Relay Control
I apologise if this is rather simplistic but it may help :
A high current relay is a very useful electrical control item. The voltage across the coil winding of a relay controls the relay contact closures. When a voltage is applied to the coil of a relay a small current in the coil, a few milliamps, produces a magnetic field that causes two contacts to touch. The two contacts are not electrically connected to the coil and can operate freely in any circuit. The contacts can take amps and are the heavy duty part of the relay doing the real work of carrying current to your electrical system from the battery to your electrical circuits.
When the coil is energised you can normally hear the contacts click together and when the coil voltage is removed you can hear them click back. The contact operation time is very quick.
In order to chose your relay you have to determine :
1. The maximum current that your battery provides to your complete system.
This can be measured with a DC current meter or calculated from the total power of your system, measured in Watts. Current = Power / Battery Voltage.
The current rating of the relay contacts should be rated higher than this maximum current as if you have inductive loads, like electric motors the current at switch on could be several times the normal running current. A relay data sheet may mention this inductive situation.
2. What coil voltage you require. The coil can be connected to your battery through a low current rated switch. So the coil voltage rating will be the same as your battery voltage.
3. The cost of the relay will depend on the current rating as this will determine the size of the contacts and the overall package size.
4. If you require more contacts then the relay must be chosen carefully. Most relays will have either normally open contacts ( N/O ) that will close when the coil is energised. You can buy relays that work the other way around, the contacts are normally closed ( N/C ) and they will open when the coil is energised. Some relays have both options ( N/C + N/O ). A drawing of the contact arrangements will normally make this clear.What contact arrangement you chose depends on what type of control actions you require.
5. From a wiring point of view you will have to use heavy duty wiring to connect the contacts and the type of connection, soldered or crimp, may be important to you. The coil wiring will be thinner wire and easier to connect. Normally the thick wires are kept to a minimum length to avoid excessive voltage drops along the wire. The coil wiring is not so critical. I would put a fuse in the battery line as a safety precaution, this will protect the battery if a huge current is taken by a faulty circuit or in cases where the wiring shorts together somewhere remote from the battery.
Remember that a relay is like a power steering system on a car. A small turning force on the wheel brings much more power into the system that actually turns the wheels. In other words a small current into the relay coil brings large contacts together that switch a much larger current into your circuits.
Note that battery operated relays are sold specifically for cars where fairly large currents, up to 40A are required in various parts of the car for lights, heaters and even audio power amplifiers etc. It would be highly likely that one of these reliable relays would do the job at reasonable cost.
A good quality in-line fuse should also be available in a plastic case.