Why use a battery isolator? In a multi-battery system, the battery with the lower state of charge will drain power from the battery with the higher state of charge. The battery isolator protects your starting battery power so that no matter how weak your accessory battery becomes, it will never drain power from the starting battery. The battery isolator also saves time by allowing simultaneous charging of both batteries. The amount of current flowing into each battery is determined by its state of charge, which guarantees that each battery will get fully charged, prolonging its life.