Why separate the batteries?

A safety aspect on board is to always be able to start your engine. You must always be able to seek shelter from bad weather or move out of the way of a ferry.

Separating a battery with a split charge relay or a DC/DC charger is a good way to ensure that the starter battery is not discharged. To minimize the risk of faults in the connection above, there is no mechanical connection between the starter battery and the starter motor.

However, when the battery bank size approaches 400 Ah, you reach a point where the batteries have to be so discharged that they are damaged before they begin to have difficulty starting an engine. At that point, it becomes more important to monitor the battery bank properly than to isolate a battery.