Relays are your friend. They're really just fancy switches, but they're better than switches. Use them any time you want to wire something directly to the battery, so you can have a switched power source. Of course, you need to have an inline fuse, too.
Relays typically have 4 terminals. 2 of them are for the larger power supply wires, from the battery to the relay and then from the relay to the item being powered. The other 2 are for the smaller "switch" wires, which just turn the relay on or off. When the relay is switched on, it allows for power to flow through the larger wires. Before this, they're disconnected.
Some advantages of this are:
- The stock wires on bikes are typically quite small. When using a relay, these smaller wires don't have power running through them except to turn the relay on or off. The real power is flowing through the larger wires.
- If the source that switches your relay is only on when the key is on (say, the license plate light) then your accessory will only be on then, also. If you don't use a relay, you could potentially drain your battery when the accessory gets left on. For instance, if you leave your grip heaters on it doesn't really matter, because they weren't really on, anyway. The relay switched them off when you turned off the key.
- For something like the headlight, the OEM wiring, with its thin wires and many extra connections, dilutes the 13 volts at the battery to 11 volts by the time it gets to the front of the bike. Since light output is directly related to power input, if you want brighter lights you will use the OEM wiring to switch the relay (turn it on/off), then run a much shorter, heavier-gauge wire directly from the battery to the relay, then from the relay to the light.
- What relays are and what they are used for. Look down the page just a little bit for their RELAY BASICS - 101.
- If you find that you have a lot of accessories hooked up and have a bit of a snarl with all the wires, this page on Canyon Chasers has good ideas on how to use relays to take care of that problem. Make sure to look at the diagram, but we believe you should use a fuse box instead of an unfused distribution block.