Here we have on offer is a 12v 100Amp Heavy duty relay. 

This relay is ideal for when setting up a dual battery system for a Auxilary battery in your 4wd or caravan applications. 

Simply connect: 

Pin 85 to earth. 

Pin 86 to Ignition. 

Terminal 30 to your Positive terminal on your original Battery and 

Terminal 87 to your Positive terminal on your Auxillary Battery. 

This will enable when your ignition is turned on that both batteries are connected, start your vehicle and both batteries will get charged and when you turn your key off then both batteries will be separated and your Accessories connected to each battery will draw off each battery that you have wired your accessories to. 

Very easy straight forward dual battery set up. 

In this listing you will receive 1x Relay as pictured.