"Russell, what type of boat an
"Russell, what type of boat and how many engines?
On my single engine (outboard) 25 footer I run 3 batteries. 1 starting battery (850 MCA) and 2 house batteries (group 27 deep cycle). I have them rigged with a combiner switch and an automatic charge relay.
The alternator charges all 3 batteries when the motor is running, only the starting battery sends power to the motor and the house batteries only send power to the cabin/assessories. If needed you can switch so that all 3 batteries can be used to start the motor in case of a low starting battery.
If you think you need 2 batteries to start the motor, you probably simply need either a new battery or a larger starting battery...."