Electrical relays are devices used to turn relatively heavy loads on/off. They are triggered by small amounts of power from an external source (usually a switch operated by the user) but can pass larger amounts of power to the load. This reduces the cost and physical requirements of the switch and can help automate some processes.

A typical setup looks like this: switch (and house power) –> relay –> load

The most common relay campers are exposed to is the solenoid for charging house batteries from the coach's alternator. When the vehicle is turned on power activates the solenoid and connects the house and vehicle electrical systems.

Relays are also useful for powering loads from LVD or Charge Controller LOAD outputs. This allows powering larger loads than the controller is rated for.

Relays come in two basic types:

1. Normally Open (NO) - the output circuit is open/incomplete when the relay is resting. No power flows. The controlled device is off. This is usually the default.
2. Normally Closed (NC) - the output circuit is closed/complete when the relay is resting. Power flows. The controlled device is on.