If you want to do this you don't need the wire to the battery. Just to the Alternator. You SHOULD use a battery isolator of some sort. A high power solenoid designed to be on all the time is your best bet here. Should be active when the car is running.
As far as the ground goes you could connect the wire directly to the engine block if you so desire. If you decide to hook this wire in your vehicle to the chassis then you will need to upgrade the wire from the chassis to the engine block to handle the extra power.
No need to connect to the primary battery at all. But like I said, make sure you use some sort of isolator.