Home Diagnosis and TroubleshootingBattery and Charging How To Test A Car Battery – And The Alternator With A Multimeter

How To Test A Car Battery – And The Alternator With A Multimeter

by Jordan Harris
how to test a car battery

Your car’s battery is responsible for two things: powering the starting motor so your car will start. And it’s also responsible for all the electrical components in your car. If your car is struggling to start, then you may have a bad battery and it’s time to replace it. In this post, we’ll guide you on how to test a car battery, as well as test your car’s alternator with a multimeter. This way, you can find out whether you actually need a battery replacement or not. We’ll also talk about everything else you need to know about your car’s battery.

How Does A Car Battery Work

Before we guide you on how to test a car battery, it would be a good idea for you to understand more about how your car’s battery and electrical system work. As mentioned, your car’s battery is responsible for mainly two things.

This includes powering up the starter motor when you turn the key and powering the electrical components in your car. That’s why when your battery is dying, you will have difficulty starting your car. It may not even start at all if the battery’s completely dead. But how does the entire system work?

Cars nowadays use a 12-volt lead-acid battery, which creates electricity from the chemical reaction between lead and acid. When the engine is off, everything electrical in your car runs from the electricity in the battery. Once the engine is running, your car’s alternator will power all the electronics in your car.

It does this while also recharging your battery, making sure it has enough electricity the next time you need to start your car. That’s why when you have a bad alternator, your battery may die more quickly.

A car’s battery typically lasts between 2-3 years, depending on the battery quality. Cars with a stop-start system may also drain the battery more quickly. If your car’s battery is dying or has died prematurely, this is usually because your alternator isn’t charging the battery properly. That’s why we’ll guide you on how to test a car battery, as well as the alternator.

We recommend watching this video from Donut Media to learn more about car batteries and alternators:

Weak Car Battery Symptoms

How do you know your car’s battery is dying? Well, the surefire way to tell is to test it with a multimeter. But before that, see if your car has any of the symptoms we’ll list down below. If you see any of these symptoms, then it’s a good idea to test your car’s battery with a multimeter.

1. Dim Headlights

If your notice your headlights are not as bright as usual, then you may have a dying battery. Or you can also check by turning on your headlights, and then turning on your car’s air-conditioning. Both of these components use a lot of electricity, so when the battery isn’t holding enough charge, it will usually dim the headlights as there isn’t enough electricity to go around.

Keep in mind that dim headlights might also be caused by an alternator that isn’t charging the battery properly. We’ll guide you on how to test both the battery and the alternator later on in this post.

2. Rotten Smell

If there’s a sulfuric or rotten egg smell coming from your battery, then this is a sign that you need to replace your car’s battery. Your car’s battery has a mixture of 65% water and 35% sulfuric acid that creates a reaction with the lead inside your battery. As mentioned, this is what creates electricity in a lead-acid battery.

Over time, some of the acid and water may evaporate which will ruin the mixture. When the mixture evaporates, it will create an unpleasant smell. In severe cases, the battery may overheat and even produce smoke. If there’s an unpleasant smell coming from your battery, best to replace it immediately.

3. The Engine Is Slow To Crank

Not everyone will notice this, but keen drivers will almost certainly notice it. Most cars will crank and start the engine easily. But if you’re battery is dying, it might crank slower and it will sound like the engine is struggling to come to life. If you notice that your engine is struggling to crank, then your battery is starting to die.

4. Swelling Battery

Your car’s battery has a boxy flat shape. As with any other battery, once it starts to swell, that means the battery is going bad. This happens either because the battery is too cold or too hold. This often happens to folks who live in colder climates and don’t drive their vehicle very often. In any case, if you see your car’s battery swelling, it’s definitely time to replace it. Immediately.

How To Check If Alternator Is Bad Red Battery Icon No charge

5. Battery Light On The Dashboard

Your car has tons of sensors and computing units that monitor the car and keep it running smoothly. These sensors also help to signal the driver when there’s something wrong with the car. One such signal is the battery icon on the dashboard, which most modern cars have.

If you see a battery icon light up on the dashboard, then it’s a sign that your battery is dying and it needs to be replaced soon. In some cars with a more complex computer, it may even tell you whether the problem lies with the battery or the alternator. In any case, if you see this warning light, it’s time to test out the battery.

How To Test Car Battery

So now you know how a car battery works, how do you test it? It’s fairly straightforward, all you need is a multimeter and a little caution. Working with batteries is dangerous since you’ll be dealing with a lot of electrical currents. You can borrow a multimeter from someone you know or buy one such as the Segomo Tools 500 for about $15.

