
You can do this by adding more battery acid into it, which will make the acidity of the electrolyte more potent, raising the specific gravity of the battery. In case the battery’s specific gravity gets too high, which can also be dangerous for the battery, you will need to decrease it by adding water into it.
It will neutralize the acid in it, decreasing the electrolyte acidity, which also reduces the specific gravity of the battery. Just to be clear and to leave a conclusive answer. If you want to increase the specific gravity of a lead-acid battery, you have to increase the acid concentration within its electrolyte.
You can do this by adding battery acid into the battery or, if possible, reduce the volume of water within the power cell. That will lessen the acidity of the electrolyte, which reduces the specific gravity of it.
The most accurate way of telling if a lead-acid battery is fully charged is by checking specific gravity on each of its cells using a hydrometer. The logic behind this is that every time the battery is discharged, its acidity also reduces.
Testers, voltmeters, and multimeters may show misleading results, but you will have a more accurate reading through specific gravity. To learn more about specific gravity, stay for a while, and join me as we go deeper on the subject matter.
Checking the Specific Gravity of a Battery With Hydrometer
If you want to check for the specific gravity of a battery, you will have to carry out a hydrometer test, a specialized tool specifically used for the process. Hydrometers come in a variety of shapes and sizes, but the most recommended is the ones with floats. This is because they are the most accurate, with very credible and reliable readings.
To carry out a specific gravity test on a lead-acid battery, here’s a step by step guide of the process, assuming that you are to use the recommended hydrometer. The ones with the glass body and float.
Step 1: Ensure You Are Wearing the Right Protective Gear
Since you will be dealing with a battery containing hazardous materials and chemicals, the first thing you have to do is make sure that you are protected.
So the first step that you have to do before carrying out the test is to wear the appropriate protective gear. You have to make sure that you are wearing goggles and rubber gloves to protect yourself from the battery acid that may spill.
Step 2: Make Sure the Battery Is Disconnected
Before you can carry out the test, you also have to make sure that the battery is not connected, and all the cables are detached.
Especially if it has a high rate of charge or discharge, this will not only keep you safe. It will also avoid damaging the battery and its interior components.
Step 3: Removing the Battery and the Caps for its Cells
Now, let’s proceed to the actual test, after making sure that you are wearing the safety gear and the battery is disconnected. You can now remove the battery from its tray and then place it on a flat surface.
From there, you have to prepare the power cell for the testing by removing the lids or caps on the top of the battery. Keep in mind that each cap or lid will be for each cell of the battery.
Step 4: Carrying Out the Test Using the Hydrometer
In this step, you are going to carry out the actual process by using the hydrometer, since the lids or caps are removed. You can now insert the hydrometer in one cell to test its specific gravity.
Keep in mind that you have to make sure to do it carefully and not applying too much pressure. After inserting the device on the cell, draw enough liquid into it and be sure to hold it still and prevent it from getting bumped.
You also have to be careful that you don’t flood the float with the battery fluid and avoid sticking the sides of the tube. This prevents inaccurate readings and distortion of the liquid inside the device.
Step 5: Take Note of the Reading and Do it for Every Cell
This is the final step for testing the specific gravity of one cell, and it is the step where you will get the reading from the float of the device. Take note of the reading since you will repeat steps four and five for the other cells of the battery.
In this part, it is advisable to label each cell with a number to make sure that you are keeping everything in check with your test.
That is how you do a specific gravity test of a battery using a hydrometer with a glass body that also features a glass float.
You have to keep in mind that specific gravity is also affected by temperature, whether you are carrying out the test with a warm or cold temperature. The result will have to undergo corrections, and you also have to avoid having the electrolyte of the battery too warm.
Corrections of the Results Due to Temperature
You already learned how to test the specific gravity of a battery, but in case you did the test with a slightly warmer or colder temperature. I mentioned that the result will still have to undergo some corrections.
To learn how to do the corrections for colder or warmer ambient temperature, you can refer below.
The corrections for the readings will be based on 70ºF. If the ambient temperature is below or above it, you will have to follow the equations listed. The factor that will be added or subtracted is a value measured at 0.003 points for every 10ºF increase or decrease.
● CF = (0.0031 x Temperature of the Cell in ºF – 23) / 1000
● CF = (0.595 x Temperature of the Cell in ºC – 12.5) / 1000
These correction factors are applicable when the cell’s temperature is 0ºF – 130ºF or -17.8ºC – 54.4ºC.
What Specific Gravity Readings Mean for Every Cell
As I mentioned earlier, one of the most accurate ways of telling the exact charge of a battery is by testing its specific gravity. The way it will tell you is by each reading of the hydrometer for each of the cells of the battery.
Now, you have to learn how to interpret the readings or be familiar with what the readings mean. Here’s a simple guide to help you with it.
First, you have to keep in mind that a fully charged cell should have a reading within the range of 1.280 to 1.300. However, if the reading is corrected based on an 80°F basis, not the 70ºF, which has the equation.
Here’s how you interpret the readings.
Final Thoughts
There you go! Those are the essential things that we have to know about the specific gravity of a battery. Just to go back to our question earlier.
When the specific gravity of a battery is low, it can be increased by making the battery electrolyte acidic since the more potent its acidity is, the higher will be the specific gravity. It can be done by adding battery acid into the battery.
