You Might Be Driving with a Dead Car Battery. Get a Battery Test.

Car batteries can last three to five years on average, but when yours dies, it usually fails without a warning.

The truth is you might already have a dead car battery.

Your battery could be 75% charged or 100% charged and still be at death's door, the same way a cracked glass can be full of water. A fully charged battery could also be so aged, so sulfated and so weak that it can only hold a little bit of power for a brief period of time.

However, warm temperatures, a healthy engine and routine drives can mask that weakness. If the temps drop, a light gets left on or you don't drive a few days, that's when your car won't start.

It wasn't that the battery died on a random day. Your battery's been dead all along. The conditions just had to be right to show it.

Batteries wear down so slowly that you might not notice. That is, if a dying battery shows any signs at all.

Car Battery Hack: Ask for This Service With Every Oil Change

One simple service, available at most repair shops, could help you avoid the entire situation.

It's called a car battery test.

It's the most accurate way to tell if a battery is dead. While it's not as well-known as getting your oil changed or brakes checked, it's just as important. Getting a battery test routinely can keep your car ready and responsive every time you need it.

If you get into the habit, it's harder for a dead battery to surprise you.

Battery tests are the real answer to “How do I know if a battery is bad?”

A car battery test analyzes how well your battery stores power. For the most accurate results, technicians use battery testers. They send a jolt of electricity into your car battery and study its response. It takes less than a minute to run a thorough analysis, and you get a simple readout:

• Good
• Weak
• Replace now

“Good” means you don't have to worry about a dead car battery for at least another six months. Winter and summer are the hardest seasons on car batteries, so test yours before then. For now, your battery is fine.

“Weak” or “Replace now” are your chance to avoid the stress of being late for work because you're waiting for roadside assistance at the least convenient time. These results mean your battery is so weak that either cold temperatures, hard vibrations or not driving regularly could kill it.

Get a battery test at least twice a year to avoid dealing with a dead battery when you least expect it.

Hearing you have a bad battery can feel confusing. After all, you drove to the repair shop. The battery started the car just fine an hour ago, right? So, what exactly makes your car battery bad? The shocking truth is that 26% of all cars on the road today will need a new battery before the end of the year. That's a lot of people driving around with failing or dead batteries under their hood.

They just don't know it. They wouldn't, at least without a battery test.

You don't have to be surprised by a dead battery anymore.

Battery testing can change your car ownership experience. For decades, car owners had no choice but to wait for a dead battery to surprise them at the least convenient time possible. That era is long gone. Repair shops have the tools to take the mystery out of when your battery will die.

Here's what you don't know about how to tell if a battery is dead.

What makes a car battery dead?

One common misconception is that a battery test is just about voltage and charge. That's like checking a tire's air pressure to see if it has enough tread for a few years.

Voltage and charge can tell you if your battery is doing OK right now. However, they will not tell you anything about the battery's overall health. A weak battery can be “100% full” right after you charge it, and the voltage may say it's fine.

However, the voltage will drop again in a few hours. Then, if you try to start it, the amount of power won't be enough to start a go-kart.

Instead, you need to look at the battery's cold cranking amps.

Cold cranking amps and how to know if a battery is about to die.

A dead battery can't hold much power for long. A healthy one can store and deliver a lot of power.

That power is measured in cold cranking amps or CCA. Every car battery's label lists the CCA rating a battery should have, and every vehicle needs a certain amount of CCA to crank the engine. If your 2012 Toyota Corolla needs 500 cold cranking amps to start, then you need a battery with at least 500 cold cranking amps, like the M-35 car battery.

Why cold? Car batteries really don't like the cold. CCA shows how many amps your car battery should be able to deliver even if it's 32 degrees below freezing. So, in other words, CCA shows how well your battery can work in the hardest possible conditions for a car battery.

Normally, a healthy car battery will show a higher CCA rating than what's on its label. If it's 70 F outside, a new 500 CCA-rated battery could show 600 CCA or more. If it's 0 F outside, testing that new battery should show a rating matches the label.

The more worn out a battery gets, the less extreme the cold has to be to kill it. Instead of dying at freezing temperatures, a worn-out battery could die at 40 F. A severely weakened battery could die at 60 F.

To sum up:

• The CCA on the battery's label shows how well your battery should work.
• The CCA from a battery test shows how well it actually does.

If the actual CCA is lower than what your engine needs, then it's about to die. It's a worn out, dying battery about to fail.

It's time for a new battery because no matter how well you charge it, if the actual CCA is still lower than what your engine needs, it's not going to start your car one day soon.

How do you test a car battery?

