Unipoint continous duty solonoids are the most common means off creating safe isolated dual battery systems.   They are simple and nearly unbreakable.

This style off battery isolator is normally wired up to an ignition feed.   This means the auxilory battery will always be completely isolated from the main battery when the ignition is in the off position, thus protecting your main (starting battery ) from being discharged.   Some poeple choose to fit an in-line switch in order to over-ride the system.

This style of isolator is not suitable for vehicles with on board computers.