Hi all; Most 4x4 that have a 2 battery set up ( one for the car and one for all of the extras including a fridge or caravan main power feed ) have a relay that would sence the accesory feed so that when the motor is running the relay between the two positive terminals of the batteries are connected and allow the auxiliary battery to be charged. This is a old school way and has been around for years. Most installs of the day just used a 30 Amp or higher headlight relay.

The other way is what is called a VSR ( voltage sence relay ) and it sences the main battery voltage and once this battery has reached a certain voltage the relay will "pull in" and allow current to flow into the auxillary battery to charge. Turn off the engine and the main battery voltage would drop a little and the VSR would automatically "drop out" These relays are also very easy to install. Just 3 connections are needed, the positive wire from the main starter battery, a main wire going to the positive terminal to the auxiliary battery ( both these wires have to be able to carry a high current charge ) and the third wire is the ground wire, this connects to the cassie of your 4x4 etc. I payed a few years ago around $60.00 for my VSR relay at the time. My 4x4 is now getting on to around 28 years old, so old school electronics and has a basic alternator, a bit of caution is needed if your 4x4 is fitted with a computer controlled alternator as you could damage the computer.