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<blockquote data-quote="helotaxi" data-source="post: 1772038" data-attributes="member: 550915">There are two reasons to isolate your batteries.&nbsp; One as mentioned above is to keep the starter battery from being drained while the car is off possibly leaving you unable to start the car.The other reason is to prevent the batteries from interacting and trying to balance charge between them with the car on or off.&nbsp; If the batteries are of a different type or different age, they will have a different static voltage.&nbsp; With the car off, the battery with the higher static voltage will discharge into the battery with the lower voltage.&nbsp; Over time this can kill one or both batteries.With a solenoid, the batteries are isolated with the car off, but will still interact with the car on.&nbsp; This usually isn&#039;t a problem.&nbsp; With a diode isolator, the batteries are totally isolated from each other.&nbsp; The system battery is the only one that is doing anything for the system and the underhood battery is basically idle.&nbsp; The alt can charge both of them but no current can flow between them with the car on or off.</blockquote>

[QUOTE="helotaxi, post: 1772038, member: 550915"] There are two reasons to isolate your batteries. One as mentioned above is to keep the starter battery from being drained while the car is off possibly leaving you unable to start the car. The other reason is to prevent the batteries from interacting and trying to balance charge between them with the car on or off. If the batteries are of a different type or different age, they will have a different static voltage. With the car off, the battery with the higher static voltage will discharge into the battery with the lower voltage. Over time this can kill one or both batteries. With a solenoid, the batteries are isolated with the car off, but will still interact with the car on. This usually isn't a problem. With a diode isolator, the batteries are totally isolated from each other. The system battery is the only one that is doing anything for the system and the underhood battery is basically idle. The alt can charge both of them but no current can flow between them with the car on or off. [/QUOTE]

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