I am helping a friend with enhancing his camper van. Situation is as follows: He may go camping for a 2weeks and want to use car grid to power lights, charge cameras, phones etc. But don't want to risk discharging battery too much(he need to start big diesel)
My idea is to wire second(utility) battery(in the back of the car) parallel to main battery. But not directly. I plan on using switches or relays to disconnect the utility battery when voltage gets below certain point. That will enable him to use remaining power in utility and ensures he can start a car. Then comes the charging/balancing the batteries. To limit balancing currents resistor(beefy one) will be placed between batteries until difference is low enough to connect them directly again.
With help of simple arduino whole system can be totally "set it and forget it". Is it really that simple or did I miss something important?
EDIT: According to responses I probably badly described what worries me. Its connecting batteries in different state of charge. I am not very familiar with lead-acid, but from my knowledge of LiPo you cannot simple connect them and let them balance on their own because you will damage more discharged battery by charging it too quickly. It also generate lot of heat in the proces and currents are tremendous. Hence the resistor between utility and main bat after discharging one bat more than the other.
Best Answer
You can buy split charge relays or controllers that come in many different types and sizes.
You can buy a simple split charge relay that has voltage sensing circuitry that charges both batteries and will keep both batteries separate so the starter battery never gets discharged below 12v. These can be found as cheap as £/$10
You can buy more sophisticated micro-controlled systems that monitor both batteries and charges each and switches between the two, they also often have inputs and outputs for extra charging sources such as extra alternators and solar panels. The can also have dedicated outputs for critical equipment such as fridges, communication/GPS devices etc. These range in price from £/$50-1000 depending on the spec.
You could also use a DC to DC charger to maintain the second battery this would be required in some modern vehicles with 'smart alternators'.