A battery isolator is a type of electric device that splits direct current and only allows current to flow in one direction, from the device itself to any number of batteries that may be connected to it. It is a common device in vehicles that have more than one battery, such as recreational vehicles (RVs), which may use one battery for starting the motor and driving activities but another battery for appliances and other devices. Using a battery isolator allows multiple batteries to be charged from one alternator but keeps one battery from draining the others when low on charge. It also has the capability of fully charging batteries according to the need of each battery.
Vehicles or with more than one battery connected to the same electrical system can benefit greatly from the addition of a battery isolator. Without such a device, when one battery is low on charge, it can drain other batteries in the system. A battery isolator allows the batteries to be connected to one system without risk of draining each other. For example, work vehicles like tow trucks with a separate battery for the winch and other systems can use those systems without worrying that they will drain the battery used for starting the vehicle.
The battery used for starting the vehicle is the most important, even if there are more batteries that support other functions. If the vehicle cannot be started, it is not possible to charge any of the batteries as the vehicle's alternator serves to charge them and it only operates while the vehicle is running. Any vehicle with a battery isolator installed can always charge its other batteries as long as there is enough power to start the vehicle.
Other advantages to having a battery isolator include the ability to use any of the batteries for starting the vehicle, if necessary. Some isolators also are switchable and allow the batteries to run on the same circuit for a boost in starting power if desired. Any vehicle that uses devices or systems that draw a significant amount of power can benefit from a battery isolator. The main drawback to most types of isolators is that the current from the alternator is split amongst the batteries, so when charging more than one battery, charging time may increase.