Connections Between House and Starter Batteries?

Question:

We have three supposedly separate battery banks on our catamaran.  One house and two starting. Why is it that I can connect a load to a starter battery positive and the house battery negative and it works?  Aren't these two different power sources?  Which battery bank would be getting discharged? - Ray

Answer:

Great question, it's absolutely normal for all battery negatives to be common and non-isolated. The behavior you indicated below is to be expected.- Jeff