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I would suggest some sort of professional installation just to be safe. This unit connects two batteries and charges from a single source. Your car's engine charges one battery. If you get this unit, you would be able to install a second battery and it will be charged off the same circuit. When you turn off the engine, the main battery will be electronically disconnected and you will be using the second battery as power source (useful for audio systems and other components that you may want to run until the battery is depleted). When you need to start the vehicle, it then automatically reconnects to the main battery which is still fully charged. Care must be exercised when installing products like these. You would need really thick wires (at least the same size as you existing battery cables - or about as thick as your pinky finger).
It is always safe to have a fused connection coming from one terminal of the isolator going to the main battery as well as another fused connection from the third terminal of the isolator going to the secondary battery.
Neither battery will be charging when the vehicle is off. With the engine running, both batteries are charged.
And yes, the main battery will continue to be isolated and fully charged when the engine is off.
Last caution, in installing a secondary battery, make sure that it is in the engine bay. If you need to install a second battety inside the car (such as in the trunk) you will need a suitable battery box, clamps and a breather hose to vent the gases from the second battery. Never install a battery inside the trunk without a box or a vent - the fumes are quite harmful
