How to Test a Car Battery With a Multimeter or Hydrometer

0

Last Updated on

You are about to read a Self-help article. Knowing how to test a car battery will enable to monitor charge, leaks as well as voltage drop of the battery. This knowledge will enable you to handle countless car breakdowns. Hence, you will avoid the unnecessary annoying costs that are associated with battery failure.

This article takes you through the process of testing for battery charge. You will learn how to use either a multimeter or a hydrometer to test for the charge. Again, you shall know how you can check for leaks within your battery. Finally, this article will take guide you on how to test for a voltage drop of your car’s battery.

And everything in between.

How to Test a Car Battery with a Multimeter

A multimeter is an electrical device that is applied in testing for resistance, voltage or current within an electric device. It is also known as a voltmeter. Besides, this device is used to test the condition of a battery. This is possible because batteries contain electric charges.

A multimeter lets you know whether or not your battery has sufficient charge to carry out its functions. It is designed for testing a battery that is suited to providing a brief power shot or maintenance-free motorcycle batteries. Let’s now see how it does the testing.

First, it is important to familiarize yourself with terminals of a multimeter. It has two electric connectors. The connectors have metal probes. It has a red wire as well as a black one. The red wire denotes a positive terminal whereas the black end is for the negative terminal.

  • Step 1: Turn on Your car’s headlights

Before you undertake any connection, ensure the headlights are on. This is very crucial since it helps to remove any surface charge that your car’s battery may have. A battery that possesses surface charge has a slightly raised voltage. It outputs an unrealistic reading on voltage-based SoC.

To correct this condition, turn on electrical loads to get rid of the battery’s capacity or rest the battery for a few hours. Turning on your car’s headlights for about 2 minutes will help in this. This is possible because the surface charge is a reversible condition, not a battery defect.

  • Step 2: Set the Multimeter to a Voltage of 20 V

At the bottom right of digital multimedia, you will see the terminals of the digital multimeter. Check the leads of the battery to ascertain that they are good— in a working state. Then, move the function switch in a clockwise direction.

Rotate the switch of the multimeter to a 20DC voltage range. Then, the screen should record 00.00 reading. Any prior reading would mean there is a fault on the digital multimeter (DMM). This would result in zero error —positive or negative— at the end of the measurement.

For accuracy, recheck to ensure it is exactly 20V. Again, a battery in a good state should have a reading of approximately 12.5 Volts. Meaning 20 DC Volts from a DMM is just okay. Keep in mind that a DMM reading of more than 20 DC may damage the battery.

  • Step 3: Turn the Headlights off

Once you have got rid of your battery’s surface charge, it becomes necessary to continue testing the battery’s charge. Before you proceed, turn off the headlights of your car.

  • Step 4: Connect the Multimeter to the Battery’s Terminals

Remove the cover of the positive terminal battery. Examine the terminal keenly for signs of corrosion. In case you identify any signs of corrosion, follow these steps to clean the terminals thoroughly:

  • Disconnect the battery cables— to avoid electric shock or extreme burns. While disconnecting the cables, start with the negative cable.
  • Remove the corrosion— use a cleaning agent such as baking soda or Coca-Cola. However, Coca-Cola is highly discouraged. This is because Coca-Cola possesses phosphoric acids and some synthetic sugars which would make the process of neutralization ineffective.

Make a solution of water and a teaspoonful of baking soda. Using an old toothbrush, rub the solution on corroded parts. After that, cover the terminals with baking soda and add a little water. You will see some bubbles after some seconds. This indicates successful initiation of neutralization, a process that eliminates corrosion.

  • Rinse and dry the terminals— use clean water to rinse the terminals then allow them to dry. You can use an air compressor to accelerate the process.
  • Apply anti-corrosion pads after the terminals have dried up. Finally, reconnect the battery to the car.

 

After cleaning the terminals of the battery, connect the positive probe of the multimeter to the positive terminal of the battery. Likewise, connect the negative probe of the multimeter to the negative terminal of the battery.

  • Step 5: Check for Signs of Charge

After connecting the terminals, turn on the headlights. This is to avail the battery with some load. Then, you can determine the following signs:

  • A battery on good health shows a voltage of about 12.6 V. However, on turning on the headlights, it may drop to 12.5 V.
  • A reading of 12.3 V shows a 75% electric charge while a 25% charge is indicated by 11.8 V. These readings are at a temperature of 26.6⁰C.
  • If the voltage drops below 5V on turning on the car, then the battery is bad and should be replaced.

How to Test the Battery of Your Car Using a Hydrometer

This is mostly applicable to the standard automotive battery. A standard automotive battery is one with caps. The caps enable access to the chambers with the cells of the battery. Such a battery has an electrolyte solution with a mixture of water and Sulfuric acid.

