Simple steps to test your own car battery

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Waking up in the morning and having a dead battery is one of the worst things that can happen to a vehicle owner. How can you prevent something like this from happening, how can you test your own battery.  

Personally, I have helped many hundreds of people in this type of situation and have experienced it myself. Let’s learn a little bit more about why batteries fail and how you can test your own battery.

With these tips hopefully you will never need to experience running late when you are running late and having to make that dreaded phone call to your boss.

Why do batteries fail

Just like anything mechanical thing that you own, there are many reasons for a battery to fail.

 The most common cause of a battery that has gone bad is age. Over time, something known as battery sulfation builds up and reduces the battery’s capacity.

 If it gets bad enough the battery will not start your vehicle when you turn the key. 

Sulfation is caused by lead sulfate crystals that build up on the lead plates inside of your battery. During normal use while your alternator charges your battery the crystal formations are caused to break up.

However, if the vehicle sits for many weeks or a month with a low charge, these crystals can build up and harden becoming impossible to remove. This build up on the plates reduces the batteries capacity. 

Repeated short trips with lots of stop and go traffic are a common cause of a low charge in your battery.

The vehicle just does not get enough time to properly charge the battery like it would with long daily commutes.

Parasitic drains can also cause your battery to become low in charge. If not repaired it will completely discharge the vehicle’s battery. 

A parasitic drain is caused by an electrical component that will not go to sleep. As we talked about in our article this can be tested easily with an automotive multimeter.

In layman’s terms this means the component does not turn off or turns on when it is not told to. This would cause the battery to drain quickly, similar to when you leave the lights on in the car.

Sulfation can be prevented by properly charging your battery in your vehicle. 

The best method to combat sulfation is proper battery maintenance. 

Keep the battery charged to a minimum of 12.4 volts and keep the temperature under 24°C. This will make sure that the crystals do not have a chance to  harden and will be removed during the next charge cycle of your battery. You will need more than a simple test light to verify battery voltage when comparing to the below chart.

chart with various open circuit voltages and the battery state of charge
Voltage and state-of-charge will vary depending on temperature and type of battery. AGM batteries will read slightly more than a traditional flooded cell battery.

Battery terminal corrosion can also limit the alternator’s ability to charge the battery. If the battery is unable to be charged correctly, it will drain over time and leave you stranded. 

Just like our bodies, batteries start to lose capacity over time and will need to be replaced when they reach the 3-5 year point as well. 

Faulty Battery Symptoms

Knowing that your battery is starting to fail, and needs tested can sometimes be difficult.

 It is important to always take note of how your vehicle runs and drives. If you do not know what normal is, it is difficult to know when you have a problem.

Reduced starting power is one of the most common signs of a battery that is starting to fail.

 It is noticed when the vehicle is hard to crank or slow to crank. It seems like there is not enough power to turn the engine over. This is a sign that the battery should be tested.

 A no start, or possibly even having the vehicle quit at the side of the road are also signs of a battery that has failed completely and needs to be tested or possibly replaced

Battery load tester vs. electronic battery tester

There are two different types of automotive battery testers used in the industry, the electronic battery tester and the load or carbon pile tester. 

They work in similar but different ways making each tester useful in different situations. 

So how do these battery testers work?

How does an electronic battery tester work?

The electronic battery tester is the most common battery tester that you will see.

It is the tester that is used at your local auto parts stores or the quick lube shop down the street. 

These testers are fast and give a printout of your batteries CCA as well as battery life remaining. 

electronic battery tester

The electronic battery tester is also known as a conductance tester. It works by sending out a small amount of current through the battery and using this to calculate the resistance inside of the battery. 

The only problem with an electronic battery tester is each battery is made differently.

To get proper results the tester needs to know the internal construction, the number of plates inside of the battery, etc. in order to calculate the results correctly. 

The electronic tester will use a “Generic” value to calculate the results. This gives okay results but can also lead to incorrect diagnosis of batteries as well.

How does an battery load tester work?


The battery load tester or carbon pile tester is the best way to assess a battery. 

It is an excellent test to use because of its simplicity.

A carbon pile load tester has a large resistor that is able to place a large amount of load on your battery. 

battery load tester otherwise known as a carbon pile tester

The battery load tester or carbon pile tester is the best way to assess a battery. 

It is an excellent test to use because of its simplicity. A carbon pile load tester has a large resistor that is able to place a large amount of load on your battery. 

Some testers have a fixed amount of current that they can draw while others are adjustable for different amounts of current draw.

What makes the test so accurate is due to the way it simulates how the starter of your vehicle draws power from your battery.

When you start your vehicle there is a draw of up to 250 amps from your battery during cranking. This can be easily simulated with a carbon pile load tester. 

Some load testers do not come with an accurate voltmeter. 

So, if testing a battery with a carbon pile I would suggest using a good voltmeter to monitor voltage when completing the test. 

How to use a battery load tester

Electronic battery testers are very easy to use so we will not go into detail on that test. 

The load test has a few simple things that need to be followed to get accurate results. 

Let’s jump in and learn how to use a battery load tester to test a battery. 

Before using any battery tester, it is important that you have a fully charged battery.

 If you do not properly charge the battery before you begin, you can get bad results. This can result in replacing a battery that only needed charged, costing you more money to keep your vehicle maintained. 

How to load test a battery

  1. Find the cold cranking amp capacity of your battery. This is what we are going to use to calculate how many amps to load the battery with during our load test.
  2. Set the battery carbon pile load tester to half of the cold cranking amps. For example, if your battery has 600 cold cranking amps. Set the load tester to 300 amps.
  3. Attach voltmeter leads to the battery terminals if you do not have a voltmeter on your load tester.
  4. Apply the current to your battery using the load button or dial that your tester is equipped with. Apply the load for 15 seconds while watching the voltage. 
  5. If the voltage drops below 9.6 volts at any time during the 15 seconds, the battery fails the load test. 

The test is that easy.

 If during step 5 the battery gets to 10 volts or below, I always recommend looking at the age of the battery. If the battery is in the 3-5 year old age range I would recommend replacing the battery because it is nearing the end of its battery lifespan. 

How to load test a battery without a tester

There are other methods of how to evaluate your battery without any special tools.

 After all, not everyone owns an electronic battery tester or a carbon pile load tester. 

This method is not as accurate as using the correct battery tester, but the results are valuable. 

To do this test we are going to try and load the battery with as much of a load as possible. We are going to do it with the vehicle’s accessories instead of a tool. 

With the vehicle not running, turn on all of the electrical accessories in your vehicle. Turn on the headlights, radio, heater fan set to high speed, rear defrost and heated seats if your vehicle is equipped with them. 

Depending on the electrical accessories that your vehicle has it will cause a current draw on your battery up to 60 amps.

 Leave all of the electrical components turned on for 2 minutes. After 2 minutes turn all items off and try to start the engine. If the engine does not start, the battery fails the test and needs to be replaced. 

While this does not tell you if the battery is going to fail soon it can help you figure out if the battery you have currently in the vehicle needs replaced. 

Hopefully this helps everyone in learning a little bit more about how battery testers work and how to test your own battery. 

John Morris
John Morrishttps://autoknowit.com
John Morris is the technical editor for AutoKnowIt.com. His years of experience in automotive repair as well as an automotive professor have prepared him to ensure that even the most technical information is accurate and concise at all times.

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