(I want to thank Terry King, a member of
BoatDesign.net for help editing these pages)
I am going to try to keep
this a simple as possible, not because I think you can't understand it.
I assume that to master the art of boat building you have to be
smart. Mainly, I don't want to end up writing a book on electricity.
I want to this to be simple, but complete enough to give you a basic
understanding of how this electrical stuff works. When I was much younger
I worked on radars and computers on Coast Guard ships and when people
asked us "how does it work?" we would say (Smart alecs that we
were) magically, mystically, wonderfully, electronically. But it's
actually a lot simpler than that. However, to some it still seems like
magic.
So what is electricity? As we all learned in grade school the world
is made up of atoms. Atoms are made up of electrons, protons and neutrons.
What we are interested in, is the electrons, surrounding the nucleus of
the atom. Electrons can be dislodged from their atom and attached to
another one giving it an excess of electrons. Excess
electrons give an atom a negative charge. Atoms with not enough
electrons have a positive charge. Through a chemical reaction
or other force these electrons can be, made to flow though a
conductor from the negative to the positive and used as electricity. So electricity is really just a
stream of electrons flowing through a conductor from point A to point B
and back to where they came from. (a complete circuit) This flow is
called current.
All of us have experienced
this either by getting a static discharge off of a door knob or other
metal object, or by seeing nature's ultimate display of electricity,
lightning. This is simple exchange of electrons from one point to another.
We use this every day in our homes, our cars, our cell phones and every
other electric or electronic device we use, but we give little thought to
what is actually happening.
In grade school most of us had a science teacher show us an experiment
where a strip of lead with a wire attached and a strip of zinc with a wire
attached, were put into a glass jar filled with acid. This is what is
called a voltaic cell, or battery. The other ends of the wire were
connected to a light and lo and behold it lit up!.
Battery
History That is about as
basic a battery as there is. What is happening here is not magic, just
simple chemistry. The acid is an electrolyte. An electrolyte is a
fluid that allows electrons to flow through it from one pole to another.. Salt water is a pretty good electrolyte. Fresh water is
not but will still conduct current and is actually more dangerous if
current gets into the water.
When you put two dissimilar metals near each other in an electrolyte, one
of the metals gives up electrons to the other metal. But this only happens
if there is a complete circuit, which is the wire and the light. So the
electrons flow from one plate to the other through the wire and the light and back to the plates. This doesn't go on forever though.
One plate will gradually disappear and soon there will be no more electron flow.
Then the light goes out. Where does it
go? It gets plated onto the other plate. Remember this, because this is crucial to how
batteries work, and how galvanic corrosion works. In fact this is called
the Galvanic Process.
If we didn't have a light in the circuit, just a wire, electrons would
still flow, but it would happen so fast that our primitive battery would
be dead in no time at all, or the wire would get too hot and melt. This is
called a short circuit. The positive side is connected directly to the
negative side. The light we put in is called a load. It
provides some resistance to the flow of electrons and slows down the
process. Also the filament in the light gives off some of the energy of
the electrons in the form of light and heat. Otherwise no work would get
done by the electrons. So for electricity to do work there needs to be
a load in the circuit. This can be lights, appliances, electronic
equipment, or motors, all of which put the electrons to work. The wire
getting hot and melting is the basic principle behind a fuse. If too many
electrons are flowing through the wire, the fuse is designed to get
hot and melt at a predetermined amount of current flow. This breaks the
circuit, or what is called an open circuit, and stops the current flow,
the same way throwing a switch does.
So now we know what current is. It is electron flow. The amount of current
is measured in Amperes, or milliamperes. The symbol for Amperes is A or
milliamperes mA.
What drives this current is called voltage. Voltage is a measure of
potential energy contained in the battery and is measured in volts. The
symbol for volts is V.
The load is resistance to current flow and is measured in something called
ohms, named after the guy who discovered it. The symbol for this is either
R or the greek letter Omega. I will just use R because the Omega symbol
is too hard to find on my computer.
Types of Electricity
Yes, electricity comes in different
flavors, AC and DC. No that is not a rock group. AC stands for
alternating current, which is what we all have in our houses, and most
appliances run on AC, and is usually 110 volts, or 220 volts for some
large appliances such as clothes dryers. DC stands for Direct
Current and is usually low voltage such as 6 volts or 12 volts. Many of
the small electronic devices we commonly use, such as cell phones,
calculators, and IPods all run on DC. Your car also uses DC and most
small boats use only DC. However, as the boats get bigger they
use both AC and DC, until you get into the ship sized yachts that use only
AC. First though, I will deal with DC because it is simpler to explain and
what most small boats use.
Direct current comes primarily from batteries. You can get it by
converting AC to DC but for now I will stick with batteries. The
simplest batteries are a single cells such a the D cells used in
flashlights, or AA and AAA cells used in most small electronic devices.
A long time ago, someone discovered that any single cell battery, no
matter how big, puts out about 1.2 volts. Over the centuries that
has been upped to about 1.5 volts. The larger you make this cell,
the longer it will last, and the more current you will get out of it
(amperes) but you still won't get any more than 1.5 volts.
So how do we get typical 12
V and 6 V batteries? These are lead-acid type batteries and
have about 2.0 volts per cell, compared to the 1.5 volts of most
flashlight-type cells. If you tie a lot of these cells together in series
(I'll explain series in a minute) they add up. So three cells is six
volts, and 6 cells is 12 volts. So actually that 12 V battery in your car
or boat is really six cells (count the vent caps) wired together to make
twelve volts.
Series means battery cells that are connected together thus;
The positive pole on the first cell is wired to the negative on the
second, and the negative on the first is wired to the positive on the
second, and so on until you have the voltage you need.
Parallel means just the opposite. All the positive poles are connected to
each other and all the negative poles are connected to each other. This
still only gives you 1.5 volts but it increases the amount of current.
This is commonly done in boats and recreational vehicles to make a large
battery bank to power all the DC equipment on board. Two or more
twelve volt batteries will be connected in parallel to increase the
current, to run more equipment, or to run your equipment for a longer
time. However, if you have something that runs on 24 volts or 48
volts, you can connect 12 volt batteries in series to get the desired
volts. In series you add the voltages. In parallel you add up the
amperages.
See the Figure below, and the photo of two deep cycle batteries in
parallel. The battery on the right is the starting battery and not
connected to the two deep cycle house batteries. However, in this instance
there is a switch which allows the house batteries to be used to start the
engine.