A battery test is not about checking corrosion or making sure the plastic case isn't cracked. It's also not about checking voltage or how charged the battery is.

The most accurate battery tests come from a conductance battery tester.

These testers send a signal through the car battery and measure how well it conducts electricity. If it meets any resistance, that's a sign of weakness hidden inside. A conductance tester also registers voltage and state of charge.

Another common battery test is with a load tester. These tests check how well the battery performs while drawing power from it. In short, a load test compares the battery's stats from before and after pulling some amperage. One drawback: If you do a few load tests on the same battery, you can drain it flat.

You probably don't have access to either a conductance tester or a load tester.

Unless you work in a repair shop, the best battery testing you can do is checking the voltage and state of charge. You can do both with an inexpensive multimeter or hydrometer.

Test a car battery with a multimeter.

A multimeter reads how much voltage your car battery has at the moment. It's not a measure of how long your battery will last. That said, checking its voltage will show you whether your car battery needs to be recharged.

1. Turn on the headlights for a couple of minutes if you just drove your car. This gets rid of surface charge, an artificially high voltage reading from the battery being recently charged, either by an alternator or a battery charger. (Turn the headlights off after two minutes.)
2. Set your multimeter to DC voltage above 15 volts.
3. Connect the multimeter to the battery terminals.
4. Check the voltage reading to this chart to see how charged it is:

Test a car battery with a hydrometer.

This will take you back to your high school science labs. To use a hydrometer, you'll need to open the top vent caps of the battery. (A flat-head screwdriver can pop them open.) However, if you have an AGM or a sealed, maintenance-free battery, you can't use a hydrometer. (Also, don't try to open AGM batteries.)

Like a multimeter, a hydrometer can only tell you the approximate state of charge, not how healthy your battery is.

1. Use the headlight trick above to get rid of surface charge.
2. Open the vent caps and insert the hydrometer's nozzle into the battery's electrolyte.
3. Squeeze the bulb to draw a sample into the hydrometer's scale.
4. Compare the hydrometer's specific gravity readings to this chart:

6 Extremely Subtle Signs You Might Have a Bad Battery

The most obvious sign of a bad battery is that your car won't start.

Batteries usually die without warning. Today's engines are sophisticated. They're smaller, lighter and easier to start than your grandparents' cars. Today's cars can compensate for a weak battery (but not fully recharge it.) which helps you get the most out of every battery. It also means that when your battery does fail, it didn't have a chance to give you any warning.

It just works one day and dies the next.

Another obvious sign: That battery warning light on your dashboard saying the voltage is low. That light could go away or never appear at all if you drive for more than 30 minutes every day. Voltage isn't a sign of how much longer you can trust your battery.

However, if you're observant, you can pick up some subtle signals from your car that the battery is about to die.

1. The headlights flicker a bit when you're idling. As soon as you start the engine, turn on the headlights. What are they doing? If they're not a solid steady beam, you might have a battery problem or a deeper electrical problem.
2. The car forgot your Bluetooth settings. Watch for electronic glitches like a window that hesitates before rolling down, a flickering cabin light or even the computer forgetting your Bluetooth connection or radio station. It may seem minor. It may also be a cry for help.
3. The idling as soon as you start seems rough. Your car draws power, usually from the alternator, to measure oxygen and fuel. In the first few seconds, though, your alternator's barely gotten going. So the battery steps in, unless it's too weak. And in that case, you'll feel slight hitches or jolts that go away as soon as you get rolling. If you do, go get a battery test.
4. The engine start didn't sound quite right. You know how your car sounds. You know what to expect. “Trust your hearing, Luke,” as a wise mentor almost said. If you can hear a slight pause that only lasts a quarter of a second, go get a battery test. You might simply need to recharge it.
5. The engine start is slow on cold mornings or nights. Weak batteries don't do cold. A healthy, strong engine can usually compensate for a dying battery. If the engine doesn't start instantly, then you might be seeing its first sign of weakness.
6. Everything seems better after you've been driving. If your car is a little glitchy or rough, but everything is always better after you've been driving for 20-30 minutes, that means your car's electronics are mostly running on the alternator — and the battery's not helping much. When the alternator's not spinning fast (like when the engine's rpm are low), the battery's supposed to step in and support the other electronics. If everything's better after the alternator's been running hard, that means your car battery isn't doing its job.

If you notice these little signs, it's a safe guess that your battery's dying. You can try a few ways to extend your battery's life.

Why guess at all? You can visit your nearest repair shop or service lane and ask for a battery test.

Not sure where to look? We can recommend a few good shops to ask for your battery test. Of course, you can also get a free battery test at an Interstate All Battery Center near you.