This 200 amp relay can be used as a battery isolator. When you use more than 1 battery in your car, you want to isolate them when the engine is not running. This prevents excess wear on the batteries because they will not be able to drain themselves.
Or use it to switch a very high current load with a small current. Use it for mobile audio, high powered air compressors, etc.
Battery isolator relay wiring diagram
Purpose: when two car batteries are wired in parallel, they have a tendency to charge/drain each other. To prevent this excessive wear, and to lengthen the life of the batteries, you should install a relay that disconnects the batteries when the engine is off. When the engine is on, the batteries should be automatically re-connected.