Even though most cars run on petrol or diesel, none of them can start without electricity. Your battery contains enough current to power a starter motor, that in turn fires your engine into life. We’re going to look at how to test, charge and replace your car battery. 

What Does the Battery Do?

There are batteries all around us, in our TV remotes, laptops and mobile phones. They all allow us to hold on to electrical power while we’re away from a plug socket. And the same is true for our cars. 

Even though all modern cars contain alternators, clever bits of kit that turn your engine’s rotational energy into electricity, you must start the engine first. In order to do this, you need a lot of electrical energy stored and ready to go. 

The battery in your car is much bigger and heavier than the one in your laptop, because it needs to generate a much bigger current to turn over your engine and make it start. Most cars use a lead acid battery made of six cells connected in series that can be recharged thousands of times. 

The battery is charged when your engine runs, so you can switch it off and still run your fans, radio and lights. It’s becoming more important to have a strong battery in modern cars that rely heavily on electronics. 

car battery

How does the Battery Work

The first thing you’ll notice when you become familiar with your car battery is the weight. Weighing anything from ten kilograms upwards, a lot of car battery use lead acid technology to hold an electrical charge.

A lead acid battery, or otherwise known as Starting, Lighting, Ignition (SLI) battery is great for producing that initial punch of electricity needed to crank your engine over. Lead plates are dipped in sulphuric acid that creates a chemical reaction. It’s this reaction that creates the electricity. 

When you turn your car key, the battery converts chemical energy into an electrical charge. This charge allows your starter motor to engage. The starter motor turns over your engine and when up to speed, the alternator kicks in and runs the electrical components in your car. 

As the alternator runs, charge is sent back to your battery to top it up, so you can start your car up again next time. 

When to Check your Battery

Checking your car battery should be done when you inspect your oil and fluids. If you’ve got the bonnet open already, you might as well take a look for any wear or damage.

Manufacturers usually recommend having your battery professionally tested at least twice a year, but you can do almost as much as a garage right on your driveway. Just remember that regular inspections and maintenance can help you avoid costly repairs or replacements. 

How to Prepare for Testing Your Battery Safely

Working on any electrical system can be dangerous. Taking the right safety precautions is essential, so follow these guidelines before you lift the bonnet:

Car batteries must hold enough power to start your car’s engine, which is more than enough to give you a terrible electric shock. Avoid touching both terminals with anything metallic like a screwdriver as you can “short” your battery. 

When performing any maintenance on your car, tie back any loose hair and remove metallic jewellery. Hair and loose clothing can get caught in moving parts, and jewellery can cause electricity to arc into your body.

Lead-acid batteries contain strong chemicals including sulphuric acid, so properly rated gloves and safety glasses are vital, especially if your battery is leaking or swollen.

We already mentioned how heavy car batteries are, so if you’re planning on removing one, wear safety shoes in case it falls. 

When you need to remove your battery, always disconnect the negative (-) battery terminal first to break the circuit.

How to Test your Battery

Visually inspecting your battery requires no equipment at all and takes just seconds to do. Knowing what to look for on your car’s battery is the first task: 

Each battery has a pair of terminals, one positive (+) and one negative (-). There will be thick cables attached to them, red for positive and black for negative. Don’t worry if you can’t see the negative terminal as they’re often hidden away under a cover. In fact, the whole battery might be kept under a cover to stop it getting dirty and dusty. 

What you’re looking for in a visual inspection are dirty, frayed or damaged cables, corroded or rusty terminals, dents or damage to the battery case, or in serious cases bulging or leaking fluids

Electrically testing your battery needs a tool known as a digital multimeter to perform properly. Widely available in car repair shops and online, they’re an essential electric diagnostic tool that measures, among other things, Volts, Amps and Ohms. 

You don’t need to be an electrician to use one though. To test your battery, follow these steps:

  1. Turn on your headlights to get rid of any “surface charge” for about two minutes, then turn them off
  2. Open the bonnet of your car and locate your battery. You might need the manual for this in some modern vehicles as it can be hidden from view
  3. Locate the battery terminals. The positive (+) and negative (-) terminals should be clearly marked
  4. Check your battery cables are properly attached and not damaged or corroded
  5. Using your multimeter, connect the red probe to the positive terminal and the black probe to the negative terminal
  6. The readout on your multimeter should be around 12.6 Volts. Anything lower than this and you have battery problems
  7. Leaving the probes in place, start your engine and watch the readout. The Volts will drop as the alternator kicks in, but if it drops below 10 Volts, it’s a sure sign of a bad battery. Turn off your engine

checking battery using multimeter

How to Charge Your Car Battery

If you can start your car, you should be able to charge your battery. The alternator works to turn the rotational energy of your engine into electrical charge, charging your battery as you drive. 

If your battery goes flat, you can use a booster pack to recharge it on the go. Units are available for as little as £15 if you shop around and can save you from having to beg a jump start from your neighbour.

Cars that aren’t driven very often are the worst culprits for flat batteries, so if you have a second car that sits for long periods, it’s worth investing in a trickle charger or even a portable solar battery charger. They both work to keep your car battery maintained over long periods.

charging car battery

How to Replace Your Car Battery

Car batteries are good for between three and five years, depending on their use. When you need to replace yours, follow these steps:

Note. Makes and models of cars have different processes, so check your service manual for exact steps.

What you’ll need:

  • A spanner and socket set to fit your car
  • A replacement battery that is compatible with your car 

The process:

  1. Remove the plastic cover from your battery 
  2. Look for any damage to the old battery or terminals 
  3. Disconnect the battery strap with the spanner
  4. Disconnect the negative (-) terminal clamps by undoing the bolts holding it down
  5. Remove the clamps from the battery
  6. Repeat steps 4-5 for the positive (+) terminal
  7. You can now lift away the old battery
  8. Place the new battery in the battery holder, making sure it’s the correct way round
  9. Follow the terminal disconnection steps in reverse, starting with the positive (+) terminal
  10. Replace the battery strap and re-do the bolts 
  11. Inspect the terminal connections and make sure they are tight and secure
  12. Replace the battery cover and try your engine

replacing car battery


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