Originally Posted by Seb
Global supply fuse means....?
Is this second fuse and thus whole din rail needed at all as there is already a trip? Could the cable just continue straight from the trip to the switches and device safely?
After I had drawn the diagramme I did wonder if it would confuse anyone...
Global supply fuse is one connected to the battery terminal, and is to protect the battery cables from catching fire should they chafe through or become damaged. It is rated at 100 amps which is enough for the engine, accessories and some more, but would blow for a major cable short circuit. It should be located as close to the battery as possible and certainly before any connections to fuse boxes or engines.
The second fuse indicated is not actually a fuse. Dohh. I should have labelled it as "from fuse 1". It is actually just a convenient way to connect wires together. You could run the cable going to the switch, strait from the breaker, but it just makes for a neater layout.
As regards batteries, this will be down to what you want it to do. Whilst the engine is running you will be drawing current from the engine. When the engine is not running you will draw current from the battery.
How long the battery can keep this up for depends on its capacity. A 45 Amphour battery will supply 10 amps for 4.5 hours at 12 volts. A70 amphour will be able to do it for 7 hours. Realistically we Ribbers dont do long periods of engine off, so a smaller battery will work fine. BUT we do like engines to start. So a bigger battery will be able to crank for longer.
As an example my 90 was reccomended to have a 100 amphour capacity but a cheapo Halfords 45 amphour has never let me down. I did have one issue once where I had been adjusting the trim gauge before launching, went to start and it didn't have enough power left to start. Jump leads soon sorted that out. I have over the winter added a second battery because I intend to do a cross channel and some extended cruising. A battery failure could be a real pain in the arse. Remember everything ran from one battery including the VHF.
A good friend of mine has a 7.5m cobra with a 150 opti. He replaced both batteries after a service and was out wave jumping, when he landed pretty hard. He lost total electrical power. Turned out to be the new battery had failed. To remedy the situation he switched to battery 2, started up and drove home.
So to sumarise your question, one battery is ok and will work fine. Two batteries gives you more power, a second reserve in case of a fault, the ability to start the engine even if 1 battery is flat. The down side is it leaves the boat with less storage and extra weight. You don't get benefits for nothing!!
Anyway i've waffled on for long enough. Hope this sheds some light on the matter.