Camper Van or Boat – secord battery isolator not Split Charging
There are some pretty sophisticated split charge systems aimed for use on boats and campers on the market. A split charge system will only charge the auxiliary or leisure battery once the starter battery or batteries have reached full charge.
If you just run a fused wire from the positive terminal of your starter battery to your leisure battery then when you drain your leisure battery power will be then be taken from the starter battery and your camper van will not start. If your starter battery is low then the starter motor can draw current from the leisure battery and if you don't have a fuse in the wire, then the wire can overheat and melt it's plastic casing.
You can buy a relay from maplin that will charge the weakest battery in the van for less than £10, but you would have to make some kind of casing for it and mount it in the engine bay near the starter battery, I didn't want to do this so I opted for the even cheaper option which is a battery isolator switch on the leisure battery. I ran a wire with an inline fuse from the positive terminal of the battery to one side of the isolator switch, and a wire from the other side of the switch to the positive terminal of the leisure battery.
It's no fun camping in the middle of nowhere if you can't be confident your vans going to start! A 12volt panel mounted volt meter will show you the condition of your starter or leisure battery.
When I park the van up at night I isolate the leisure battery by turning the switch. This means that anything that runs off the leisure battery can only consume power from that battery and one it is drained the starter battery will not be used. In the morning I start the van and allow it to charge the starter battery before opening the isolator switch to let the leisure battery charge.
I have also fitted a panel mount digital voltmeter with a 3 position switch, this can display the charge of either battery or be switched off. It's always useful to know how much power you have in your batteries and when you are traveling at night in the rain with headlights and heater running the alternator does not produce enough power to increase the charge in either of the batteries.
Unlike a split charging system, you decide when to charge the leisure battery. It adds the risk of human error(forgetting to isolate the battery!) but there is less wiring and less to go wrong that a split charge relay system.