Batteries & Fuses
Think of a battery like being a water pump. Instead of pumping water, the battery pumps electrons, Figure 1.
We rate water pumps in Volume = Gallons/Minute and
Head (or Pressure) = how high a column of water will the pump lift.
Likewise we rate batteries in Amps (electrons/sec),
Volts (Pressure) and
Amp/Hr (Number of Amps that can be delivered in 1 Hour).
1 Amp = 6,241,000,000,000,000,000 electrons/sec...it's a boat-load of electrons!
A battery is really made of 6 cells. Each cell is made up of lead plates, lead oxide and sulfuric acid. The cells are hooked together in series and the resultant voltage is the sum of the individual cells. (Much the way D cells are stacked together in a flash light.) Figure 1B shows the concept.
The total battery voltage will vary depending on the chemicals used in the construction, temperature, and degree of charge. Even though the battery voltage is not really 12V we call this a 12V battery.
Lighting a Light Bulb
When we attach a light bulb across the terminals of our battery, electrons will flow from the negative terminal to the positive terminal through the light bulb, Figure 2. The filament of the light bulb resists the flow of electrons. This resistance creates light and heat. The rating of the light bulb will determine how much current (Amps) will be draw at a given voltage (12V).
Let's give our light bulb a rating of 1 Amp @ 12V.
As we increase the number of light bulbs connected to the battery the current coming out of the battery increases. You can see from Figure 3 that as the number of light bulbs increase the number of Amps drawn from the battery increases.
The type of hookup you see in Figure 3 is called a parallel connection. In parallel connections the voltage across each light bulb will remain 12V but the current draw from the battery will be the sum of all the light bulb currents (and electronic devices.)
So how does this affect our boat electrical system? To understand this we need to add another concept, the Fuse.
When working around your battery you should wear safety glasses. The current draw out of a storage battery can be enormous. If you slip you can vaporize a probe or melt a screwdriver.
A Fuse is a simple device that breaks if too many Amps pass through it. There are many styles of fuses but we will use the glass tube fuse since that is what you will most likely encounter on older sailboats. Figure 4.
If our fuse is rated at 2 Amps then it will melt if more than 2 Amps flows through it thus breaking the circuit. Take a look at Figure 5.
Let's add a Fuse to our example circuit. See Figure 6.
At this point everything should work fine but what happens if we add another Cabin Light onto the circuit? See Figure 7.
As you can guess, this is a case where it would be okay to increase the value of the fuse. Often people will add additional lights and then begin blowing fuses. Be careful if you are blowing fuses and you have not added any additional devices to the circuit this would be an indication that something has gone wrong with one of the devices in the circuit or with the wiring itself. You need to find the cause of the blown fuse and repair it.
When you begin to look at the course on Electrical Systems Trouble Shooting there is a potential pitfall that you may run into. Usually when a Fuse Blows in a circuit it signals DANGER. Something has malfunctioned and the Circuit is drawing too much current. However, in our Cabin Light Example. The problem was created when we added the additional Cabin Light. In this particular case it is okay to increase the value of the Cabin Light Fuse.
In actual practice the Fuse in use is usually several amps above the expected current draw. So you might get away with adding 1 or 2 Cabin Lights without blowing the Fuse. But, if you were to add 3 or more Cabin Lights you will most likely blow the Fuse.
We have some LED Cabin Lights aboard the Christopher-Jin. They are very "green" and easy on the battery. Typically LED lights will last a very long time and draw only 1/5th to 1/20th of the power of an incandescent light bulb, which is good for the running potential of your battery.
But there are trade-offs. LED's are usually not as bright and they are lacking in the magenta portion of the light spectrum. Which translates to, they tend to cast an un-earthly glow. The red LED's do work very well for night-vision Cabin Lighting. So, most of our Cabin Lights are incandescent giving the cabin a warm cozy feel.
Compact Florescent Lights
(At home, we are watt-misers. 95% of our light fixtures have compact florescent light bulbs in them. These days, they are color balanced to match incandescent lights and they are instant on. I'm not sure how well they work in low voltage systems aboard ship.)
You should now understand the basics of Batteries and Fuses. It's time for us to move on to trouble shooting the electrical system.