12v Guide

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

basic circuitThis 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.


basic circuit - now with fuseThis 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.


more advanced 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).

circuit showing engine 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

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Note to anyone using a Laptop aboard

You probably all know this but it is much more efficient to step your voltage from 12V to 19V (or whatever your laptop uses.) If you step it up to 240V using an inverter and then back down to 19V using the transformer in the mains plug you're just dumping all the extra voltage as heat. You can get these nifty devices from Maplins/Halfords etc that are for running laptops in a car and take the voltage directly from 12V to 19V (most are adjustable in case your laptop runs at another voltage.) Happy surfing!

12v to 19v

Since my inverter went up in smoke a few weeks ago. We have realized that the only item that we require (regularly) a 240v supply are the lap tops. Fridge/freezer, TV and music systems are all 12v and phones are charged through the USB point on the TV . So the news that I can easily get 12v to 19v is a joyous revelation.

However I will not be telling my wife that soon I will not require the obligatory pub visit to charge my laptop as I still require a regular small charge myself.

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Thanks for posting this tutorial on how to read electronic diagrams and for explaining the basic diagram components.
I've been trying to understand a specific diagram I have and this is the best step-by-step I've found on the internet so far.
As I'm new to electronics and I plan to build a solar renewable energy system, I'd be very interested in reading the last part "Circuit showing solar panels or wind generator", but I feel like it's missing. Also, I couldn't find the missing tutorial part anywhere on your site.

Please, send me a link with the guide to solar panels circuit diagrams!
Cheers and thanks in advance!