Why Isolate your Batteries?
Many vehicles have multiple batteries: one dedicated for starting the engine and the other to power accessories. Without a Battery Isolator, a battery with a lower charge will draw power from a battery with a higher charge, until the batteries equalize. This problem is known as multiple battery drain and could leave you with dead batteries.
A battery isolator will prevent current from flowing from one battery to the other and each battery will act as an independent power source. this will ensure that the battery you dedicated for starting never gets drained.
A battery isolator will also allow you to simultaneously charge both independent power sources at the same time. The amount of current which flows into each battery is then determined by the battery's own state of charge (based on voltage regulator settings).