Originally Posted by Nicks
And making 2 6v's into a 12v is just connecting positive to positive and negative to negative and then only pull loads from one battery?
That is parallel connection. You want series connection.
Boat ground to negative terminal on one battery. That battery positive to the negative of the second battery. The second battery positive to the boat's positive connection.
Series connection, voltage adds so 6V + 6V = 12V BUT capacity in amp hours stays the same. 220 amp hour at 6V + 220 amp hour at 6V = 220 amp hour at 12V
Parallel, voltage stays the same but the amp hours add up. So 12V 220 amp hour + 12V 220 amp hour = 12V 440 amp hour.