Any one over the age of 40 probably knows, as you get older your eyes just don’t work as well as they use to. So it was no surprise to me when I stated to have trouble seeing while ridding the old FXR after dark. I just figured my night vision was going. Then one night I saw a reflection of the headlight in the garage window and realized it was looking kind of dim. Of course my first thought was that the charging system has a problem. How could this be, Cycle Electric Inc components are not supposed to do that. So I whipped out my trusty voltmeter and checked the charging voltage at the battery. It brought a smile to my face to see 14.2 volts, right where it should be. So why is the headlight so dim. I took a voltage reading at the headlight and found only 9 volts. The difference in voltage between the battery and headlight was due to voltage drops. Excessive voltage drops result in a low voltage condition that can cause problems for voltage sensitive electrical components. If there is too much voltage drop between the voltage regulator and the battery the battery will not charge properly. This leads the question, what is a voltage drop? What causes it and how do you track one down?

What is Voltage drop

A voltage drop occurs whenever electrical current passes threw a point of resistance. To recap from last months article we learned Voltage is the force that makes electrons flow and Amperage is the actual flow of electrons. Resistance restricts the flow of electrons. A good analogy that can help you understand this is to compare the flow of electricity in a conductor to the flow water in a pipe. Voltage can be compared to water pressure. Amperage is measured in electrons per second where as water flow is measured in gallons per minuet. Electrical resistance restricts the flow of electrons. This can be compared to a partially clogged pipe restricting the flow of water. Imagine a 1” water pipe with a section in the middle that is reduced down to ¼” then increased back to 1”. One side of the pipe is connected to a water supply that delivers an unlimited amount of water at 50 PSI (pounds per square inch). The other end of the pipe has a valve on it. The ¼” pipe in the middle is the resistance. When the valve on the end is closed and no water is flowing the pressure will be 50 PSI on both sides of the ¼” resistance. As the valve is opened water will start flow. As long as the water supply can supply enough current the pressure on the up stream side of the ¼” restriction will stay at 50 PSI. The pressure down stream of the ¼” restriction will start to drop. The wider you open the valve the more water current will increase and the pressure after the resistance will drop. When the valve is closed and the flow of current stops the pressure will equalize. Pressure on both sides will be the same and there will be no pressure drop. What we learn from this is if there is no current flowing there will be no voltage drop. As the current increases, so does the voltage drop.

Where do voltage drops happen?

  The fact of the matter is that all conductors have resistance. Some just have more than others. Resistance is measured in Ohms. The resistance of a wire depends on the material the wire is made of, the size or gauge and length of the wire. 12-gauge copper wire has 1.588 ohms per thousand feet. 16-gauge wire has 4.016 ohms per thousand feet At 68° F. The 12-gauge wire has less than half the resistance of the 16-gauge wire. For low current applications a small wire will do. As amperage requirements increase it is necessary to use a larger wire to keep the voltage drop within reason. Not to mention the energy lost when the voltage is dropping is turned in to heat. An excessive voltage drop can make enough heat to melt insulation and even burn up wires. The resistance of the wire should not change over time. The main source of resistance in the wiring system is at connections.  Every place there is a connection there is a place for a bad connection. This is where you should look for excessive voltage drops. The resistance of a new clean connector depends on the contact surface areas and the amount of force between the two surfaces. Over time the resistance can increase due to corrosion and possibly becoming lose. To calculate the effect of increasing resistance compared to voltage drop we can use ohms law. Ohms law states that it takes one volt to push one amp through a one-ohm resister. Using this formula, if you know two of the components you can find the third. For instance if you have 10 amps flowing through a 1 ohm resister there will be a 10 volt drop across the resister. A vary small resistance can make an unacceptable voltage drop. A derivative of ohms law states I X R=V. V is the voltage drop across the resistance. I stands for intensity, which is another term for current. R is the resistance measured in ohms. Lets look at some scenarios.

 In the case of my FXR the headlight draws 5 amps.

     5amps X .6 ohms = 3 Volt drop

This means the circuit had 0.6 ohm between the battery and the headlight. Some of this was between the positive terminal and the light bulb and some was between the bulb and the negative battery terminal.

In the case of an electric starter that draws 100 amps with .05 ohms in the circuit

100 amps X .05ohms = 5 volt drop

That would leave 7 volts at the starter motor, which would be unacceptable.

The average ohmmeter is not accurate enough to read tenths of an ohm. This is why it is better to use a voltmeter to find bad connections that cause unacceptable voltage drops.


