There are two major benefits to installing a headlight relay into your lighting system. One is brighter, headlights for safer driving at night. The second is the extended life of (up to 50%) of your original headlight switch. To better understand why adding a relay into the headlight plugheadlight circuit works so well, you first need to understand the problem at hand. Electrical friction, officially known as "resistance" is created whenever heavy electrical current travels through a wire. A by-product of resistance is heat. The greater the distance involved the more resistance (and more heat) that will be created. As the resistance builds, the voltage within the circuit begins to drop.

A drop in battery voltage will have an immediate effect on the brightness of the headlights. For example, a 12-volt charging system is designed to maintain 13.5 volts at the battery. When 13.5 volts is available at the battery, the headlamps will provide 100% of there rated output. When the operating voltage falls to 12.8 volts, because of high resistance within the circuit, the headlamps will provide just 83% of their rated output. When the voltage drops to 12.5 volts, the headlamps will provide just 63% of their rated output. You experience this as dim headlights.

Service Life of the Headlight Switch

Besides the brighter headlights, a relay installed in the head light circuit will greatly extend (up to 50%) the service life of the headlight switch. The headlight switch in a typical 6-volt system passes 22 amps of heavy current through the switch. In contrast, when a relay is installed into the circuit, the amount of heavy current that passes through the headlight switch is reduced to less than one amp.