With two batteries and a battery switch to choose between them, you can always start the boat, even if one battery has been run flat by the lights and fish finder. In this video I add a second battery to the Green Machine. The cables have to be heavy enough to carry the starting current; see measuring voltage drop.

On a bigger boat the battery banks and switches can be confusing. Pete and I work out how the banks on his Halvorsen 36 are wired together and what each switch does, so they get used properly and faults are easier to find later.