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How to Test a Car Battery With a Multimeter

Updated on July 29, 2016
A neglected car battery can leave you stranded.
A neglected car battery can leave you stranded. | Source

Knowing how to test a car battery with a multimeter for common battery issues may not seem important, but battery cell problems, corroded terminals, acid buildup, and battery case damage are among the main culprits behind starting and charging system problems. Use your digital multimeter for some simple tests to find out the working and physical condition of your battery, and then clean your battery to help prevent the most common battery issues.

If your battery discharges after some time or doesn't seem to accept a charge, there's a chance your battery is behind this issue; but you also may be dealing with a starting or charging system problem.

This guide will show you how to use a digital multimeter and other simple tools to help you troubleshoot, inspect, and even clean your battery, if necessary.

You can check your battery for leaks using a voltmeter.
You can check your battery for leaks using a voltmeter. | Source
Index
WARNING - Measuring Electrolyte Temperature
How Can You Tell If Your Battery is Charged?
1. Battery Voltage Test Using a Multimeter
2. Battery Charge and Overall State of Health Test
II. How to Check Battery Terminals
III. How to Do a Battery Leak Test Using a Multimeter
IV. Inspecting the Battery Case
V. How to Clean a Car Battery
1. Cleaning the battery case
2. Cleaning the Battery Terminals
3. Cleaning the Battery Tray
VI. Reinstalling the Battery
Quiz - Test Your Car Battery Knowledge

Warning

When measuring electrolyte temperature never use a metallic thermometer. Sulfuric acid may have an adverse reaction with the thermometer and even cause an explosion.

I. How Can You Tell if Your Battery Is Charged?

You can use one (or both) of two methods to learn the state of your battery using a couple simple tools.

With the first method, you can quickly check if your battery has enough juice for the starting system of your car to fire up your engine using a digital multimeter. This is a quick way to learn the state of your battery, specially on so called free maintenance batteries, which don't have removable caps.

The second method uses a hydrometer. Unlike the digital multimeter, the hydrometer can reveal not only the state of charge of your battery, but also its overall health. You'll know if your battery has failed. But you can only use this tool on batteries with removable caps. You can buy an inexpensive hydrometer at most auto parts stores or online.


1. Battery Voltage Test Using a Multimeter

You can quickly check your car battery charge using a multimeter. Some free maintenance batteries come with a charge indicator, a sight on the battery top that tells you if the battery is fully charged (green dot), in need of a charge (no color visible), or faulty (yellow dot).

Whether you free maintenance battery comes with an eye indicator, you still can test it to reveal the general state of charge in a minute, using your digital multimeter.

1. First, set your digital voltmeter to 20 DC volts.

2. Touch the negative (black) battery terminal with the negative (black) meter probe.

3. Touch the positive (red) battery terminal with the positive (red) meter probe.

4. Ask an assistant to turn on the headlights to provide the battery with a light load.

5. Check your voltmeter reading.

At about 80F (26.6C) in temperature:

* A reading of about 12.5 or higher, means your battery
has a good charge.
* A reading of about 12.3, means your battery is about 75% charged.
* A reading of 11.8 or lower, means your battery is about 25% or less charged.

If you got a low reading, slow charge your battery to improve the chemical reaction of the battery and repeat the test. If your reading doesn't go higher, replace the battery.



2. Battery Charge and Overall State of Health Test

Also known as electrolyte gravity test, it'll tell whether your battery is fully charged, undercharged, or even faulty.

For best results, use a hydrometer with a built-in thermometer. Some hydrometers are self-adjusting; other hydrometers come with a conversion table to help you make the adjustments to your reading when performing the test at different ambient temperatures.

When doing the next test, wear acid-resistant glove and goggles:

1. Remove the caps from the top of the battery.

2. Submerge the tip of the hydrometer into the first cell of the battery and squeeze the hydrometer bulb.

3. Release the bulb to allow electrolyte to enter the hydrometer needle.

4. Read the electrolyte specific gravity as indicated in the instructions on the tool's package.

5. Make a note of your reading and perform the same test on the rest of the cells.

6. Compare your results to those of your tool manufacturer instructions.

Basically, if your readings fall between 1.265 and 1.299, your battery is charged. When your readings fall below 1.265, your battery is undercharged. In most cases, a slow or trickle charge will help restore the charge and improve the chemical reaction in the battery. However, a difference of 25 to 50 points or more (a point equals 0.001), between any of your readings, indicates that the battery is sulfated and you need to replace it.

Checking Battery Cables

II. How to Check Battery Terminals

This is one of the main causes of "dead" batteries. Dirty, corroded, or loose battery terminals will lead to hard-to-start or no-start issues. Sometimes, though, these type of problems are hard to spot with a simple visual inspection.

Here, you'll check your battery terminals for voltage drop to know the state of your battery connections using a voltmeter.

1. First, disable the ignition system. Do this by temporarily disconnecting the ignition coil or removing the fuel pump fuse or relay (check your car owner's manual or your vehicle repair manual to locate the coil or fuse, if necessary). This will prevent your engine from starting.

