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How to test a car battery

Test a car battery

A voltage test doesn’t tell you the battery’s condition

A voltage test tells you the battery’s State of Charge (SOC). But it doesn’t tell you the battery’s State of Health (SOH). To measure SOH, you need a digital battery tester.More on that later.

How to test a car battery state of charge with a voltmeter

Set your voltmeter dial to DC volts >12 or 15. Attach the red lead to the positive (+) car battery post (not the cable terminal) and touch the black lead to the negative (-) car battery post. At 70° a fully charged battery will read around 12.6 volts. For other temperatures, see the chart below.

If the battery test at or above the stated voltage, that does NOT, by itself, mean the battery is good. All you’ve done at this point is to confirm the state of charge. The voltage test does not confirm car battery amperage capacity, whether there’s a voltage drop on the posts/terminals or whether there’s a small battery drain across the case. In other words, a battery can output proper voltage but may not be outputting enough amps or have other issues.

battery temperature charge chart

Use a voltmeter to test a car battery capacity

Now we’re going to use the same setup to test the car battery’s ability to output amps. Leave the meter hooked up and have a friend start the engine. The voltmeter will dip during cranking and then bounce back up to a near-normal reading. This happens quickly. If the meter shows voltage dipping to less than 10-volts during cranking, the battery is in poor condition and should be replaced.

Test voltage drop

Set meter on the lowest DC volts setting. Connect

battery voltage drop test

Battery voltage drop test

one lead to the negative battery terminal and the other lead to the negative battery post. Crank the engine. The result you see is battery voltage drop between the battery post and the battery terminal. Repeat the test on the positive post and terminal. A reading of 0.1 volt or higher is an indication of excessive resistance which can decrease amps while cranking and affect recharging. Clean the battery terminals and posts according to this article.

A digital battery tester tells you more than a voltage test can

A digital battery tester measures the battery’s internal resistance and performs a simulated load test. Internal resistance is a major indicator of the battery’s overall condition and the load test tells you how well the battery will perform in cold conditions.

To use a digital battery tester, you first enter the cold cranking amp (CCA) specification listed on the battery’s label. Then you press the start button. The tester will first check the voltage. Then it will conduct a simulated load test, which used to be the gold standard of testing. Then it will conduct an internal resistance test, and finally a conductance test.

What is a battery load test?

A battery load test is basically a stress test for the battery. The tester loads the battery to 1/2 of its CCA rating for 15-seconds. The voltage must remain above 9.6-volts during the test for it to succeed. Like the voltmeter test, this test tells you the battery’s condition at that time, but doesn’t give you any idea of internal resistance. So it can’t be used to predict battery life.

Why a digital battery tester is more accurate

A solid-state battery tester like the Solar BA9 is a great way to test the overall condition of your battery. This tester checks several types of batteries, from the traditional flooded lead-acid (SLA) found in most cars, to Absorbed Glass Mat (AGM) flat plate, AGM spiral wound and even gel cell batteries used on electric start lawnmowers.

The tester conducts a simulated load test, conductance and resistance test. When a car battery tests good using this type of charger, it’s an indication that you can count on the battery to perform into the future. Repeat the battery test each season for peace of mind

Why is internal resistance important?

As the battery ages and the lead plates deteriorate and form sulfate crystals, internal resistance between the plates and main bus increases. Increased resistance is an indicator of remaining battery life. In other words, high internal resistance is a sign of internal deterioration. Most digital car battery testers report the results in terms of pass/fail rather than Ωs.

Examples of digital battery testers

Midtronics is considered top-of-the-line in the auto trade. But their products are pricey. Expect to pay at least $200 for a Midtronics battery tester. A less expensive and equally as accurate is the Solar BA9 battery tester (around $55 on amazon).

midtronics battery tester

Battery testers to avoid

Several companies make a combination of volt and load tester. In the industry, these are referred to as battery toasters because the load test uses heating coils similar to those used in a toaster. They’re cheap, but they’re not very reliable because the resistance of the coils changes based on their temperature. Perform the test and you’ll get one reading. Perform it 15 seconds from now and you’ll get a different reading.

©, 2018 Rick Muscoplat

Posted on by Rick Muscoplat


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