A car uses quite a lot of electricity to work the
and other electrical equipment.
If the power came from an ordinary
, it would soon run down. So a car has a rechargeable
and a charging system to keep it topped up.
The battery has pairs of lead
immersed in a mixture of sulphuric acid and distilled water.
Half of the plates are connected to each
. Electricity supplied to the battery causes a chemical reaction that deposits extra lead on one set of plates.
When the battery supplies electricity, exactly the opposite happens: the extra lead dissolves off the plates in a reaction that produces an electric
The battery is charged by an
on modern cars, or by a dynamo on earlier ones. Both are types of
, and are driven by a belt from the
consists of a
- a stationary set of wire
windings, inside which a rotor revolves.
The rotor is an
supplied with a small amount of electricity through
(contacts) touching two revolving metal
on its shaft.
The rotation of the electromagnet inside the stator coils generates much more electricity inside these coils.
The electricity is
- its direction of flow changes back and forth every time the rotor turns. It has to be
- turned into a one-way flow, or
A dynamo gives direct current but is less efficient, particularly at low
speeds, and weighs more than an alternator.
A warning light on the
glows when the battery is not being adequately charged, - for example, when the engine stops.
There may also be an
to show how much electricity is being generated, or a battery-condition indicator showing the battery's state of
How the alternator works
Moving a magnet past a closed loop of wire makes an electric current flow in the wire. Imagine a loop of wire with a magnet inside it.
of the magnet passes the top of the loop as the
passes the bottom of it. Both passes make current flow in one direction round the loop.
The poles move away, and current stops flowing until the south pole reaches the top and the north pole the bottom.
This makes current flow again, but in the opposite direction.
A car alternator uses an electromagnet in order to boost output of electric current.
How a dynamo works
In a dynamo the electromagnets are stationary, and are called the
coils. The current is produced in an
- another set of coils wound on to a shaft and turning inside the field coils.
The principle is the same as that of the alternator, but the current goes to a
- a metal ring split into segments that are touched by carbon brushes fitted in
guides. Two segments touch a pair of brushes and feed current to them.
As the armature turns, the current changes direction. But by then another pair of commutator segments has come under the brushes, and this pair is wired up the other way round - so the current coming out always flows in the same direction.
Regulating the current to the battery
The current from an alternator is rectified into direct current by a set of
that allow current to flow through them in one direction only.
To charge the battery the voltage supplied to it must not be too low or too high.
The alternator has a transistor-operated control device that regulates the voltage by supplying more or less current - as required - to the electromagnet.
The rectifier and regulator are usually inside the alternator housing, but on some alternators they are outside, mounted on the alternator body.
A dynamo does not need a rectifier -there is a
in a separate box, which has
One relay controls the voltage level by briefly cutting off the current in the field coils.
The second relay prevents the dynamo from overcharging and damaging the battery.
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Modern cars are designed to have a constant through-flow of fresh air that keeps the interior atmosphere pleasant even with all the windows shut. The incoming air can be heated by the engine to keep the windows clear of mist and the car interior at a chosen temperature.