Be sure to follow safety measures religiously to avoid painful accidents. Follow these steps to test a car battery:

How To Test A Car Battery: 1. Prepare The Battery

How to test a car battery

The first step is to locate the battery. Most cars will have their battery in the engine bay, but some cars – such as 3-Series BMWs – will have their battery in the trunk. Consult your manual if you’re not sure where the battery is. Next, turn on your headlights for two minutes to get rid of any surface charge the battery may still have. This will help you get a more accurate reading.

Before proceeding, it’s a good idea to remove any metals or jewelry from your body. Additionally, make sure you’re working in a dry area and there is no moisture anywhere near the battery.

Next, inspect the battery terminals. The positive terminal usually has a red cover over it with a “+” sign, while the negative terminal has a black cover with a “-” sign. Inspect for any signs of corrosion on the terminals.

If there are any, you will need to clean it first to make sure you get a correct reading with the multimeter. You can scrub off the corrosion using fine-grit sandpaper. Also, it’s a good idea to wear gloves to prevent exposure to harmful chemicals and battery acid on your skin.

Also, note that a car’s battery can emit an acid smell. This rarely happens but if it does it would be a good idea to call a professional mechanic instead to replace and dispose of your old battery.

How To Test A Car Battery: 2. Prepare And Connect The Multimeter

How to test a car battery

The multimeter has several settings for different tests. To test a battery, what you want to do is to set it to the 20 volts setting. Simply rotate the dial on the multimeter and you’re good to go.

The next step is to connect the multimeter to the battery. The multimeter will have two probes: red and black. Simply connect the red probe to the positive terminal and the black probe to the negative terminal. Make sure that the probe and the battery terminals are in contact.

How To Test A Car Battery: 3. Measure The Battery

Once the probes and terminals are connected, your multimeter will immediately start taking a reading. A healthy battery will read around 12.6 to 12.8 volts when the car is off. If your battery’s voltage is around that number, then the battery should be fine. However, the next thing you’ll want to do is test the battery’s cold-cranking amps.

How To Test A Car Battery: 4. Testing Cold Cranking Amps

For this next step, ideally, you need someone else to start the car. When the car starts, the reading on the multimeter will drop as the battery cranks the car. Have someone else start the car, and pay attention to the reading on the multimeter.

If it drops to below 10 volts, then your battery isn’t in great shape. In this case, it might be a good idea to start saving for a new battery. If it drops to below 5 volts, then really it’s time for a new battery immediately. If it drops to about 10 to 11 volts, then this is perfectly normal and your battery is healthy.

This video from ChrisFix will give you a great visual guide on how to test a car battery:

How To Test Alternator

While you’re testing the battery’s health, it’s a good idea to test your alternator while you’re at it. Identifying whether or not you have a bad alternator will help keep your battery in good shape and prevent your car from dying while you’re on the road. No one wants that. The steps are similar to how to test your car’s battery health:

  1. Prepare the battery, making sure there is no corrosion on the terminals. Make sure that you’re working on a dry area and that there is no moisture around the battery. Remove any metals or jewelry from your body as well.
  2. Connect the multimeter probes to the battery terminals.
  3. Turn on the engine and analyze the reading on the multimeter.

If you have between 14.2 to 14.7, then your alternator is charging the battery correctly. If it goes beyond 14.7 volts, then this can cause damage to your battery. Anywhere between 13.2 to 14.2 is also fine, but you might not have enough charge for when you turn on accessories like the headlights, radio, and air conditioning. In this case, it would be a good idea to have your alternator either replaced or serviced.

If you need a visual guide, watch this helpful video from ChrisFix:

Car Battery Replacement Cost

So, you’ve tested your battery and turns out it needs to be replaced. How much is it? Well, most car batteries will cost you around $80 – $150. However, if you drive a luxury car, a new battery can cost as high as $200. This is because luxury cars often have more electrical components and require a larger and more powerful battery to operate the car.

The battery requirements for each vehicle differ depending on the vehicle’s make and model. It’s a good idea for you to research battery buying guides and find out which battery would be best for your vehicle.

The labor cost to replace a battery is usually already included in the battery price and most auto repair shops won’t charge extra. However, if you want to replace the battery yourself, you should watch this video below:

How To Extend Car Battery Life

Now that you’ve bought a new battery, you’re probably going to want to get the most out of it. Here are some things that you can do to extend the life of your car’s battery:

  1. Limit short drives. Quick drives in your vehicle will prevent the battery from fully charging. Over time, the battery will slowly go flat and will die. If you drive for short periods often, avoid using electronic accessories that consume a lot of power such as air-conditioning. This will help your battery to charge faster as you drive along.
  2. Fasten your battery. If your battery isn’t secure, it could vibrate which can result in internal damage and short circuits. Check your battery terminal and mounting bracket regularly, especially if you drive on bumpy roads frequently.
  3. Clean the corrosion. As mentioned, your battery terminals can corrode over time. Keeping them clean will help the battery to stay in good shape. You can clean them by using sandpaper or scrub them with a toothbrush dipped in a mixture of baking soda and water. Then rinse it with cold water, and be sure to dry it thoroughly with a clean cloth.
  4. Test your alternator. As mentioned, the alternator charges the car’s battery while the engine is on. If the alternator isn’t charging properly, then your battery will go flat sooner than it should. Make sure your alternator is always charging the battery at the appropriate level.

