How To Test A Car Battery – An Easy Step-By-Step Guide

When you go to the doctor, whether there’s something wrong or because you’re due for your physical, he or she will run a few tests.

Maybe you’ll get sent to have blood taken at a lab? Or perhaps it’ll be something uncomfortable involving a rubber glove? Tests can provide a lot of information on the health of your various systems. (Continue reading when you’re done squirming.)

The same goes for your vehicle. You can tell a lot about how your car is functioning by running a few tests. One of the easiest and most important things to check on is your battery.

A car battery being tested with a multimeterGetting stuck with a dead or failing battery is a serious pain in the you-know-what. By staying on top of its condition, you can hopefully avoid a frustrating situation down the road.

In this easy to follow, step-by-step guide on how to test a car battery, we’ll cover some of the signs to watch for that might indicate it’s unhealthy, in need of a test, and potentially needing you to take the time to charge it.

Ready to play doctor?

How Do I Know If My Car Battery Needs Testing?

Regular testing of your vehicle’s battery is a great idea, even if there are no outward signs of trouble. As with your health, preventative maintenance is always a better idea than waiting until there’s a problem.

But, since many (possibly most) of us are reactive rather than proactive, here are some telltale signs that your battery is in need of a check-up:

  • Turns over slowly when starting
  • Headlights and/or dash lights flickering or dimming when starting or idling
  • Power windows slow when the motor is off
  • Signs of acid leaks or corrosion on the battery

If you keep a battery in storage that’s not being used, it’s good advice to leave it attached to a quality battery maintainer. This will make sure the charge doesn’t drop too low and will keep it in top top condition, instead of slowly degrading due to lack of use.

How To Test A Car Battery With a Multimeter, Voltmeter or Conductance Tester

Many people own a multimeter – or Voltmeter – for testing appliances, electronics, and household wiring. They’re popular with hobbyists, too. Guess what? You can use them to check the state of charge on your car battery, too!

Here’s how:

  1. Put on safety goggles and gloves. (You can never be too careful with a car battery.)
  2. Uncap the battery terminals on your car battery. Clean off any corrosion that may be present.
  3. Turn on your digital multimeter and set it to the closest setting to 12 Volts DC that’s higher than 12V. This will probably be somewhere from 15V to 20V. Some units won’t have a number, so set to DC Volts or Voltage.
  4. Touch the black lead to the negative terminal (black).
  5. Touch the red lead to the positive terminal (red).
  6. Check your multimeter reading.

How To Test A Car Battery With A Load Tester

A load tester is a type of dedicated car battery tester that can tell you a lot more about the state of your battery than a multimeter can. As the name implies, it can also test your battery under load to tell you if you’re still getting the Cold Cranking Amps (CCA) the manufacturer promised.

Digital testers will require varying degrees of input from the user ranging from CCA to battery type, group and size. Consult the owner’s manual for complete instructions.

Here are general instructions for how to test your car’s battery using a load tester:

  1. Put on safety goggles and gloves.
  2. Uncap the terminals. Clean off any corrosion that may be present.
  3. For digital load testers only, set the CCA value (Cold Cranking Amps) if necessary. You can find the CCA marked on your battery.
  4. Connect the black (negative) clamp to the black (negative) terminal.
  5. Connect the red (positive) clamp to the red (positive) terminal.
  6. Check the reading for voltage at rest. For analog displays a needle in the green zone is good, yellow is weak, and red is bad. A digital display will show the volts, and an indicator light will show if it’s bad, weak, or good.
  7. Press and hold the load test button for as long as the manufacturer recommends (usually 5-10 seconds, otherwise the resistor will overheat). Some digital units will take this reading automatically.
  8. Check the reading for CCA. On an analog display, look for the green zone marked closest to the CCA of your battery. Anything below is weak or bad. On digital units, there should be an indicator light or display to tell you if it’s bad, weak, or good.
  9. After a load test, check your volts at rest to see if the test depleted the battery noticeably.

How Many Volts Should A Fully Charged Car Battery Have?