The test is also known as an electrolyte gravity test. The test shows charge, undercharge or battery faults. It is recommended that you use a hydrometer with an in-built thermometer. Again, it is advisable to put on gloves and eye protection devices. The devices should be resistant to harmful sulfuric acids.

Testing the Battery Fluid Using a Hydrometer

  • Step 1: Remove the Battery Caps

You should never try to remove the caps while the car is on the motion. Instead, you should park the car first. After this, remove any jewelry on your body. This is because jewelry can conduct electricity. This would result in a fatal scenario. Besides, you should have worn your gloves as well as eye protective gear.

Next, move to the battery. Batteries that have covers also possess a plastic cover top which makes it difficult to view the battery. Some batteries contain a hole with an exposed pole—that subtends from the cover— whereas others will the two ends covered.

In case a pole extends from the hole it is marked as negative. This shows that the battery post has a negative polarity. On the other hand, a positive pole remains covered for safety reasons.

Remove the latch on the side of the cover. By doing so, the plastic should freely get unattached. Next, raise the plastic cover straight up as well as of the battery. Don’t touch the battery, especially in a situation whereby it is cracked or when it leaks.

  • Step 2: Sink the Tip of the Hydrometer into the First Cell

As soon as the hydrometer is inside the cell, squeeze the bulb of the hydrometer. This is to make sure no liquid remains in the hydrometer — after it has been dipped into the cell of the battery. Additionally, this ensures all the air has been pushed out.

  • Step 3: Release the Bulb

Let go of the bulb. By doing this, you ensure a portion of the electrolyte finds its way into the needle of the hydrometer.

  • Step 4: Take Readings of the Specific Gravity

After you release the bulb of the hydrometer, the electrolyte rises up the float of the hydrometer. The readings on the wall of the hydrometer represent specific gravity. The readings — specific gravity obtained—correspond to charge in the battery.

Reading between 1.265 to 1.299 denotes a fully charged battery while a specific gravity of 1.200 to 1.265 is 75% charged. Check the table below for more information:

Specific gravityCharge of the battery
1.265 to 1.299Fully charged
1.235 to 1.26575% charged
1.200 to 1.235Half charged
1.170 to 1.20025% charged
1.140 to 1.170Almost running out
1.110 to 1.140Discharged

 

Generally, a reading that falls below 1.265 is considered an undercharge. Mostly, a trickle or a slow charge will support the restoration of the charge as well as help in boosting the chemical reactions inside the battery.

Conversely, a change difference of 0.025 to 0.050—that’s 25 points to 50 points since a point is 0.001 specific gravity— or more between any two specific gravities is an indication that the battery should be replaced. This is because such a difference denotes that the battery is sulfated.

 

How to Use a Multimeter to Do a Leak Test on Cover of a Battery

Apart from the charge of a battery, you can also test the leak on the cover of a battery. You can do this by using a multimeter. This is also known as testing for Dead Cells. The common cause of dead cells — also referred to as a dead battery — is the accumulation of deposits on the cover of a batter.

The accumulation of acid and dirt material on the battery terminals as well as across the battery will gradually lead to leaking of the charge. A multimeter checks the leaking point of a battery. In carrying out Leak Test:

  • Fix the multimeter to a low setting on the voltage scale
  • Turn the multimeter on
  • Hold the negative terminal of the battery with your multimeter’s black probe.
  • Hold the dirty cover of the battery using the red probe.

Any little amount of voltage that the multimeter records show that the battery power has begun leaking —due to a buildup of deposits on the battery. However, if you obtained a multimeter reading of zero voltage—but still believe that a certain device drains your battery’s power— it is advisable to carefully examine the circuit. This will enable you to locate the device that may be draining your battery’s power.

 

How to Use a Multimeter for a Voltage Drop

Additionally, you can use a multimeter to test terminals of a battery for a voltage drop. In this case,

  • Switch off the ignition system;
  • Touch the positive terminal of the battery using the red probe of the multimeter. Next, touch the negative probe of the multimeter to the negative terminal the battery;
  • Start the engine and register the reading on the multimeter.

A 0.5 V register shows that the terminals are dirty. Hence, you should clean or examine the state of the battery post as well as the terminals.

  • Check the other battery terminal. Touch the multimeter’s black probe to the negative terminal of the battery then turn on the ignition. Likewise, a 0.5 V or more multimeter record shows that there is possible damage or the terminal is dirty.

 

Final Note

Both multimeter and hydrometer are crucial in testing conditions of your car’s battery. Apart from the condition of the battery, you can use the two devices to test for defects within the battery.

These tests depend on the type of battery and condition to be tested. You should first decide on what you want to test. Then, using the steps outlined in this article, carry out your car battery’s tests.