Rick's Free Auto Repair Advice

How to test a car battery

Test a car battery

I have lots of other cool car battery posts on this site
How to clean car battery terminals and posts
How a car battery works
How to replace a car battery cable terminal
How to charge a car battery
What are cold cranking amps?
Who makes the best car battery?
What is car battery reserve capacity? 

Use a voltmeter to test a car battery

You can use a voltmeter to test a car battery. It’s not the most accurate test, but it can tell you the current status of your battery. I’ll show you how to use a voltmeter to test your battery below. BUT, the test results cannot be used to determine the condition of your battery going forward. For that you need a real battery tester that conducts a simulated load, capacitance and resistance test. I’ll get into those towards the end of this post

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.

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 battery 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.

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

©, 2018 Rick Muscoplat

Posted on by Rick Muscoplat


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