Alternator Repair Cost

If it turns out you also have an alternator issue, then you will need to address it as well. If you leave it as it is, your new battery will die prematurely. The tricky part is knowing which part of the alternator system you need to replace, as the cause may vary. There are usually three causes as to why an alternator isn’t charging the battery properly:

  1. The alternator itself is worn out and isn’t operating as normal.
  2. The drive belt or pulley – which is what powers the alternator – is worn out and isn’t running properly.
  3. Wiring problems with the alternator. The alternator houses your car’s negative ground wire. If any of these wires were to come loose, or short out, it can cause the alternator to not function properly. More seriously, the electrical current can surge uncontrollably, and cause even more damage to the alternator.

We wrote an article about alternator repairs in great detail (and figuring out how to fix an alternator), and you can read about it here. But to give you an idea of the cost, an alternator replacement will cost anywhere between $200 to $1,000 (you can save a bit on labor if you learn how to change an alternator and changing the alternator). A cheaper option would be to buy a refurbished alternator, which can be as cheap as $150.

However, make sure you buy a refurbished alternator with a warranty, as there’s no telling how long it will last. Also, keep in mind that you can’t just plug and play any alternator. You have to make sure that the alternator can fit and will run on your car.

Meanwhile, if you have a problem with the drive belt or pulley, then the replacement will cost about $160 including labor.

Saving Cost: DIY Alternator Replacement

Of course, another way to save money on repairs is to do the job yourself (and save on the alternator replacement labor cost). We don’t recommend repairing your old alternator yourself as it’s quite complicated unless, of course, you have a degree in electrical engineering. Meanwhile, also check out our guide on how long does an alternator last to learn more.

However, if you’ve bought a new or refurbished alternator, here’s how you can replace the alternator yourself:

  1. Disconnect the car battery.
  2. Disconnect the wires that are connected to the alternator.
  3. Remove the drive belt from the pulley.
  4. Remove all the bolts that hold the alternator in place.
  5. Swap it out for the new alternator that you’ve purchased. Reinsert the bolts to hold the alternator in place, then reconnect the drive belt and the alternator.
  6. Reconnect the battery and turn on the car. Afterward, you should test out your battery using a multimeter to see if the new alternator is charging the battery properly.

As always, follow safety precautions before working on the battery and alternator. Here’s a quick video on how to replace a car’s alternator:

Dead Car Battery

So let’s say you’ve tested your car’s battery, found out that it’s not in great condition, but simply haven’t gotten around to replacing it, and now it’s dead and your car won’t start. What do you do? Well, if you’re at home, then you can probably buy it online and have it delivered. Or maybe ask your neighbor to take you to the nearest auto shop and buy a new battery and replace the old one.

But if you’re out on the road, you’re going to want to jumpstart it so you can drive to the nearest auto shop and replace the battery. Here’s a quick guide on how to jumpstart a car with a dead battery:

How To Jump Start A Dead Battery

Jump start 12V

You will need a jump lead and a second car. The battery voltage of the two vehicles must be identical. A car with a 12-volt battery must not be bridged with a truck battery with 24 volts. If you don’t have another vehicle, you can buy some of the best jump starters that suit your vehicle and keep it in the car at all times.

Turn off the engine on both vehicles, take out the gear and pull the handbrake. Check whether all the electrical consumers of your car (e.g., lights or rear window heating) have been switched off. Don’t forget to turn on the hazard lights if you’re on the side of the road.

Then connect the red cable to the positive pole of both batteries. Clamp one end of the red jump leads to the positive pole of the empty battery and connects the other end to the positive pole of the donor battery.

The black cable procedure is different: it belongs to the negative pole of the battery that supplies the current. Connect the other side to an unpainted metal part on the engine block of the breakdown vehicle, not to the empty battery’s negative pole. Once the cables are correctly attached, you can start the helper car.

Now start the car with the empty battery. An attempt should not take longer than 15 seconds. If the start is successful, let both engines run for about 3 minutes.

Then disconnect the cables in reverse order: first, the black negative cable and the positive red cable. Now your car should have enough juice to drive to the nearest auto repair shop. Avoid using electronic accessories that consume a lot of power such as air-conditioning.

