How To Test A Car Battery

Testing your car battery is easy. All you need is a battery load tester and a digital volt meter. 

Here’s how to test a car battery.

Testing The State of Charge

Here’s how to test a car battery using an hydrometer.

First, clean the battery and make sure there is no obvious physical damage or bulging of the sides. Replace the battery if damage is observed.

If you have cell caps that can be easily removed, remove them and check to see if each cell is filled to the bottom of the battery top, like in the photo below.

If not you can fill them with distilled water. Be careful not to over fill.  Fill them only to the bottom of the battery top, not all the way to the top. You can use battery hydrometer, like the one in this photo, to determine the specific gravity of each cell in the battery.

Each cell should read within .050 from your highest reading to the lowest reading.  If not, put the battery on a [easyazon_link asin=”B000NCG7QQ” locale=”US” new_window=”default” tag=”carbattery0e-20″ add_to_cart=”default” cloaking=”default” localization=”default” nofollow=”default” popups=”default”]battery charger.[/easyazon_link]

Most battery chargers will have a red and green scale indicating the charge of the battery. Continue to charge the battery until the charger is in the green part of the scale.

After charging, use the hydrometer to see if all cells are within .050 of the highest reading to the lowest reading.  You need at least a charge of 75% for the next test.

If you have a hydrometer reading of 1.217 or higher, and a voltage reading of 12.4 or higher you have a good charge on the battery.  At this point, you can load test the battery.

Test With a Load Tester

Here’s how to test a car battery using a load tester.

You have to first remove the “surface charge” of the battery.

The surface charge is simply a 12 volt or more voltage reading that remains on the battery after it is charged.

This can give you a false reading of 12 volts when the battery is actually reading lower.

Simply hook up your load tester to your battery and press the test button for a few seconds. Another way if you have not removed the battery from the car, is to just start the car and immediately turn the engine off. Do this twice.

If the voltage reading is below 12 volts it is likely that that the battery will not charge up and is no longer usable.

If the battery holds a charge, you can apply a load test by pressing the test button. A load test of 10 seconds should be enough to get a good or bad reading on the tester.

Technically, you should be able to apply a load of half the cold cranking amp rating of the battery for 15 seconds and the battery voltage should not drop below 9.5 volts.

With most small testers, like the one on the left, all that is necessary is to load the battery for 10 seconds and observe the needle.  It will read good, weak, or bad.

While applying the load test take your digital volt meter and take a voltage reading.  Any reading below 9.5 volts is indicating a weakened battery.

Ideally, after removing the surface charge, and while load testing for 10 seconds, you should expect to see a voltage reading of  9.5 volts or more.

But any reading below, you should replace the battery.

At this point, hopefully you have a healthy battery that has passed all the tests.

If you have one that has failed the test, check the following in addition to replacing the battery for good measure.

• Loose or corroded battery connections. See here on how to check and clean.
• Check alternator drive belt and ensure it is not loose or broken.

Points To Keep In Mind 

• If your battery still turns the engine over but is weak according to your tests, you should be aware that your battery could fail soon. I like to carry this [easyazon_link asin=”B00D42AFS8″ locale=”US” new_window=”default” tag=”carbattery0e-20″ add_to_cart=”default” cloaking=”default” localization=”default” nofollow=”default” popups=”default”]portable jump starter[/easyazon_link] in my glove compartment, just incase my battery needs a jump.
• Any sudden change to cold weather and temperature can often kill a weak battery. Also check the label on the battery and see if you are at the end of the life span of the battery.
• Manufactures will warranty their batteries for different lengths of time. Typical life spans are 36 months 48 months and 60 months.
• If you are 30 months of a 36 month battery that is testing weak, you are on borrowed time as far as the battery is concerned. I would get a replacement in such a case.
• A warning sign that your battery is getting to the end of its service life is cranking speed.
• When you start your car cold, as in first thing in the morning, is the cranking sound before the engine starts vigorous or labored. If you can describe the sound as labored then your battery is weak.
• Again, test and check the items already mentioned.
• Another quick check is to start the car with the head lights on, preferably at night. If the head lamps dim significantly then a battery failure is looming.

I hope you found this information on how to test a car battery useful.  Please leave a comment or question below.

Thanks For Visiting!

Edward Verheyden is the founder of CarBatteryHelp.com.  He has a 30+ year mechanical background and is the main reviewer here.  He lives in San Diego CA with is wife and two daughters.

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