Car batteries provide the power needed to start your engine and run the electronics.

Read on and find out what a car battery does, how you charge one and the signs that your battery is going. 


What Does a Car Battery Do?

A car battery provides the power needed to start your vehicle.

Car batteries use a wet cell design.

Most petrol and diesel car batteries have 6 cells, which all produce roughly 2 volts of energy – 12 volts in total.

There are two types of plates – positive and negative – in the battery, which are made of lead and lead dioxide.


How Does It Work?

The cells in car batteries store and transform chemical energy into electrical energy.

The plates in the cells are submerged in sulphuric acid, which triggers a chemical reaction between them, thereby producing electrons.

When you switch the ignition on, this signals to the battery to start the chemical reaction.

The electricity produced flows from the battery to start the engine and to power the spark plugs.

The alternator can then take over, supplying most of the electrical current to the electrical systems and keeping the battery charged.

Your car battery is rechargeable and can charge as the engine runs.


How Do You Charge a Car Battery?

To charge a car battery, you will need a toolbox.

Park your car in a ventilated area – you should not try to do this in your garage.

With the engine turned off, you will need either a trickle charger or a smart charger – preferably a smart charger.

The battery has two terminals – one positive (+) and one negative (-).

If you don’t see any corrosion or damage, you can remove the covers or terminal protectors from the battery.

If you do spot some damage, you should have your vehicle looked at by a mechanic instead.

Connect the positive (red) charger clamp to the positive terminal of the battery, and the negative (black) charger clamp to the negative terminal of the battery.

Make sure that these are correctly attached before you continue.

Set the appropriate charging mode and voltage.

You can then plug the charger into a power outlet before turning the charger on.

Give it time to charge the battery – this can take several hours if the battery is completely depleted.

When the battery is fully charged, you can turn off the charger and unplug it from the power outlet.

You will then need to disconnect the charger clamps from the battery terminals, starting with the negative clamp and then the positive clamp.

Replace the protectors and test the engine.

Alternatively, you can ask a mechanic to do this for you.


How Long Does It Take to Charge a Car Battery?

It can take between 10 and 24 hours to fully charge a car battery.

This can vary depending on the age and model of the car battery.

Do not disconnect the battery before it has fully charged, as this could cause the battery to run flat.


How Do You Test a Car Battery?

Check the Voltage

To test your car battery’s health at home, you can use a voltmeter.

Turn off any car accessories that could be draining the battery, and then remove the battery terminal covers.

Connect the positive side of the voltmeter to the positive (red) terminal first, then connect the negative side to the negative (black) terminal.

You can then check your reading.

A healthy battery should read between 12.4V and 12.7V.

If your reading is closer to 12.3V, then your battery may be approximately 75% charged.


Headlight Test

You can also carry out a headlight test to check your battery health.

Turn the headlights on.

If they look darker than they should be, then your battery could be low.

If your headlights get brighter when you put your foot on the accelerator slightly, then you could have a faulty alternator.


Battery Health Indicator

Many modern cars have in-built battery health indicators, which have an arrow that will turn red or green depending on the condition of the battery.

Red indicates low voltage, whilst green indicates it is in good condition.

Nothing will show when the battery is flat.


Visual Inspection

You can carry out a visual inspection of your battery whilst it is in the engine bay using a torch.

If you can smell sulphur or see bubbling or dried liquid, then you need to have your car looked at by a professional mechanic.

If you don’t see any cracks or liquid, then you can remove the battery from the engine bay to take a closer look.

For your safety, remember to remove the negative (black) terminal first, and the positive (red) terminal last.

Any bulges can indicate that the battery is overcharging, and any cracks mean that your battery needs to be replaced immediately.


Signs Your Battery Is Going

Your battery may need to be checked by a technician if you notice any of the following warning signs:

  • Engine cranks slowly
  • Clicking sound
  • Dimming lights
  • Frequent jump starts
  • Electrical systems losing power
  • Sulphuric smell

Any of these signs could indicate that your battery is failing.


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Why Is My Battery Flat?

A flat car battery can be caused by the lights being left on, a faulty charging system, corroded or loose connections or old age.


red battery warning light on black background

If you see a red battery warning light, chances are your car won't start!


How Do You Start a Car with a Dead Battery?

To start a car with a dead battery, you can jump start it.

You will need a live battery – usually another car – and jump leads.

Attach the crocodile clips to the correct end of each battery.

Then wait 5 minutes and start the car with the live battery – this should start to recharge the dead battery.


How Do You Disconnect a Car Battery?

Whilst disconnecting a car battery can be a simple process, you should always consult a mechanic if you aren’t confident doing this yourself.

To disconnect a car battery, you will first need to open the bonnet and locate the battery.

Using a wrench or pliers, loosen and remove the nut that is holding the negative terminal in place.

You should always disconnect the negative terminal first.

Once the nut has been removed, you can gently lift the terminal off of the battery post.

Carefully tuck the disconnected terminal away from the battery to prevent accidental contact.

Then do the same for the positive terminal.

To prevent accidental contact, you can insulate the disconnected terminals using electrical tape or terminal protectors.


How Do You Reconnect a Car Battery?

Please note that certain vehicles have electronic systems that will take some time to reset after the battery is disconnected.

With the car parked in a safe place, you should wait a few minutes before attempting to reconnect the battery.

You can then plug the battery back into the vehicle, always starting with the positive terminal.

Place it over the positive battery post and secure it with the nut, then do the same with the negative terminal.

Once the battery is reconnected, ensure all the connections are secure.

You can then try and start the car and check if the electrical systems are working well.

If not, book an appointment at a local garage so that they can do this for you.


What Car Battery Do I Need?

To find out which car battery you need, you can look in your owner’s manual.


How Much Does a Car Battery Cost?

In the UK, a car battery replacement can cost anywhere from £100 to £300.


How Do You Maintain Your Battery?

Corrosion can build up on the battery terminals over time, which can hinder the flow of electricity.

To maintain your battery, you should make sure that the terminal connections are tightly secured, clean and free of dirt and debris.

Going for regular half hour drives can ensure your battery has enough time to recharge.

If you don’t use your car that often, you may want to buy an intelligent charger which you can leave connected for a long time without damaging the battery.

When you start or park your car, make sure that you switch off all the electricals.

You can dip the clutch when you start the car, too, to reduce the load on the battery and starter motor.

You should also test the battery regularly by performing battery voltage and conductance tests.


How Long Do Car Batteries Last?

Car batteries can last 5 years or longer if they are well-maintained.

If you tend to make lots of shorter journeys, then the battery may not be getting the chance to fully recharge and will degrade faster.


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