Many cars are provided with start batteries and an and an alternator as a back up for more power needs. The chassis batteries help start the engine, the headlights, wipers, computers and other stuff. The need for a battery isolator comes when the other charging devices don’t dissipate the starting battery, which is crucial to start the engine. And you will no allow this to happen and make other devices use up all the power.
Why you need an isolator
People may think that an isolator will not be required, and alternator will fill up for it as it will help charge up itself and the other battery when the engine is running, but this done at slower pace and the need to quicken pace of charging. But all this aside, there would be chances that the alternator stops working and the other electrical components will try to draw power from both the batteries and they end using all the power of the start battery. Draining both batteries is not advisable, and you will soon have no devices in the car working.
The isolator will allow the flow of power in one direction, thus it will prevent the use of power from all sources and save at least one of them up for back up, if you run out of power. The diodes help the charging of the both the batteries to happen simultaneously. It will thus prevent one battery using up the other and vice versa.When the start battery is being used then the power from the alternator is not allowed to be supplied and is blocked by the battery isolator. When the charging is required, then both the batteries are charged together.
There are certain issues that isolators are not so preferred. The reason when two batteries are being charged, it will at an even slower pace. The voltage is lost when it passes through the isolator. And this loss is seen when the battery is undercharged, and this could lead to a complete damage to the battery. But it is most useful when you will have to manage high current loads.It always ensures that the main battery is always charged how much ever the other battery charge is dissipated.