There are many options for dual battery systems:
Manual isolating switch.
The switch allows current to flow between the start battery and the house battery. When the switch is turned off it prevents the current flowing between the start battery and house battery. If the house battery drains it will not drain the start battery.
Voltage sensitive relay (isolating switch). This device turns the isolating switch on and off when it detects a voltage on the input side (start).
When the voltage on the start side is high enough it will allow current to flow between the start battery and house battery. When the voltage on the input (start) is low enough the current is prevented flowing between the start battery and the house battery.
When the vehicle is running the alternator provides enough voltage to trigger the relay, which connects the start battery and house battery together. When the vehicle is turned off, the alternator does not produce voltage and the relay disconnects the current flow between the start battery and the house battery. Thus the house battery can drain and not drain the start battery.
Ignition relay (isolating switch) which allows current to flow when the ignition is turned on with the key and prevents the flow when the ignition is turned off with the key.
DC-DC charger which has a built in isolator. An example of this is a Redarc BCDC1225D charger. This charger is a MPPT Solar Regulator (9V to 32V), isolator and 25 Amp DC-DC Charger in one unit.
If your vehicle has a smart alternator you will need to have a compatible DC-DC Charger like the Redarc BCDC1225D. When smart alternators increase and decrease the voltage it causes a Voltage sensitive relay (inside the Redarc DC Charger) to fluctuate which causes the DC charger not to charge correctly. To overcome this, a trigger based on the ignition switch has to be installed so that the DC charger knows when the vehicle is turned on or off.
A manual isolating switch is the cheapest solution for a dual battery system but requires you to remember to manually turn on and off the switch.
A voltage sensitive relay costs more than a manual isolating switch and automatically turns on and off the switch. If you have a vehicle with a smart alternator you will need to override the switch by connecting the blue wire to an ignition source. The Redarc SBI12 has this feature and instructions to setup.
The ignition relay costs cheaper than a voltage sensitive relay and will work on vehicles with smart alternators.
The DC-DC Charger is the most expensive option, however provides the most value, because it contains an isolator, solar regulator and DC-DC Charger. Note. Not all DC-DC Chargers have a solar regulator built in.
If you are not sure what solution best suits your needs please feel free to call us on 1300 377 128 and our team will help you out.