En 
SvSeparate batteries
How to ensure that the engine always starts
Being able to get underway safely is a critical safety aspect on board. You must always be able to start the engine—whether to seek shelter from bad weather or to maneuver out of the way of a ferry or other traffic.
Separating a battery using a battery isolator relay is an effective way to ensure that the starter battery is not discharged. To minimize the risk of faults in the configuration described above, there is no mechanical switching device between the starter battery and the starter motor.
However, when the battery bank capacity approaches around 400 Ah, a practical limit is reached. At that point, the batteries must be so deeply discharged that they risk damage before they begin to have difficulty starting the engine. In such systems, it becomes more important to properly monitor the battery bank than to isolate a dedicated starter battery.