2. With your meter's red probe, touch the positive battery post. Now, touch your meter's black probe to the cable terminal connecting to the same battery post.

3. Ask an assistant to crank the engine. If your meter registers over 0.5 volts you need to clean or check the physical condition of the battery post and terminals.

4. Now, test the other battery terminal. This time, though, with your meter's black probe, touch the negative battery post. Now, touch your meter's red probe to the cable terminal connecting to the same battery post. Ask your assistant to crank the engine and check the voltage reading on your meter. If it is more than 0.5 volts, you need to clean or check your battery terminal for damage.

III. How to Do a Battery Leak Test Using a Multimeter

Buildup on the battery cover is one of the main causes of "dead" batteries. As dirt and acid mix and accumulate across the top and terminals, the charge slowly begins to leak. To check whether your battery has reached the leaking point, use a multimeter.

Set your voltmeter to a low setting on the voltage scale. Turn on the voltmeter. Then touch the battery negative terminal (-) with the black probe from your meter, and touch the dirty battery cover with the red probe. If your meter registers even a small amount of voltage, buildup has begun to leak battery power. Head to the section below How to Clean a Car Battery to remove the leak. If you get zero volts reading from your meter but you still suspect that some electrical device is draining power from your battery — aka parasitic draw — check this other article, Car Battery Drain, to locate the circuit and the device discharging your battery.

Checking a Car Battery

IV. Inspecting the Battery Case

Now it's time to check the physical condition of your battery case. A damaged battery case can not only prevent the battery from working properly, but eventually will kill it. For this, you'll need to remove the battery from your vehicle.

1) First, disconnect the battery cables starting with the ground terminal and then the positive terminal. Now, unscrew the hold-down mechanism to remove the battery from the tray.

2) Make sure the hold-down mechanism works. If there's missing hardware, replace it. This will prevent the battery from bouncing and getting damaged when the vehicle is moving.

3) Place the battery on a workbench or similar surface. Carefully examine the battery case for potential damage. Check for a bulging side or cover, cracks around the case, or damaged terminal posts. If your battery is damaged in any way, replace it.

4) Overcharging and internal short circuits will cause a battery to swell as the acid turns to gas. Thus, if you find signs of bulging, check the charging system as well.

5) Now, check the battery cables. Check for frayed, cracked or worn insulation around the cables and the condition of the cable threads. Replace them as necessary.

Using Baking Soda to Fight Acid

A mixture of baking soda and warm water can help you fight corrosion on your battery.
A mixture of baking soda and warm water can help you fight corrosion on your battery. | Source

V. How to Clean a Car Battery

1. Cleaning the Battery Case

You can use a simple procedure to clean the battery case. For this, you'll need to prepare a mixture of 8 ounces of warm water for one tablespoon of baking soda. This will help you neutralize and lift up acid and dirt off the battery case and terminals.

Put on your goggles and rubber gloves and, using a soft brush, apply the solution across the battery top and around the sides of the case. If your battery uses cell caps (maintenance type batteries), don't let the mixture seep under the caps and mix with the electrolyte inside.

Wipe the solution using a clean rag. Continue applying the cleaning solution until your see no signs of buildup.

2. Cleaning the Battery Terminals

Just as you did with the battery case, remove dirt and corrosion off the battery terminals using the baking soda and water solution.

To make your task easier, pour the mixture in a foam cup or similar disposable cup and dip the battery terminal in it for one or two minutes. Then use a battery post cleaning tool to finish removing the corrosion off the terminals. Repeat the procedure until you see both terminals free of corrosion.

3. Cleaning the Battery Tray

Inspect the condition of the battery tray. Make sure it has no missing screws, no cracks or missing pieces and no signs of corrosion. If necessary, use the same solution to remove dirt and corrosion from the tray.

Make sure the hold-down mechanism on your battery is working.
Make sure the hold-down mechanism on your battery is working. | Source

VI. Reinstalling the Battery

After cleaning the battery case, terminals and tray, reinstall the battery.

1. Carefully place the battery on its tray.

2. Secure the battery to the tray using the hold-down mechanism.

3. Connect the battery terminals. This time start with the positive terminal and connect the negative or ground terminal last.

4. After connecting the terminals, spread a thin layer of petroleum jelly around the top of the terminals and posts. This inhibits corrosion buildup around the terminals.

If your tests showed your battery seems to be in good health, you may have a problem with the starting or charging system. This other article on Troubleshooting Car Starter Problems shows you how to do some quick tests without using tools.

Once you know how to test car battery common problem areas, you'll service the battery faster next time around. And servicing your battery regularly goes a long way. You make sure your battery provides all the necessary power to the starting system, ignition system and other important electrical circuits as necessary, especially during the cold months. Even more, with a little maintenance you'll help prolong the service life of your battery and stretch your car maintenance budget.

Test Your Car Battery Knowledge

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