Rick's Free Auto Repair Advice

Test a car battery

How test a car battery

If you do an Internet search for “How to test a car battery” you see post that tell you to use a voltmeter. That’s partially true. Testing your car battery’s voltage tells you its current state of charge. But it doesn’t tell you the battery’s state of health. In other words, a voltage tests only tells you the charge level at that moment in time. But a voltmeter alone can’t tell you the battery’s condition or give you any kind of prediction of remaining life. For that, you need a digital battery tester.

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 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

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.

battery tester

Old fashioned battery tester

If you don’t own a digital battery tester, see this post on how to test your car battery with a voltmeter.

What to look for in a battery tester?

The battery tester mus work with multiple battery types

Start/stop vehicles are equipped with enhanced flooded batteries (EFB) or absorbed glass mat batteries (AGM). These batteries required a tester that has an AGM/EFB setting. If you use a battery tester that was designed for standard lighting ignition (SLI) flooded batteries, you will get a false result.

Even withing the AGM category, spiral wound AGM batteries test differently than flat plate AGM batteries. So you need a battery tester that can test: Flooded, AGM flat plate, AGM Spiral plate and Gel Cell

The battery tester must test cold cranking amps — CCA

This test tells you how your battery will perform in cold weather.

Reverse polarity detection

You don’t want to damage your tester if you hook it up backwards

Over-voltage protection

You don’t want to damage your tester if you hook and system voltage is too high

Charging system test capabilities

A good charger is capable of checking for proper charging voltage with and without loads as well as testing for AC ripple

State of Health

State of health is determined by internal resistance

Cranking test

A cranking test tells you the condition of the battery and the starter

©, 2020 Rick Muscoplat

Posted on by Rick Muscoplat


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