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How to test a car battery

If your car battery's out of juice, you'll need to charge, jump or replace it.

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Testing and maintaining your car’s battery is easy, but you will need a couple of tools. If you need to test your battery right now and you don’t have tools available, check for stores within walking distance, ask a friend or nearby good Samaritan or call for roadside assistance.

How to test a car battery

A fully charged battery should read at least 12.7 volts. Testing a car battery is simple and inexpensive. To get started:

  1. Get a battery tester. You can pick one up for about $10 online or for a little more at a local auto shop.
  2. Pop the hood. There’s usually a lever near the driver’s side door, dash or floor. If you’re having trouble, check your manual.
  3. Find the battery. The battery is a box-shaped component that’s about a foot wide. On top, you’ll see two terminals or bolts. In some cars, it’ll be hidden under plastic trim in the engine bay, so check your manual if you’re having trouble.
  4. Connect the tester. Once you’ve located the battery, take your tester and attach the colored clips to the corresponding posts. The red connector needs to be clamped on to the terminal with a plus sign, known as the positive post. The black one should go to the negative post.
  5. Perform a battery test. With the engine switched off, a full battery should read 12.7v (short for volts) or above. A reading of 12.5v or less could mean your car will be difficult to start up. 12v means the battery is dead.

How to use a battery tester to check the alternator

With the engine running, you can also use your tester to check the condition of your alternator. When the engine is idling, the alternator should be pumping out at least 13.5v to the battery posts. Anything less and your alternator may be faulty and need replacing.

A significantly higher reading should also be investigated, though modern cars may have a different threshold. Check with your manufacturer to be sure.

How to jump-start a battery

If your battery is dead, you may be able to get your car started by jump-starting it. You’ll need jumper cables and a second, running car pulled up close to the dead car.

  1. Place one of the red, positive clips on the dead battery’s positive terminal.
  2. Place the other red, positive clip on the good battery’s positive terminal.
  3. Place one of the black, negative clips on the good battery’s negative terminal.
  4. Attach the last black, negative clip to an unpainted metal surface in the car’s hood — not to the battery.
  5. Start the working car’s engine and let it run for a few minutes.
  6. Start your car’s engine and let it run for a few minutes.
  7. Disconnect the cables in the opposite order than they were connected (black first, then red).

Jumper cable warning

Don’t let the negative and positive jumper cables touch each other when the other end is connected to a battery.

Some newer cars should not be jump-started. Check your owner’s manual or call your car manufacturer to make sure it’s safe before jumping a car.

How to charge a battery

If you don’t have a second car available or your battery is too far gone for a jump, you’ll need another device to charge your car. These are relatively inexpensive and readily available from local auto shops or online.

  1. Turn the engine off
  2. Connect your charger in the same way as the battery tester.
  3. Then, plug the charger into the wall.
  4. Wait several hours.

If the battery loses power rapidly or you still experience difficulty starting the engine after it’s charged, you need a new battery.

Is a dead battery covered by car insurance?

Yes, if you have roadside assistance included with your insurance policy, you can call for help and get a battery jumpstart or get a new battery delivered to you, usually within a few hours. It typically takes about an hour to replace a completely dead battery and even less time to jump one that just needs some juice.

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How to prevent a dead car battery

To keep your car healthy and prevent your battery from dying:

  • Check your battery on a regular basis.
  • Drive your car regularly. If it’s sitting in a garage for weeks or months, the battery will likely die.
  • If you haven’t driven your car in a while, charge your battery before starting it.
  • If the terminals on the battery have a white powdery substance on them, it needs to be cleaned. If you don’t know how to do it, take it to your local garage.
  • Turn off the radio and any lights before leaving your car.
  • Ask your mechanic to check your battery and replace it if necessary when servicing your car.

How does a car battery work?

Your car’s battery contains metal and acid, and the reaction between the two creates chemical energy. The battery converts that chemical energy into the electrical energy needed to start your engine. It’s also where the electrical power comes from to operate your radio, A/C fans and fancy accessories like parking sensors.

How does a car battery stay charged?

Your engine has a component called the alternator that does all the hard work. When the motor is on, a belt spins a pulley on the alternator, which creates an electrical charge. This power feeds back into your battery, and since the chemical reaction is reversible, the battery recharges.

What causes car batteries to go flat?

The most common causes of battery failure include:

Causes of a battery failureSolution
Leaving devices charging or electrical systems switched on.Leaving anything electrically powered on while your engine is off will cause the battery capacity to drop. This includes leaving electrical devices plugged into the USB outlet or cigarette lighter receptacle in your car. If you like to take your vehicle camping, or for extended off-road traveling, consider fitting a dual battery system.

Solution: Avoid leaving your headlights, interior lights, radio and aftermarket electrical components on when leaving the vehicle. Recharge battery.

Battery charge level is too low. The battery is probably flat. Use a tester or charger to verify the battery’s voltage. On most vehicles, you’ll have a 12v battery (ATVs and motorcycles may have a smaller power source). Verify the reading for the voltage is over 12.7 volts.

Solution: Charge, jump or replace the battery.

Hot weather.Hot weather can be just as bad for a car battery as cold weather. Batteries contain a liquid chemical solution that will evaporate in hot climates. On the side of your battery there should be a level. If the fluid is below that level, you’ll need to top it up using distilled water through one of the filler plugs.

Solution: Check battery fluid level weekly in hot climates.

Failing alternator.The alternator is driven via a belt from the engine. As it spins, it creates an electrical charge and keeps the battery at full capacity. Over time, the alternator’s internal parts wear out, making it less effective.

Solution: Use a multimeter across the battery terminals with the engine on to verify an output of 13.5V. If you get a different reading, take your vehicle to the garage to have the alternator inspected.

Corrosion buildup on terminals.The two posts of a car battery are made of a metal alloy. Over time, a white, powdery corrosion can form on the terminals. This deposit can disrupt proper electrical contact and cause electrical issues.

Solution: There are instructions in the owner’s manual on how to safely remove the battery cables connected to the positive and negative terminals. Cleaning away the corrosion may resolve electrical problems. If you’re unsure of how to do this, consult a mechanic — improper cleaning could damage the vehicle’s electronic system.

Battery case is damaged or bulging.The battery case should not be deformed or misshapen in any way. Additionally, it should not be cracked or leaking.

Solution: Replace battery.

Cables are loose.Loose wires or worn-out insulation can cause electrical issues with your car.

Solution: Have an automotive electrician examine and repair the wiring.

Old ageEventually, even if with proper care and maintenance, a battery will die. You’ll know this is the case when you rule out all other alternatives. There may also be a date listed on the battery outer casing.

Solution: Replace your battery.

Bottom line

If your car isn’t starting and you think the problem is the battery, test it to see if it’s putting out enough voltage, and then charge or jump it if it’s not.

It’s also a good idea to choose a car insurance policy that offers roadside assistance so that you’ll have help if your car ever refuses to start when you’re away from home.

Frequently asked questions about car batteries

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