You can use this as a Split Charge relay allowing you to charge your living batteries from the alternator when your engine is running, so you can charge up while you move along. Wire the alternator ignition wire to the positive spade, earth (negative) the other spade, and then connect the positive of each set of batteries each to their own one of the large terminals. (assuming both batteries are negative earthed to the vehicle, otherwise you will need to add a common negative to them as well)
For Split Charge Do not use on Euro V/VI engines, or engines with smart alternators or regenerative brakes. These newer systems do not give a constant voltage and are not suitable for a split charge relay.
Dump Load - Make use of your excess power
You can also use this relay to work as a dump load, you can set the load terminals on one of our charge controllers to come on at 14V and off at 13.6V and then wire the load terminals to the spade terminals on the relay. Then connect your high load (upto 200A) to the large terminals and the charge controller will then turn the dump load on when the batteries are full so you can put the excess power into say a water heater or something useful. This could be a water heater, or you could divert the charge controller power to a back up battery bank.
Switch charging between battery banks
You can use relays to enable you to switch the charge from your solar to different battery banks. By using a relay for each battery bank you can then use an On-On switch (available here) to alternate the relay's state and turn one relay on and the other off at the same time therefore moving the incoming power from one battery bank to the other. As these are 200A relays you can do this with any of our charge controllers or even with multiple charge controllers.