This is just a quick review of concepts like voltage, current, and resistance. Many people have a "sense" for what these things are and how they relate to your motorcycle's electrical system. Here is a refresher, with some useful practical applications at the end:
There are many different ways to "think" about electricity, depending on how complicated the situation is that you are trying to understand. The "water hose" analogy is useful for the simpler concepts, although it is not perfect.
Think of a battery as a pump. It pumps electricity out of its positive terminal, and sucks it in through its negative terminal.
The battery "pump" pushes electricity down the wires. If there is nothing connected to the battery, it still pushes, but the electricity doesn't go anywhere. If there are a lot of things connected to the battery, it pushes just as hard, but this time lots of electricity moves.
The battery's voltage is simply how hard it pushes the electricity.
Batteries are designed to be "pumps" that always have the same "push" (voltage) no matter how many things are connected to it.
(As a practical matter, there are limits. The battery can only push so much electricity. And, as the battery gets used up, its voltage drops, until someone comes along and recharges the battery. Lets ignore these facts for now.)
For the purpose of this discussion, our motorcycle battery always has a "push" equal to 12 volts. Whether lots of things or nothing at all is connected to it.
Sometimes voltage is referred to as a "potential", do you see why? It has the "potential" to move electricity if things are connected to it.
Current is simply "how much" electricity moves. For example, if the battery isn't connected to anything, no electricity moves, and current is zero. Remember, the battery is still pushing with 12 volts, its just that nothing's moving.
Once you connect something to the battery, like a headlight, the electricity moves. Now you have current. How much current you get depends on how "hard" it is to "push" the electricity through the headlight and how much "push" you have.
If it is easy to push electricity through the headlight, you get a lot of current. If it is hard to push electricity through the headlight, you get less current. Current is measured in Amps. One amp isn't much current, one hundred amps is a lot.
(Remember the water hose? If your pump is trying to push water through a pin-hole, not much actual water will come out. If the pump is pushing through a fire hose, you'll get lots of water. A stronger pump will push more water in both cases.)
Current is usually called "I" in equations, and is measured in amps (which is short for amperes). I don't know why they use "I," but they do.
When you hook up a headlight to your battery, you hook up two wires. All of the electricity that gets pushed out of the battery must get sucked back in, otherwise no electricity would move.
For the headlight, the battery pushes 12 volts out of the positive terminal, down a wire, through the headlight, then back down a wire to the negative terminal of the battery.
The negative terminal of the battery (where the electricity gets sucked back in), is connected to the frame of the bike. This way, the electricity can always find its way back to the battery. The word "ground" is used. Our motorcycles have "negative ground," because the negative terminal is connected to the frame of the bike.
This is an easy one. If its hard to push electricity through something (a headlight), it is said to have a lot of resistance. If it is easy to push electricity through, it has less resistance.
(In the real world, almost every thing has some resistance. Even big wires, although the resistance is small, its not zero. Only superconductors have absolutely no resistance, and there aren't any on your bike!)
Resistance is measured in Ohms. One ohm isn't a lot of resistance, 100,000 ohms is a lot of resistance.
Something that is connected to a battery is often called a "load." If the the resistance of the load is small, lots of electricity flows, and it is called a "big load." If not much electricity flows, it is a "small load."
How does resistance relate to voltage and current?
This one is really easy. Lets say you have a headlight that has 2 ohms of resistance. You hook it up to a battery that pushes with 12 volts. You already know that electricity will flow with a "current." How much current?
(Oh, no an equation!)
Its a super simple one:
V = I x R (Volts equals Amps times Resistance)
Here, 12 volts = "I" amps times 2 ohms. So, "I" is equal to 6 amps! Easy, huh?
This equation ALWAYS works. If you know any two of volts, amps, and resistance, you can always find the third. You can flip the equation around three different ways:
V = I x R I = V / R R = V / I
OK, so what's a Watt?
After all, when you buy a headlight, it doesn't say on the box how many ohms it is.
One way to think of "Watts" is a combination of push and how much moves. It is a way of measuring how much of the battery's "oomph" gets used up.
("Oomph" is a technical term meaning "power" not "voltage" which I call "push.")
Watts just makes it easy to compare how much power something uses. For example, a lightbulb in your house gets 120 volts of push, more electricity moves than if you connected that same lightbulb to your motorcycle, where it would only get 12 volts. You would be right in thinking that the bulb would be a lot dimmer on your motorcycle.
We solve all of these confusing issues (!?) by introducing the "Watt," a measure of "oomph." Watts are simply volts times amps.
W = V x A
Example: You buy a motorcycle headlight that says on the box that it is 60 watts. Since you know that the bike provides 12 volts, you expect 5 amps (60/12) to flow through the bulb. (60 = 12 x 5)
A 60 watt lamp made for your house, uses the same power (same number of watts), but since it runs on 120 volts, it is designed to flow only 0.5 amps of current, (60 = 120 x 0.5)
This should make it clear as mud. Both lamps have the same watts, or "oomph," so they make about the same amount of light. But, they work at a different current and voltage!
