Car batteries are essential parts of all cars. Every driver is aware of what a car battery is, and most know how to change one. However, not everyone is aware of how car batteries work, how long they last and the warning signs that one is breaking.

How does it power the engine and in-car electronics? How important is it? Should you know more about it? Below, we explain what a car battery is, how they work and what to do if you run into problems.

how do car batteries work?

Car batteries have one essential role: supplying a power source to start a car. Sure, it may power our car radios and other electronics too, but nothing quite as important as getting the car running.

So, how does a car battery work? Well, much like other batteries, it converts chemical energy into electrical energy. The name of this reaction is known as the lead-chemical reaction.

Each car battery consists of lead and dioxide plates that float in sulphuric acid. These plates are in a battery’s 12 cells. After switching on a car engine, a reaction occurs between the lead and dioxide, catalysed by sulphuric acid. Turning on the ignition “tells” the battery to start this reaction.

The reaction produces electrons. These electrons form electricity, which moves to the starting motor. The battery also powers the spark plugs.

This process provides the engine with ample energy to start the combustion process. However, the car battery isn’t responsible for maintaining this process. Instead, a car alternator maintains the process. The role of the alternator is why jump-starting a dead battery works; the battery only begins the process.

how long do car batteries last?

This is a difficult question to answer. Their lifespan varies depending on environmental factors such as climate, mileage, driving style and electronics use.

Broadly, batteries last between three and five years. However, if you frequently take short trips, drive fast or have a host of electronics on during your commutes, then that could be significantly shorter.

To better maintain your battery, try to avoid having lots of electronic devices on at one time. Do not think about changing your commuting or driving habits for the sake of a battery, but maybe put the phone charger away when the sat-nav is on.

If you’re asking, “how long does a car battery last without driving?” then the answer is significantly shorter than 3-5 years. When driving, the alternator charges the battery. Without this charge, a car battery lacks the energy to start the ignition process.

In a non-driven car, a perfect car battery can only last 2-3 months.

how to check a car battery

Many drivers have been caught out by a flat battery. However, you can regularly check your car battery to ensure this doesn’t happen to you.

Before we explain how to check a car battery, you need to know when a car battery needs to be replaced.

A warning sign that your battery is on the way out is a decrease in performance. Of course, if you hear the “clicking” sound coming from a battery, then that means it’s already dead.

Unfortunately, without equipment, there isn’t much you can do to test a car battery’s health. The best a driver can do is ensure it is connected properly.

To check a car battery, you need a car battery tester (voltmeter). These are readily available and you can find one for roughly £20. These test a car battery’s health by checking its voltage.

Instructions will differ slightly on a product-by-product basis, but for most car battery testers, you will need to:

  1. Turn off the engine.
  2. Attach the red lead to the positive terminal.
  3. Attach the black lead to the negative terminal.
  4. Wait for the voltage to be recorded.

Now, the voltage gives an excellent indication of a car battery’s health. You will want to see a voltage higher than 12.4v. Ideally, it should be 12.7v.

If it’s under 12.4v, give your car a short drive and check it again with the engine off. Test it again after it has been charged by the alternator.

If the reading is still low, then you will need to charge or replace your car battery.

How to charge or replace a car battery

You can buy a car battery charger from most automotive stockists and stores. We would recommend researching which charger is best for your car model, either through the manufacturer’s website, instructions or via an independent garage.

Before charging, always read the instruction manual of both your car battery and the car battery charger. We would also recommend buying an automatic car battery charger, as this switches off once the optimum voltage has been achieved.

To charge a car battery, simply:

  1. Ensure the engine is off.
  2. Affix the charger ports to positive and negative terminals. Only attach them to the terminals.
  3. Ensure the charger cables are untangled. Coiled-up cables could cause overheating issues.
  4. Turn the car battery charger on. Do not turn the charger on before the cables are properly affixed.
  5. If you have an automatic charger, it will switch off when the car battery reaches optimum charge. If not automatic, make sure to remember to switch it off yourself, before disconnecting.
  6. Test the battery again to ensure it has been fully charged.

To replace a car battery, follow these steps:

  1. Ensure the engine is off.
  2. Disconnect internal cables from the positive and negative terminals. You may need a socket wrench to do this.
  3. Locate the mounting screws for the battery itself – these are usually found on the bottom/sides of the unit.
  4. Lift the battery upwards in one swift motion, being careful to not scrape it on your car’s engine bay.
  5. Unpack your replacement battery and take note of its new mounting positions.
  6. Re-seat the new unit in place of the old one, and secure in place.
  7. Repeat steps 2 & 3 in reverse.

While changing a car battery can be simple for some, if you’re unsure, get in touch with a mechanic.

common problems with car batteries

Unfortunately, like other aspects of cars, car batteries do suffer from problems from time to time. Here are some of the most common to help you quickly diagnose issues:

slow starting

When you first start your car, the battery is used in conjunction with the starter motor to help engage your car’s ignition and spring it to life. Sometimes, this process may feel a bit slower than normal for no apparent reason. While starter motors can go bad, a more common fault in this process would be linked to your battery.

If your car is taking a few extra seconds to kick over in the mornings, this could be a sign that your car’s battery is going bad. To fix it, try either jumpstarting your car and seeing if there’s any change in speed, fully charging the battery for four or more hours, or even replacing the battery outright.


Corrosion occurs on metal surfaces after long-term exposure to liquids. Car batteries can get exposed to all sorts of liquids while in use, including those contained within the batteries themselves. While running, the sulfuric acid within batteries release hydrogen gas. This gas reacts with the moisture and salt in the air, causing corrosion on the terminals and on the protective casing.

Corrosion can cause your battery to stop working entirely, reduce its overall charge level, and cause issues with the ignition. You can spot corrosion via a blue-looking film on the battery casing or contacts. Fixing it can be as simple as cleaning the affected area while checking for any leaks, or just replacing the unit.

low fluid levels

That’s right, even your car battery has fluid inside. Most car batteries have this fluid added during the point of manufacture, never to be topped up again. If your car’s battery fluid has run low, it’ll likely fail to hold a charge long enough for your car to be usable, meaning it will need to be replaced.

harsh weather

The cells within batteries only really perform when they are granted a stable climate. Extreme heat transitioning to extreme cold is enough to toy with the chemical composition of the electrons contained with batteries, forcing them to break down at a faster rate.

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