A test result is useless if you don’t know what you’re comparing it to! Although there is variation from one battery to another, a fully charged 12V car battery will read at around 12.6-12.7 volts at rest.

With the engine running this should climb to anywhere from 13.7 to 14.7 volts.

Here are some guidelines for battery depletion levels:

  • 12.5V – 90%
  • 12.42V – 80%
  • 12.32V – 70%
  • 12.2V – 60%
  • 12.06V – 50%
  • 11.9V – 40%
  • 11.79V – 30%
  • 11.58V – 20%
  • 11.31V – 10%
  • 10.5.V – 0%

If your battery reads lower than about 12.45V, it probably needs recharging. You can do this by running the vehicle, or hooking up to a car battery charger.

A Note On Temperature and Battery Charge Level

The charge levels I’ve used are based on an ambient temperature of 80F (26.6C). As the temperature drops, the reading for a fully charged battery climbs.

At 0F (-17.7C) the reading will be more like 12.84V. At 140F (60C) it’ll be about 12.5V for a 0.34 voltage drop.

If you want to calculate 100% charge at whatever temperature it is where you are, add 0.024V for every 10 degrees below 80F, or subtract the same amount for every 10 degrees above 80F.

Remember, these numbers are always approximations.

How To Check A Car Battery With A Hydrometer

Most batteries these days are of the sealed, or “Maintenance Free” variety. Some, however, have caps you can open to access the cells inside so you can top up the distilled water as it evaporates.

With this access, you can also use a hydrometer to test the specific gravity and health of the battery.

Without getting into detail about specific gravity, here’s what you need to know about using a hydrometer to check your car battery:

  1. Fully charging the battery with a charger or by running the engine.
  2. Allow it to settle for 2-3 hours.
  3. Put on protective goggles, gloves, and clothing.
  4. Disconnect the negative cable to eliminate any possible load.
  5. Open the caps on the top of the battery to expose the six cells inside.
  6. Squeeze the bulb of the hydrometer.
  7. Insert the nozzle into the fluid inside the first cell.
  8. Release the bulb to draw fluid into the chamber.
  9. Allow the fluid to settle and take your reading. Green, red, and white zones indicate good, bad, and weak levels. You can also take a numerical reading.
  10. Insert nozzle back into the cell and squeeze the bulb to return the fluid.
  11. Write down your results.
  12. Repeat steps 6-11 for the remaining 5
  13. Close the caps and carefully clean up any acid you may have dripped.
  14. Analyze your results.

Another note on temperature: It’s best to perform this test when the ambient temperature is between 60 to 80F (15.5 to 26.6C). For every 10F (12.2C) above 80F add 0.004 to your reading, and subtract the same for every 10F below 80F.

What Is A Good Reading For A Car Battery With A Hydrometer?

A reading of about 1.265 in each cell indicates a healthy, fully charged battery. If the reading is lower but consistent between all the cells, you should be able to charge the battery and correct the problem.

However, since you should theoretically have fully charged before testing, there may be an issue with holding a charge, or there’s a problem elsewhere in the system, such as an alternator issue, or a parasitic draw on the battery.

If there is a difference of 0.025 to 0.050 between one or more cells and the rest, your battery may need service or replacement.

Paging Doctor Twelve Volt

There, that wasn’t so bad! Testing your own car battery can save you time, money, and aggravation. And it’s a lot easier than, say, checking your own prostate.

Now that you know how to do it, you can help out a pal by either offering to do the same for them, OR you can just stay at home and send them the link to this article. That’s what friends are for.

If you have any questions or comments, be sure to fire them off to us in the comment section below. We love to know you’re out there! We hope you found this article helpful, and many thanks to you for reading!

From childhood go karting and motocross, to collecting and obsessing over scalextric, matchbox and radio controlled cars, I've always had an obsession with cars. Learning through manuals, books, trial and error, and more knowledgeable family members, I've also enjoyed tinkering with the mechanics and electronics of any vehicles I've owned. Now, over 3 decades later, I've started this site as a place for me to share my knowledge, to teach others how to care for and maintain their vehicles themselves, at home, so they can get the most of their vehicles and save a pretty penny compared to always seeking out professional help.

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