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Glossary of terms you will come across during your Phase 2 training.

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In plant vehicle charging systems, an increase in the speed of the alternator gives a corresponding rise in the voltage of the alternator output. We need the voltage of the alternator to be higher than that of the battery, but we cannot allow this voltage to rise too high.  If it were to rise too high, it would damage electronic components and other electrcial devices such as bulbs and motors. To avoid this, we use a voltage regulator, which does not allow system voltage to rise above a pre-determined figure. For a 12V system, this is normally in the region of 14.5V, but always check the manufacturer's manual for more exact information.

Check here for more on voltage regulators.


Durite voltage sensitive relay, VSRAs the name suggests, this relay is sensitive to the voltage of the electrical supply that operates it.

These types of relays are used in split charging systems.

Taking the example of the Durite 0-727-33 Split Charge Relay, this is a voltage sensitive relay.

The relay simply detects the rise in voltage from the vehicles charging system when the engine is started. When this voltage rises above a pre-determined value, the relay closes and allows the alternator to charge the secondary (auxiliary or leisure) battery.

When the voltage of the alternator drops (when the engine is switched off), the relay opens, thus ensuring that secondary loads cannot discharge the engine battery.

These are relays capable of handling high current (at least 30 amps).

The switching coil requires a certain minimum voltage to work. This voltage is usually in excess of 13.5V. This ensures that the relay will only close and connect the auxiliary battery when the alternator is running and capable of supplying electricity.

When system voltage drops below a pre-determined value (in the region of 12.6V), the relay contacts open. This ensures that the auxiliary battery is taken out of the circuit when the alternator is not supplying electricity.

Shown here on the right is a voltage sensitive relay made by Durite, and shown below is a circuit diagram for a typical split charging system.

Typical circuit diagram for a split charging system

Read the page on the Durite website

Read more about split charging systems on CP Fitters here


Voltage spikes are sudden, sharp, large rises in circuit voltage.

Spikes can be caused by a number of different factors, but you should remember that voltage spikes can do serious, permanent damage to electronic components.

Caution: Voltage Spiking Produced by Relays and Solenoids

It should be noted that, when we de-energise a relay or solenoid by switching off the supply to the switching coil, the associated magnetic field around that coil will suddenly collapse.

We know that a magnetic field moving across a conductor will produce electricity. In a relay or solenoid, the magnetic field is quite strong (strong enough to close the contacts or solenoid), and there are a lot of turns of wire in the switching coil.

This means that, when we suddenly switch off a relay or solenoid, the magnetic field moving rapidly across the large number of turns in the switching coil, will produce a significant voltage spike that can harm any electronic devices that may be in the circuit.

You can read more about relays and solenoids here on CP Fitters


A voltmeter is an electrical instrument used to measure the force that pushes electrons around a circuit. They come as stand-alone units, but they are usually incorporated into mulitmeters along with ammeters and ohmmeters.

Voltmeters have a very high electrical resistance, and once they are set to the correct range, they can be safely directly connected into a circuit. For example, the internal resistance of a voltmeter is so high that it can be connected directly across a battery, from positive to negative. In this case, the voltmeter will simply read and display the voltage across the battery.


This is a Valve Regulated Lead-Acid Battery, sometimes also referred to as sealed lead-acid (SLA), gel battery, or maintenance free battery.

Although the term 'maintenance free' is a misnomer, these batteries do not require constant maintenance (topping up with de-ionised water). They still require the other normal inspection and maintenance routines that all vehicle and machine batteries require.

All lead-acid batteries, when they are being re-charged, will produce gasses. That is why we limit alternator charging voltages to approximately 14.5V. If we go over that voltage, or attempt to charge a battery in excess of 10% of its Ah capacity, excessive gassing will occur.

Up to a point, in a VRLA battery, these gasses are contained, and eventually can recombine with the electrolyte. This leads to a build-up of pressure in the battery. If this pressure exceeds a safe limit, a valve will open, allowing the excess gasses to escape - hence the term 'valve regulated' lead-acid battery.