I have a 1999 18' Bayliner with a 120HP Force outboard on it. I want to install a second battery with Blue Sea's Add-A-Battery. (Why? Because I want some insurance when I'm out on the Bay of Green Bay. I've had close calls in the past)
Notice that in their diagram the common busbar connects the two battery negatives, and provides a connection for the negative leads to other devices. If you simply have a cable connecting the two battery negatives, you can attach to either battery negative terminal. There is also a negative busbar under the instrument panel, but that would be too far away from ACR to make sense to use.
'88 Bayliner 1700 Capri Bowrider, 85 HP Force O/B, "Sea Weasel"
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The battery negative primary distribution bus is identified in the Blue Sea Systems literature as "COMMON BUS BAR." If you have a non-metallic hull, the battery negative primary distribution bus is usually bonded to the engine block. The engine block is usually bonded to something in contact with the sea, often a galvanic corrosion sacrificial anode. Sometimes a special electrode is used to provide a contact with the sea, often made from a very noble metal like bronze. This would constitute a ground on a vessel.
By the way, if you buy all of those additional components recommended by Blue Sea Systems in their installation diagram, you will find you have spent more money on fuses, terminals, bus bars, added wiring, and so forth, than the cost of the battery switch and the automatic combining relay. Following the Blue Sea Systems recommended installation results in a rather complicated and expensive system.
You can order cables, lugs, fuse holders, buss bars, etc. from many different marine suppliers. I don't think you will find large battery cables with in-line fuse holders built in. Car stereo installation shops usually carry large gauge wire and fuse holders but it's questionable how well it will hold up in a marine environment. It might be worth a look. Another business that will usually have or be able to make battery cables is an automotive battery supplier. Also, an electrical business that deals with solar systems, specifically off grid systems, will be familiar with large gauge battery cables. They typically use these to hook inverters up. I happen to be in this business.