A starter relay switch is an electrical component found in vehicles that completes a circuit between the battery and the starter motor when the ignition switch is activated. The starter relay isolates the ignition circuit from the much higher current that the starter motor requires to operate.
Nearly all vehicles with internal combustion engines have an electric starter motor that initiates engine revolutions until the normal combustion process begins. These motors draw a great deal of current from the battery, often as much as several hundred amperes. Starter relay switches are often used in conjunction with an additional component called a solenoid to prevent this current from running through the ignition circuit.