How Does A Car’s Electrical System Work?

Wondering how automotive electrical systems work? This is the ultimate guide on a car's electrical system that gives you the best overview.

The circuitry involved in a modern car’s electrical system is mind-boggling. Even the experts who design it sometimes struggle to understand what goes wrong.

A battery and alternator power a car’s automotive circuit. They send an electrical current through ever-more-complex routes to run appliances. These include the headlights, radio, spark plugs, sensors, and ECU (among many more).

The rise of EVs has also urged manufacturers to invest in electrical development. The benefits are there for all to see.

So, how does a car’s electrical system work? Answering that in any great detail is a futile task, but this article should give you a good overview of the ins and outs.

Table of ContentsShow

Some Electrical System Terms You Should Know

Electric System Terms

When discussing automotive circuitry with anyone, specific terms will always come up. Here’s what some of the most common mean.

  • Coulomb (charge) – the combined negative charge of about 6,250,000,000,000,000,000 electrons. Yes, really.
  • Amp (current) – how much charge passes a point per second. It’s the flow rate of charge.
  • Ohm (resistance) – how much resistance (a little like friction) appliances have to the current.
  • Volt (potential) – pressure is a good analogy. Imagine blowing air through a partly blocked tube. Higher volts is like applying more pressure to this tube.
  • Cell – a cell uses a chemical reaction to create an imbalanced charge, producing a voltage and powering a circuit.
  • Battery – batteries are made up of two or more cells. Car batteries have six 2.1V cells, leading to a “12V” battery.
  • Ground/Earth – circuits only work if they’re complete. The wiring must return to the battery’s other terminal. In a car, electricity flows to appliances through wires. It then returns to the terminal through the car’s chassis. This is automotive “grounding” or “earthing”.
  • Appliance – any component of an electrical system designed to be powered. Examples include light bulbs, spark plugs, and the radio.
  • Electromagnetism – when current passes through a wire, it creates a magnetic field. Likewise, generating a magnetic field around a wire induces a current.
  • Alternating Current (AC) – the electrons quickly and constantly change flow direction. You find AC in your home’s electrical system.
  • Direct Current (DC) – the electrons flow in the same direction. You find DC in battery applications, including cars.

A Quick Lesson In Physics

Okay. To be clear before starting, you won’t need to head off to college to get a physics degree before reading this. This section serves as a gentle reminder or an introduction to basic circuitry.

It’s fascinating – and slightly frightening – to note that nobody knows what electricity is. We have many theoretical models, yes, but in terms of its physical substance? Nobody – not even the top minds in the leading institutions worldwide – knows.

Ohm’s Law is the fundamental principle covering everything physicists and electrical engineers investigate. It explains the inherent relationship between Volts, Amps, and Ohms (named after him).

Ohms Law
V = voltage, I = current, R = resistance

Ohm’s Law states that current (A) requires a certain amount of potential (V) to overcome encountered resistance (Ohms).

While Mr. Ohm discovered this relationship in 1827, Mr. Coulomb noted the link between current and charge back in 1785. Hence, we measure charge in Coulombs.

Coulombs law
F = electric force, k = Coulomb constant, q = charges, r = distance of separation

Coulomb’s Law measures current (A) as the amount of charge (Q) passing a given point per second.

Car Electrical System Charging Components

Car Battery and Car Electrical System

Car electrical systems are complicated. Extremely. However, you’ll find two charging components in every modern vehicle: a battery and an alternator.

Note: before the 1960s, manufacturers installed generators, known as dynamos, rather than alternators.

Here’s a little more information about how exactly they both work.

What Is A Car Battery?

Although you’ll have heard that a car battery puts out 12 Volts, it’s not entirely true. In reality, it contains six 2.1V cells. In total, these make 12.6 Volts, so you should look for this reading when testing.

Car batteries are lead-acid types. Two lead plates are submerged in an electrolyte of acid and water. Chemical reactions that then occur generate an imbalanced charge across the two plates. When combined with the other cells, these two plates and charges translate into the positive (cathode) and negative (anode) terminals.

A circuit is generated by connecting the anode to the cathode through wiring. In a car, this wiring first passes through the rest of the vehicle’s appliances. The electrons from the anode power them.

Contrary to a common slip of the tongue, batteries don’t directly hold electricity. They store chemical energy. This energy can be converted into electricity whenever needed.

