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Ground circuits — Part 3

Generator electron current, dual battery ground cable
Friday, May 5, 2017 - 07:00
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The advantage in tracing electron current is you sometimes discover unusual things occurring in the circuit, and in this example, we find an unusual way that generator electron current flows when the engine is running if the battery negative terminal is grounded to the engine block and to sheet metal at the same time. Let’s call this circuit a dual ground electrical system. The electrical system uses two separate ground cables when the generator is producing a charging voltage. One cable is the engine ground and the other cable is the accessory ground or sheet metal ground.

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Do not be confused by the two terms of generator charging voltage and generator electron current. Voltage and electron current are two distinct different values in any circuit. In any circuit you have voltage readings in volts and millivolts that indicate voltage conditions in a circuit and you also have electron current readings in amps and milliamps that indicate electron current conditions in a circuit. Our focus in Part 3 focuses on generator electron current through the ground circuit. This is going to show us something unusual and often overlooked when a battery has two ground cables.

During engine run, the generator provides the electrical energy to electrically operate vehicle circuits and at the same time recharge the battery. The generator produces charging voltage which is measured with a voltmeter at the generator terminals. The charging voltage also appears at the battery terminals which may be easier to access on some vehicles measure the charging voltage. When the generator is producing sufficient charging voltage it supplies electron current to all vehicle circuits. Figure 1 below illustrates the generator is operating and supplying electron current to all vehicle circuits.

Figure 1


Electrons leave the negative terminal of the generator which is the housing of the generator bolted to a bracket bolted to the engine block. Electrons are attracted to the positive terminal of th­e generator, “+GEN,” through all electrical circuits that are turned ON and provide a path for electron flow. Electrons always flow from the negative terminal, through the circuit, to get to the positive terminal of the same voltage source.

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All Electrons “channel” through the engine block to G100 and travel up the engine ground cable.

The term “channel” refers to the way electrons travel (migrate) through the metallic mass of the engine block. When electrons hit a solid object, they generally scatter more or less randomly as they collide with individual atoms of the engine block. They will follow the most direct path in the engine block to arrive at G100. This electron flow must not migrate through engine bearings or the water jacket of the engine which would cause deterioration by electrolysis and degrade the bearings or cause a leak in the water jacket. That is why the generator is often mounted on the engine block where it may be hard to get to. Placing the generator at a specific location is required to prevent electron current from interfering with critical engine functions as electron current migrates through the block. That’s also a good reason not to ground a high current electrical circuit to the engine block without the manufacturers specific instructions to do so at any particular point on the engine block. Selecting the wrong place on the engine block to ground a high current circuit may cause problems later down the road.

Back to electrons flowing up the engine ground in Figure 1. Electrons traveling up the engine ground cable make contact with the sheet metal ground cable at the battery negative terminal post where they are both connected together. Most electrons travel down the sheet metal ground cable and continue through the sheet metal ground circuit to supply electrons to all vehicle circuits turned ON just as battery electron current did.

All vehicle circuits continue to function during this brief transition period of electron flow from the battery to the generator. The transition takes less than a second although there will be a change in electron current from low battery voltage providing low electron current when cranking the engine to a higher voltage (charging voltage) providing higher electron current when the generator comes online. The transition from battery electron current to generator electron current as the generator comes online is fairly smooth and is higher when the generator is the provider because it operates at a higher voltage than the battery voltage.

Most of the electron current that travels up the engine ground travels down the accessory ground cable to supply all circuits. A very small amount of generator electron current enters the battery at the negative terminal, travels through the battery and exits the battery at the positive terminal which returns to the generator +GEN terminal. The small current flowing through the battery is used to recharge the battery.

If you would like to explore the subject in more detail, we recommend our book “Electrical Troubleshooting SHORTCUTS.” You can also click here to read about “SHORTCUTS.”

In the next email we will discuss measuring ground electron current and some do’s and don’ts. You might be surprised with what we share. Chances are it will be something you were never told before.

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