Batteries do self discharge when not being used. They also sulphate. So best to use a battery maintainer. The batteries may still have enough charge left if the spring to start the boat if nothing is done and batteries are left in the boat. But now you are trying to charge low batteries with the alternator.
As the batteries get more expensive it becomes more important to use a battery maintainer. At $80 a pop in Walmart not a big deal to replace a car sized marine battery. But at $200 to $300 for a large marine battery times 2 or 3 or 4 batteries a battery maintainer starts to look like a good idea.
As the batteries get more expensive it becomes more important to use a battery maintainer. At $80 a pop in Walmart not a big deal to replace a car sized marine battery. But at $200 to $300 for a large marine battery times 2 or 3 or 4 batteries a battery maintainer starts to look like a good idea.