But what happens when all the acid has reacted with the plates? Eventually, lead sulfate will coat them both, reducing the acid to water. There are no longer any particles to form spare electrons in the anode.

Now how do we get charge then? That’s where the alternator comes in.

What Is A Car Alternator?

A car alternator is like plugging your phone in while using it.

Its primary function is to generate a charge to top up the battery. It also takes over the running of certain appliances, if necessary.

The alternator, wired into the car’s circuitry, reverses the chemical reaction in the battery. The ions bonded to the plates return to the lead/water solution. From here, they can be reused again and again.

The alternator generates a charging current through the principle of electromagnetism. Current is sent to a rotor. The rotor contains a coil of wires, inducing an electromagnetic field. As the rotor and its field spin within the stator, a charge is induced in the stator.

Three coils are used. This leads to a three-phase current, keeping a constant high AC output. A regulator ensures the alternator’s output is kept relatively consistent. Finally, a rectifier converts the AC into the DC that the car uses.

How Does A Car Use Electricity To Start?

Car Clicks But Won't Start

When you turn the key in the ignition (or press the button) to the START position, a switch completes the circuit to the starter motor. This switch is conveniently known as the ignition switch.

With the circuit complete, electrical current flows from the battery to the starter motor. It then uses a solenoid to jut out its pinion and mesh with the engine’s huge flywheel.

The solenoid completes the circuit to the motor itself. The battery then powers this motor with an enormous current (up to 350A!). The pinion gear then spins, torqueing the flywheel round.

This gives the engine the momentum it needs to start and run under its own steam.

When you let go of the key, the switch opens, disabling the circuit. The starter motor stops spinning, and the pinion gear disconnects. It’s now redundant until the next time you come to start your car.

Of course, the engine needs electricity to start. Since a running engine drives the alternator, the only place it can draw this from is the battery.

Once the car’s started, the alternator will restore chemical energy to the cells. In normal circumstances, this doesn’t take long. However, it might take a half-hour drive if the battery was depleted before you started the car.

It’s then fully charged, ready to start the car again next time.

How Does A Car’s Electrical System Work?

Battery Alternator and Appliances

A car’s electrical system is a three-pointed concept. See the graphic above.

When the engine’s running, all the components contribute to the circuit (including the wiring that connects them).

For example, see how the battery and alternator combine to send current to the appliances? But that the alternator’s also charging the battery?

Remember that the outputs of all these devices are constantly varying. Sometimes these variations are slight. Other times, they can be huge.

If the battery’s output momentarily drops, the alternator will compensate. Likewise, if the alternator’s charging current reduces, the battery will have to work harder to power the appliances.

There is constant variation all the time. Only top electrical engineers have a thorough grasp of every aspect. Even then, we – as the human race – don’t understand the fundamentals whatsoever.

What Appliances Are Electrically-Powered And How Much Charge Do They Use?

Every manufacturer is different. However, this section includes a list of typical electrical appliances. It also states a standard current range for each one.

A good battery should have a capacity of 48 Amp-hours. That is, without any alternator input, it can power 48A for one hour, or 24 Amps for two, etc.

  • Battery (charging current) – 45A to 200A (this must be regulated – too much, and the battery will overheat and explode)
  • Sidelights – 0.5A
  • Headlights (low-beam) – 7A to 10A
  • Headlights (full-beam) – 10A to 15A
  • Average ignition coils/spark plugs – about 5A per coil
  • Wiper motor – 2A to 4A
  • Radio and speakers – 40A

These readings are all variable depending on the manufacturer’s designed system.

4 Facts You Didn’t Know About Car Electrical Systems

Here are a few facts you didn’t know about car electrical systems to conclude this article.

1. Current Is The Dangerous One

Car Electrical System

Current is the killer. Remember the definitions? The higher the current, the more electrons flow. Voltage is the “driving force” or “pressure” (by analogy) these electrons use to pass through something.

Current is the dangerous aspect, but without a high enough voltage it can’t pass through you. Therefore, a high current/low voltage system (such as that found in a car) is unlikely to shock you. You should still always take extremely careful precautions, as things can always go wrong.

Warning signs on electricity pylons, for example, might say something like “400,000 Volts – Danger of Death”. The danger isn’t the Volts themselves, but the current this potential drives through you.

Currents as low as 0.1 Amps can kill you if maintained for more than a few seconds.

For reference, low-beam headlights might need about 6A to 10A. The starter motor might easily be a few hundred Amps.

