The positive side of the battery is labeled +BATT (say “plus BATT”). The electrical charge at the battery positive terminal is described as B+ (say “B plus”). The positive sign indicates a positive charge where electrons are needed. This electrical terminology indicates there are more electrons at the negative terminal than at the positive terminal. The difference between the two battery terminals is referred to as battery voltage, or a “potential difference” or EMF for electromotive force. EMF describes the force that moves electrons through a circuit. For purposes of our discussion let’s just call this force battery voltage and keep it simple. (KISS = keep it simple student)
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Battery voltage is measured with a voltmeter to indicate the difference in voltage between the two battery posts. If a vehicle battery is fully charged, which means the negative terminal is full of electrons and the positive terminal has very few, the voltmeter indicates 12.66 Volts between the battery posts. That will be a topic for later study.
Several circuit components are connected together to form a complete vehicle electrical circuit in Figure 1, representing a typical vehicle type voltage source (battery and generator). An ignition switch controls the voltage applied to vehicle circuits. Lamp circuit #1 operates when the ignition switch is CLOSED and switch S2 is CLOSED. Notice that Lamp Circuit #2 is connected to Hot-At-All-Times B+ which is a direct connection to B+ supply. Lamp circuit #2 will operate any time switch S3 is closed regardless if the Ignition Switch is ON (CLOSED) or OFF (OPEN).
So far, we have discussed electrons accumulated at -BATT. They aren’t moving at this point with all switches OPEN. The electrons are stored and available at -BATT and are measured for quantity by the value of battery voltage. The higher the battery voltage the higher the state of charge. It’s entirely another story when electrons start moving. Moving electrons reveal some important facts about circuits you may not have realized before.
Our focus in this series of 9 articles is on the ground side of the circuit and the electron current that flows through the ground side of the circuit. Notice all the circuits are connected to the “ground side,” which is nothing more than the negative side of the power source. There are two “legs” in this electrical system which is sufficient to illustrate the principle of how electrons flow through a vehicle circuit. Of course, electrical systems have many circuits connected to ground. However, we can illustrate the electron current principle with only two legs in the electrical system.
There are three ground connection points in this schematic. One is the engine block. The sheet metal is divided into the other two separate grounds which are connected with a ground strap. That is a total of three places where an electrical circuit can be grounded. Being grounded means being connected to the negative terminal of the voltage source.
The generator as a voltage source will be discussed later. For now, let’s stay focused on the battery as the voltage source and the supplier of electrons to the circuit. The negative post of the battery, -BATT, is connected to the engine block by two cables. One cable is the engine ground cable connected to the engine block. A second cable is called the accessory ground or sheet metal ground and is connected by a cable from -BATT to sheet metal (1). A ground strap connects sheet metal (1) to sheet metal (2) which completes the ground circuit on this vehicle.
This arrangement allows either the battery or the generator to supply the electron current to operate vehicle circuits. When the engine is NOT running, the battery provides the electron current provided the battery has sufficient state of charge. When the engine IS running, the generator takes over and provides the electron current provided it is working properly. In a properly operating vehicle electrical system the generator supplies all the electron current and the battery becomes a load on the generator as we shall see as this series continues. Next time in Part 2 we discuss how battery electron current flows through the circuit.