How to Test Car Battery with a Multimeter

  1. A low or dead battery is often the cause of a car not starting.
  2. Most vehicle owners do not check their battery until it fails.
  3. Regular preventative maintenance suggests conducting a car battery voltage test at least twice per year using a multimeter.
  4. A multimeter is an electronic measuring instrument used to gauge volts, amps, and resistance from an electrical source.
  5. The most common automotive application for a multimeter is to test the strength of a car battery.
  6. To use a multimeter, the vehicle battery should be located and prepared by cleaning the positive and negative terminals of any dirt or corrosion.
  7. To measure the battery load, the multimeter dial should be turned to the “20 volts” setting, and the battery must be allowed to rest without surface charge before measuring.
  8. The multimeter has two probes, red and black, which are used to contact the positive and negative terminals, respectively.
  9. If the multimeter reading shows less than 12.2 volts, the battery’s resting voltage is weak and most likely needs to be charged or replaced.
  10. Utilizing a multimeter can help predict and prevent imminent failures that often occur without warning in a car’s battery and electrical system.

How To Test A Car Battery: In Conclusion…

Great! Now you know how to test a car battery, alternator, and everything you need to know about your car’s battery. If you drive a new car and the battery is still new, then you generally don’t need to worry.

Most alternators will last for at least 80,000 miles, and some will last until around 100,000 to 150,000 miles. If your vehicle is around this mileage, it would be a good idea to check the alternator every few months or so to help keep the battery in good shape.

Frequently Asked Questions

If you’re still curious to learn more about how to test a car battery, our FAQs here might help…

How Many Volts Is A Car Battery

A car battery’s voltage isn’t usually definite or kept at a constant rate. Although it may be called a 12V battery, its actual voltage range varies between 12 to 14 volts. In particular, a battery’s resting voltage should be around 12.6V. This is the idling voltage reading when your car is turned off and isn’t running. While the engine is running, the battery might run between 13.5 to 14.5 volts or thereabouts. Its voltage is subsequently boosted by the alternator. This is necessary, as many electrical and electronic functions in your car require power from the battery. Meanwhile, anything below 12.4V is generally a good sign that your battery’s charge is depleting. At 12V, your battery is only 50% charged, while at below 12V, your battery is practically flat.

How To Charge A Car Battery

If you’re going to leave your car idling for too long, sooner or later, the battery will die completely. To avoid it depleting fully, it’s a good idea to hook up your car to a battery charger to maintain its charge. Battery chargers essentially trickle in small currents at a time to keep the battery topped up when you need it. To charge a battery, a good first step would be accessing the battery. Then, ensure that all in-car electronics are turned off. Once you’re ready, grab the battery charger. Now, connect the positive lead to the battery first, followed by the negative lead. This is crucial, as you need to follow this order – positive first, then negative. When that’s done, plug the battery charger into a wall outlet, and turn the device on to charge your car’s battery.

How Long Does It Take To Charge A Car Battery

The length of time taken to charge up a car battery depends on several factors. First off, what’s the charge rate of the battery right now? If the battery is nearly depleted, it would naturally take longer to charge it up than a partially depleted battery. Secondly, how fast can your battery charger charge? A typical battery charged will often charge at around 4 to 8 amps. At this rate, it would usually take between 10 to 24 hours to fully charge a car battery. However, there are more powerful battery chargers out there that could charge at a rate of 20 amps. At this rate, it could take you as little as just 2 to 4 hours to charge a 12v battery. On the flip side, most cheaper chargers only charge at 1 to 2 amps, requiring around 2 days or more to fully charge.

How Often To Replace Car Battery

The lifespan of a car’s 12V battery can vary. Just to be safe, you could have it replaced every 2 to 3 years. This is especially the case if you’re using your car a lot, or if your car regularly draws plenty of electricity. For example, cars that have an abundance of in-car electronics and other electrical components will force the battery to work harder. Thus, stressing it out and prematurely wearing it. However, some other estimates say that the aforementioned figures are too early. They claim that a battery is easily able to last between 4 to 5 years. A general rule of thumb would be to replace it every 4 years. But, also bear in mind the numerous symptoms that might indicate a weak battery, and have it replaced accordingly.

How To Tell If Car Battery Is Bad

There are several ways to tell if your car’s 12V battery is bad. Usually, a faulty or weak battery would immediately impact your car’s electrical and electronics accessories. So, take a look and see if the headlights are running dim, or if the engine is slow to crank. Some cars also have dedicated battery warning lights on the dash, which would light up if it notices something wrong with the car’s battery. Otherwise, you can inspect the battery visually. Should you notice a rotten egg smell, have your battery replaced as soon as possible. The latter can sometimes be at risk of overheating and catching on fire. You may also take a close look at the battery itself. If you spot any swelling on the battery housing, it would be a good idea to have it replaced, as well.

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1 comment

Zachary Tomlinson 05/18/2023 - 2:42 PM

I find it fascinating that testing your car battery could ensure that it can handle whatever you throw at it as you drive. My friend is thinking about spending more time with nature by purchasing an RV for his family. I should suggest that they find a store that sells retail RV batteries and have them tested before a trip!


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