That’s why it’s so important to only work on electrical systems if you know what you’re doing. You’re unlikely to arc the current through yourself due to the lower voltages found at most points in an automotive circuit, but it’s possible.

Stay safe. Use a mechanic. Don’t mess with wiring when the battery’s connected or even anywhere nearby.

2. How To Protect Your Car’s Electrical System

Driving car

Fuses protect your car’s electrical system by breaking if the current passing through them will be damaging to the appliance or the car as a whole. While they’re a wonderful, replaceable failsafe, it’s better to avoid the situation in the first place.

The best way to protect your car’s electrical system is never to use any appliances when the engine is off. The one exception? The starter motor (of course).

Any amount of electricity you use begins the chemical reaction in the battery. When the engine’s switched off, the alternator isn’t reversing this reaction.

Eventually, the battery will be so depleted that there won’t be enough power for the starter motor. You’ll need to jump-start the car.

If this happens regularly, the battery will start to struggle. Once this happens, there’s more strain on the alternator. Everything becomes unsettled.

Another helpful tip is to take your car for a drive at least once per week. This’ll ensure the battery’s at full capacity. It’s also good for your engine, tires, brakes, exhaust, and steering.

3. Why Do You Have To Jump-Start In A Specific Order?

businessman using jumper cables to start a car in parking lot

When you jump-start a car, you must always connect the positive terminals with the red cables first. You must attach the first cable to the working vehicle and the other end to the car with the flat battery. You must also disconnect them last, in the reverse order.

But why? What’s different about it compared to the negative terminal and black cables?

In short, the reason for this is to avoid the risk of sparking and short circuits. Both situations can be damaging to a car and dangerous for you, too. If a battery leaks flammable liquids or gases, you should never jump-start it. But, if you do and cause a spark, it could explode.

Remember that the electrons “leave from” the negative terminal and “return to” the positive. (In reality, the “hosepipe” model isn’t how circuitry works, but it’s enough for now.)

The electrical potential leaving the working battery should be 12.6V. The electrical potential at its return to the positive terminal is 0V (to all intents and purposes).

The danger is always with the last clip you connect. Before attaching it, it essentially “holds” the voltage from the other cable. They’re connected through the battery terminals on the other car.

Attaching the negative cables first induces an electrical potential of 12V in the positive one. It’ll spark when you connect it to the battery. Accidentally dropping the clip onto the metal engine or chassis will create a dangerous short circuit.

However, connecting the positive leads first means the negative cable has a potential of 0V. As a result, it’s much safer to handle. If you drop it, it’s much less likely to spark.

You should connect the negative cable to the “dead” car’s chassis rather than the negative terminal. This means the current to the rest of the vehicle will be travelling through the discharged battery. If you connect to the terminal, the current will pass through the battery cable but not the battery itself.

4. Why Is It Called A 12V System In A Car?

best car battery brands

As mentioned earlier, a car battery produces 12.6V. 12.6 rounds to 13, so why do we call it a 12V system?

It’s because older cars used three 2.1V cells. Combined, these put out 6.3V – a reading manufacturers rounded to 6V.

Car batteries now use double the amount of cells (six instead of three), so, to represent this, it’s known as a 12V system.

Even though it isn’t really.

How Does A Car’s Electrical System Work? Conclusion

Car Wont Start But The Radio And Lights Work

Each aspect of a car’s electrical system can be delved into in ever-closer detail. The intricacy involved in each component and appliance takes a team of dedicated engineers years to produce.

In many cases, their efforts have paid off. Most of us just don’t take the time to appreciate a well-designed wiper motor or headlight casing!

The most crucial element of automotive circuitry is that everything works together. As long as you do your small part to maintain this balance, your car’s system should be fine for many years.

As a takeaway point, remember this. The worst thing you can do is use the lights or radio while the engine’s off! If you want to get more comfortable, just turn the ignition. Sure, it’ll use a few cents of fuel, but you’ll face far fewer problems with the electrics.

You Might Like These Too

Types Of Car Sensors And What They Do
Types Of Car Sensors And What They Do
Benjamin Kitchen

Ben is an automotive author from England. With experience in a fast-fit garage, he's an IMI-qualified light vehicle technician. He aims to help drivers worldwide with common automotive problems. You’ll often find him working with his 1.2 Vauxhall Corsa – it may have a tiny engine, but in eight years it's never once let him down!

More About Benjamin Kitchen