A battery isolator helps to ensure that the starting battery has sufficient power to start the engine and recharge the batteries if, for example, loads on the auxiliary battery (e.g., refrigerator or navigation lights) cause it to be drained, or if an auxiliary battery fails. How does dual battery isolator work? Generally built with diodes, isolators ensure that charge is evenly distributed between multiple batteries and the alternator. … By using diodes to allow current to flow only in one direction, a battery isolator prevents a fully charges main battery from passing current to a partially charged auxiliary battery