connect or disconnect ?

An isolator is NOT needed..........as long as you don't forget to unplug the anderson plug..

But normally, you shouldn't need to unplug if its only overnight........the combined capacity of both batteries should be plenty for overnight ( depending on what you are running in the van )

A multimeter is what you need...that way you get an idea of what your system is doing, and roughly how long you can leave things hooked up..

Well the same thing could be said about having a key with a remote to unlock/lock your car..... can't you just use the key.... in simple terms it comes down to convenience.

Depending how you car and van are wired for 12v you may need or not need to disconnect.... if it has been wired in a way that the 3 way fridge will run on 12v when the car is not running then yes you have to disconnect.

Your average caravanner that we see through our workshop doesn't have a dual battery set up or an isolator fitted meaning when they stop at a country town for a look chances are their 3 way fridge is going to keep running flattening the starter battery in about 2 to 3 hours.

Some installs use an ignition trigger to a relay so when the car is turned of it stops power going to the 3 way fridge.

Some people still prefer this method but it's much cheaper and better to install a electronically controlled isolator.

One other thing; 6mm cable is way under sized for what you are trying to achieve.

Please do not let this little question hijack Johnno's thread but I have a related question.

We are getting a third battery and an isolator installed in the car in few weeks.

My question is: Is there any reason why we cannot control the isolator with a connection to the starter solenoid? So that all batteries crank the motor without the drivers intervention.

Please do not criticise the choice of three batteries as we have our reasons.

Thanks

PS If the moderators consider this OT then please mods let me know and I will start a new thread. I cannot find it.