A relay is an electromechanical device used to turn on an accessory, they are typically used to remotely turn on a high current device.. The great thing about a relay is you can mount it near your device lets say a bilge pump and run much lighter gauge wires up to your dash for the actual switch...
A relay simply has a electromagnetic coil inside it that when current is applied it engages a set of contacts completing a circuit..
Relays come in many shapes and sizes, the most common are the bosch style automotive relays that typically have 5 poles.
The 5 poles on the relay are numberd and each has a specific function.
Terminal #30 is considerd the common terminal, and is normally connected to your accessory ie bilge pump for this example.
Terminal #87a is connected to #30 when the relay is NOT energized or at rest, for our example it would not be used.
Terminal #87 is only connected to #30 when the relay is energized and would allow us to send power to our bilge pump when the switch at the helm is turned on.
Terminals #86 and #85 represent the coil when 12v is applied to one terminal and ground to the other the relay will entergize moving the internal contacts..
That being said lets look at a simple diagram.
Here we have a two way switch mounted at the helm that would commonly be found with a bilge pump. It has an OFF setting, Auto setting, and On setting for manual use. Its main terminal would have a fused lead coming from our fuse box with +12 volts on it. The terminal that is hot when switched to AUTO would be wired into the bilge pump's float switch for automatic operation, the other side of the float switch would then be connected to terminal #86.
Our ON terminal would also be connected to terminal #86 so that we can switch on the bilge pump at will, or in case the float switch fails.
Terminal #87 is connected via heavy gauge wire to the battery and fused.
Terminal #85 is connected to a ground
Terminal #30 is then connected to the bilge pump and will supply 12v+ to the pump whenever the relay is energized..
When we set the switch to auto, 12v travels down the blue wire into the float switch, when the water rises enough to engage the float switch 12v+ travels to terminal #86. Since #85 is connected to ground the coil inside the relay energizes and closes the connection between #87 and #30 turning on the bilge pump.
*This example can be adapted to many things, turning on livewell pumps, high current lighting, amplifiers, and battery isolation (with a high current relay).*
This example will show you how to use a high current relay to add a house battery for a trolling motor, stereo system, underwater lighting etc. A relay can be used to isolate the house battery from your cranking batteries manually with a switch at the helm or isolation can be automatic if you have a circuit that is only live when the ignition is turned on..
A standard bosch style relay is too small to handle the current supplied by a battery so you will need an ignition protected relay rated for high current use such as this one.
This Relay / Solenoid is a 4 pole version of the previous example it functions exactly the same except it does not feature the #87a terminal for a connection when the relay is at rest.
Okay, so we have our main starting battery on the left, it is connnected to our normal electrical system, starter, bilge etc. Our new deep cycle battery will be connected to the amplifier, trolling motor or other high draw accessories. We would then put our high current relay in line between the positive terminals of both batteries. Our relay is grounded near the battery and a light gauge wire can be run up to the helm and either connected to a switch or the ignition circuit.
When power is supplied to the coil on the relay the contacts close and the batteries are connected. Enabling both to charge, when power is cut the connection is broken isolating the two batteries.
While this is not the ONLY way to accomplish battery isolation it has it's pro's and cons..
PROS 1) better than a diode isolation system - diode isolators have a voltage drop preventing full current from reaching the batteries and can possibly shorten the life of your batteries. 2) allows you to use high draw accesories without fear of being stuck with a dead battery.
CONS 1) if wired to the ignition you could possibly end up with two dead batteries if the alternator was to die. 2) if wired to a switch you have to remember to engage/disengage the relay for charging.
An upgrade to this setup would be a voltage sensing relay that automatically closes the contacts when voltage from the alternator is detected.
