There are so many electrical components in the modern motorcar that most of us scarcely notice them when we’re driving around. If you’ve got a luxury item or two, like heated seats or a heated windscreen, you might take note – but some of the more impressive items are now so familiarly that they’ve become almost mundane. You wouldn’t get very far, for example, without windscreen wipers or headlights. It might be difficult to imagine life without even non-essential items like your car’s radio. And essential items like the car’s starter motor are even more underappreciated. Without them, we’d have to start our cars by crank – and risk broken wrists and worse, like the early adopters more than a century ago.
Of course, we don’t need to charge our cars (that is, unless we’re driving electric cars). So where does the energy that powers all this stuff come from? The answer, naturally, is the petrol tank of your car. Let’s take a look at how your car takes the energy that comes from burning petrol and turns it into the electricity that turns your headlights on and off.
The home of all of the electricity in your car is the battery. Without it, life would be almost impossible. For one thing, your headlights would immediately switch off whenever you stopped the car, which would make junctions of darkened country lanes nightmarish.
Each electrical component of your car is wired to the positive terminal of your cars battery. The negative terminal is attached to the other side of each component via the body of your car, in what’s called an ‘earth return’ circuit, which will earth each part of your car.
In most modern cars, the battery puts out twelve volts of direct current – which is around the same as many household appliances, like computers. Voltage is the terms used to describe the force available to drive the current around the circuit – it’s distinct from current itself, which is measured in amperes, and is drawn by devices rather than being pushed by the battery.
Of course, the voltage of a battery wont’ tell you much about how long it’ll last for. This is instead measured in amp/hours. A twenty-four amp/hour battery, for example, will draw one amp for an entire day. If the car is drawing twice that, it’ll last for half as long, and so often.
Life would be tremendously annoying if the battery didn’t recharge itself as you were driving along. This is the job of the car’s generator. Generators take the motion of the car and translate it into electricity which can be used to power all of the car’s devices.
Confusingly, there are two different sorts of generator. In older cars, a device called a dynamo might be used; however, in newer ones you’ll find another sort of device called an alternator.
Inside an alternator are wires which coil around an electromagnet. The two don’t touch each other – the electromagnet is instead fixed to the motion of the car’s engine. As the engine spins, so too does the magnet, and by the process of electromagnetic induction, a current appears in the wires. This current is then used to charge the battery.
There’s a complicating factor here, however. The energy outputted by the alternator comes in an alternating current, which switches rapidly back and forth between positive and negative. In order to translate this into direct current which can be used to charge the battery, and special device known as a rectifier is used. This consists of a number of capacitors and transistors, which work to remove the bottom part of the signal and smooth the signal to the point that it can be used to charge the battery.
While dynamos provide direct current, they’re far heavier and more cumbersome. Since a rectifier circuit weighs a negligible amount, it’s perhaps unsurprising that the alternator is now favoured.
If the generator in your car begins to fail, then you’ll slowly begin to run out of charge. If you notice that the battery light has come on, and you haven’t accidentally left the headlights on overnight, then you’ll need to seek out a replacement alternator. Fortunately, you can buy starter motors and alternators online with relative ease – just be sure that you match yours with the vehicle you’re going to be buying. Whether you’re looking for a Mercedes, Audi, Ford or BMW alternator, there are specialist online retailers willing to provide.