A basic introduction to the 12v system

This introduction assumes you know (or can work out) what a battery, battery terminals, wires etc look like, and also that you know that electicity needs to go round a full circle back to the battery (a circuit). If you don't know this then please ask somebody to go through that kind of stuff with you.

Disclaimer (because there's always one): Don't do anything silly. This introduction is to help you get your head round things - it's always worth getting somebody to show you this stuff in real life, and always, always get somebody to double check your electrical installations.

Safety: The biggest safety concern with 12 volts is the fire risk. This usually occurs when wire which is too thin is used. The wire can overheat and cause insulation or other materials near by to carch fire. This rarely happens, and is unlikely in a boat where most of the devices used are relatively low current.

Please see the battery section for safety concerns about batteries.

Basic Circuits

This diagram shows a very basic circuit. The symbol on the left is the battery and the twirly bit on the right is a light. In real life this would be a battery with a wire going from the positive terminal to the light, and then from the light to the negative terminal.

This is the same circuit - only now we have a fuse near the positive terminal of the battery. A switch is a good idea too (and is required by the Boat Safety Scheme). We put a fuse in to protect the wires and the gadgets we have on the circuit.

Now we have two lights on two separate wires - for example if one light was on the port side of the boat and the other on the starboard.

You can see we have a fuse on each of these lines. This is smaller than the fuse near the battery and only has to protect that one part of the circuit.

Circuits showing the engine

The above circuits just show us the battery and some loads (ie the lights). But on a boat we usually have the engine attached in order to charge the batteries (and the batteries are used to start the engine too).

The engine can be seen on the grey circuit on the left. Most of us have a more complicated setup with two banks of batteries that are somehow joined (using a switch, diode or relay) when the engine is running and charging the batteries.

Circuit showing solar panels or wind generator