How voltage gets used up:
Here's where this all gets useful.
You've got a battery, making 12 volts. From the positive terminal you connect a wire to a headlight. From the headlight, you connect another wire back to the negative terminal on the battery. The electricity flows in a loop, and you have a circuit. The light goes on.
Lets make the wire to the headlight red and the wire returning to the battery black.
Remember that I said that everything has resistance? Well, there are three things here that have resistance: The red wire, the headlight, and the black wire.
How much electricity (current) flows depends on the TOTAL resistance of all three of these things. (Remember this!)
Certainly, the wires don't have much resistance, but they do have some. In this example, lets say that the red wire has 0.2 ohms of resistance, the headlamp has 2 ohms of resistance and the black wire has 0.2 ohms of resistance.
How much current flows? Well, the total resistance is 2 + 0.2 + 0.2 = 2.4 ohms. So go back to our equation (V=IxR), and you will see that 5 amps will flow when 12 volts is turned on. (Because 12 volts = 5 amps x 2.4 ohms.) Since the electricity has no where else to go, there will be 5 amps in the red wire, 5 amps in the headlight, and 5 amps in the black wire (this makes sense!)
A really useful way to think about this is to think about the voltage getting "used up" in each of the resistive elements (the red wire, the headlight, and the black wire) as the current flows through.
From the positive terminal of the battery, you start with 12 volts. 5 amps flows through the red wire, (you already figured this out!).
Since the red wire has 0.2 ohms of resistance and flows 5 amps of current, you can easily calculate that you've "used up" 1 volt. (Because V=IxR, 1 volt = 5 amps times 0.2 ohms!).
In fact, you can use your multimeter to measure this voltage. Place one probe at the positive terminal of the battery, and the other where the red wire meets the headlight. It will register 1 volt!
If you do the same for the headlight, you will calculate that it "uses up" 10 volts. (Because V=IxR, 10 volts = 5 amps times 2 ohms!)
Do this a third time for the last element, the black wire, and you find that it uses up 1 volt. (Because V=IxR, 1 volt = 5 amps times 0.2 ohms!)
Now go back and add up all the voltage that was "used up" by the circuit:
Red wire (0.2 ohms) 1 volt used Headlight (2 ohms) 10 volts used Black wire (0.2 ohms) 1 volt used ---------------------------------------- For a total of 12 volts!!!!!
This is no accident. This always works (although it can get complicated with branching circuits). In fact, it has a name: Kirchoff's voltage law (or KVL for short). Its the law, so don't break it.
Why is this useful?
Well, you might not be running around your bike with a multimeter and calculator to figure things out, but a general understanding of why this works WILL make it easier for you to figure out what's going on with your bike's electrical system.
If you have a bad connector in one of your circuits (like the headlight), this connector might be adding resistance to that particular loop.
Not only connectors, but also bad switches, fuses, and wires themselves can add resistance to the loop!
As you saw earlier, adding resistance will reduce the amount of current that flows through the loop.
The added resistance will also "use up" some of the voltage, so that the thing that you are trying to get electricity to will get less volts.
When the thing you are trying to get electricity to gets both less volts and less current, it gets less of what it needs. (Less oomph!)
Headlights will be dimmer, ignitions will not get enough "Oomph", and it will use less watts than it is designed for!
Use your multimeter to track these things down!
How to use your multimeter to track these things down:
Lets say, for example, that you want to know if your ignition is getting all the "oomph" it needs to run properly. (This is a very common problem.)
Ok, so the ignition, as you know, gets its electricity from the positive terminal of the battery. You also know, that it returns its power to the negative terminal of the battery. The question you are trying to answer is: How much voltage is getting used up on the way to and on the way back from the ignition.
With your multimeter set to measure "volts," and your bike running, you place one lead on the positive terminal of the battery, and the other on the ignition, where the power comes in. (Conveniently, this happens to be the big red wire on the ignition.)
Now, look at the multimeter, it will tell you how much voltage is getting used up on the way to the ignition.
Do the same thing for the path back to the battery. Place one probe on the black wire that comes out from the ignition, and the other on the negative terminal of the battery. This shows you how much voltage is getting used up on the way back to the battery.
If, for example, you find that you loose 2 volts on the way to the ignition, and 0.5 volts on the way back, you know you have a problem. The more serious problem is obviously on the way to the ignition. This means there is some unwelcome resistance on the way to the ignition. Suspect connector corrosion first (see the FAQ), but it could also be switches or wires.
Always remember to measure both the "to" and "from" paths. Because a loose or bad ground connection can also cause a problem.
[Tip: In this example, if you don't find much voltage getting used up (less than a volt or so altogether), and you are still having trouble with your ignition, check that the battery voltage is high enough. Then you can suspect things like ignition coils, wires, etc. In no time you'll be tracking down, and fixing resistive drops all over the place, and your bike will love you for it.]