Plus these two conductors will carry the current of all your electrical loads combined so they are typically fairly beefy cables. All you need now is some space a few basic tools a little knowledge and another battery.

12 24 Wiring Diagram For Boat With Onboard Charger And 3
Wiring 3 batteries in a boat. Even a small boat 3 5 loads wed recommend at least 12awg wire for this. With two batteries and a 1 2 all two bank manual switch already installed in your boat you have all the basics in place to add as much battery power as you want. The most obvious and most common is simply to use it to turn off your battery when in storage so that your battery does not drain as quickly and is ready to go when you are. An manufacturer of marine hardware and accessories. For example if a 3v and a 9v battery were wired in parallel the output voltage would be 6v 93 divided by 2 however the current will be total amperage of all. Run marine grade electrical wiring from the black negative terminal on the right battery to the black negative terminal on the left battery.
An ordinary auto battery would do for starting and lights but for running a radio and other electronics something with a little more of a deep cycle capacity will be needed so the battery doesnt go flat while youre fishing and listening to the radio and leave you stranded when you try to. The battery should be a combo startingdeep cycle battery usually sold as a marine battery. Unlike wiring batteries in series when batteries are wired in parallel the voltage does not increase the output voltage is the average voltage of all batteries in the circuit. This is sometimes a long wiring run on a boat. Place a second battery along with the old one or two new batteries side by side in the engine compartment and secure with straps battery boxes or bungee cords. There are many ways to use a perko battery on off switch.
10awg for larger boats 5 10 loads is normal.