Electrics and autos have gone hand in hand for decades, and the infusion of electrical components into new 4WDs is increasing daily. Whether you may be adding a new accessory or simply having to troubleshoot or repair an electrical problem, some basic knowledge of electrical theory and practice can be handy.
Most vehicles we drive are equipped with, and operated from, a 12-volt battery. Along with the alternator, this source provides all the electrical energy needed on our vehicle. The battery is used for starting purposes and can supply electrical power when the engine is not running. Once the engine has been started, the alternator can be actuated by the voltage regulator to supply the electrical needs of the vehicle and recharge the battery as needed.
There are two basic properties we deal with when discussing electrical systems: voltage and current. Voltage (volts) is the electrical potential and can be analogous to the water pressure in a tank. The greater the voltage, the more energy we can typically supply. Thus, a fully charged battery or properly operating alternator will provide brighter lights than will a lesser electrical source. Current (amps) is a measure of electrical flow through a conductor and can be compared to water flow in a pipe. Increased flow capability means that greater energy is available at the far end of our wiring path. As an example, this becomes especially important when trying to get that last ounce of winch pull when you're stuck in the mud.
An electrical circuit consists of a complete loop or path. That is to say that in an operating circuit, current flows from the positive side of the battery to the device being powered and then back to the negative side of the battery. If this path is interrupted at any point, the circuit is broken and the device will not function.
This brings up two points concerning how electrical circuits are wired. First, note that a switch can be placed inline to engage or disconnect the device of interest. Such a switch is generally placed on the positive wire leading to the device such that the downstream wiring is left unenergized when the switch is off. Second, since the same current flows around the entire loop, both positive power wires and ground wires must be sized to accommodate the current draw of the device. If a single wire is used to connect several devices to ground, then the wire must be of sufficient size to support the total current flow of the three devices. On most vehicles, the frame and body are tied to battery ground and serve as a large return ground path for many electrical devices.
As current flows through a wire, it is subjected to a slight resistance in the wire. This resistance causes some voltage loss, or drop, as the current travels through it to the end where your electrical device resides. If you use too small a wire, the voltage drop across the wire will be significant, and the voltage available at our device will be somewhat less than our full battery voltage. Performance of that device can suffer.