Testing for voltage drops


 Since Voltmeters measure a difference in voltage the easiest way to track down a voltage drop is with a voltage meter. Simply measure across two points in a circuit while current is flowing threw it. The reading you get will be the voltage drop. You need a meter sensitive enough to read small voltages. See the section on voltage meters later in this article. My old FXR has been ridden hard and put away wet for 20 years. This makes it the perfect bike to use as an example for voltage drops. It was ten years ago when I had dim headlight problems. The figures in this example are as it is now. This is how I tracked the voltage drops down.

In order to keep the voltage closer to normal operating voltage without the motor running I attached a battery charger to the battery. With the battery charger on a 5 amp charge and the lights on, battery voltage measured 13.76. I took a voltage reading directly across the headlight. I did this by removing the bulb from the head light shell but left the wire socket plugged in to the bulb. With the high beam on I took a voltage reading across the negative pin and the high beam pin. This time I got an 11.72-volt reading. The 2.04 voltage drops is not as bad as I had back in 98 but will work of this example.  Voltage drops can be on the positive and negative side of the circuit. To check the negative side I measured from the negative pin on the headlight and the negative terminal on the battery and got a reading of .7 volts. To check the positive side I put the black lead on the high beam pin and the red lead on the positive battery terminal and got a reading of 1.37 volts.  Every place there is a connection there is a place for a bad connection. So I started measuring each connection point. Any connection with more than .1 volt drop I cleaned and tightened. The power starts at the battery. It goes threw the main circuit breaker to the ignition switch than back to the lighting circuit breaker. From there it goes threw a connector at the headlight shell to the high/low beam switch then to the headlight. Every connection had some drop. Some of the factory crimp on terminals had higher than normal drop across them. I did not get a substantial drop at any one point on the positive side but a couple tenths of a volt here and there can add up. After cleaning up some connections I got the drop down to about 1.2 volts and decided to live with that.

Voltage meters

Most technicians and home mechanics will be measuring voltage with a Multi meter. A multi meter is a meter that can perform many functions. Most can measure voltage and ohms. Some can measure amperage. Meters can be Analog or digital. Analog meters have a needle that swings and points to a number. Digital meters display a reading on a screen similar to a calculator. When measuring amperage and voltage you will need to determine if you will be measuring alternating current (AC) or direct current (DC) and select the proper setting. Some meters will only read AC or DC but most will do both. On most analog and older digital meters you need to select the proper range for what you want to measure. The choices may look like this DC 2V, 20V, and 200V. It is important to choose a range that is higher than the voltage you will be connecting it to or the meter may by damaged. If you are not sure what range you need it is best to start on a higher scale and work and your way down. The lower scales will give better resolution and accuracy. You cannot read accurately to a tenth of a volt on the 200-volt scale. Most modern digital meters are auto ranging. All you need to do is select the appropriate function such as DC volts (DCV) and the meter will automatically select a range to give the best resolution. The most important thing (VERY IMPORTANT) is to read the indicator to see what scale the meter is reading in. This is usually in the upper right hand corner of the meter.

Resolution and accuracy

When measuring the voltage of a household appliance, a volt or two doses not matter. When trying to track down a .1-volt drop you will need a meter with better resolution. It is kind of like trying to time out 30 seconds on a watch with no second hand. In order to accurately measure tenths you need to measure hundredths.

 Some meters are temperature compensated and calibrated to national standards and some are just accurate to within a volt or two. For some jobs the accuracy and resolution of the meter dose not matter, for other jobs it dose. At Cycle Electric Inc we have many different meters. We mostly use fluke 73 and fluke 79 meters. They are a good quality made in the USA middle of the road meter. We also have a Fluke 7-600, which is a low-end meter we use for shop maintenance. This meter is good to check for a blown fuse on the air compressor but has many shortcomings. The voltage range starts at 4 volts so it is not good for low voltage readings. The ohms scale only goes up to 400 ohms. When put it across a 500-ohm resister it will read OL or open. Before I know this I thought I had an open circuit when the meter was just out of range. This steered me in the wrong direction on my diagnosis. Low-end meters are also venerable to interference from RFI or radio frequency radiation. This is the noise you can hear on your radio. It can come from many places such as arcing brushes or solid ignition wires. This interference can give you a faulty reading on your meter. The classic case is when someone calls me and said they have something like 50 volts at the battery. The first thing I ask them these days is “did the head light blow out”? When they say no I tell them they do not have 50 volts at the battery. Most of them will insist the meter works just fine on other bikes. This is usually traced to solid ignition wires emitting RFI and interfering with their digital meter. In order to put significance to the reading you get on your meter it is important know and understand its resolution and accuracy.

Let the road you choose to ride bring you happiness.