Hi all, if there was any question about my being an idiot, I will now erase all doubt. I am going back and forth between two good books on boats and 12v systems. Looking at pictures of batteries in series, it appears that my house batteries are not truly in series. My set up has the positive cable from BOTH batteries going to one side of a large fuse. The other side of the fuse has the lead from the battery charger and the main positive cable from bank one that goes to the selector switch.
Apparently, I should have only one positive cable from one of the batteries to the fuse, with a short cable from one battery positive to the other battery positive terminal. Is this correct? If so, when I add a third battery to this bank, I should only have to run a cable from the new battery's positive terminal to the middle battery's positive terminal (which already has a cable to the first battery's positive terminal. Until I hear from you guys I will assume this is correct.
That's one way of doing it. The only reason I can see for doing it that way is they can use smaller, and generally cheaper, battery cable. Batteries in parallel double the current (batteries in series double the voltage) so the wire going to the fuse would have to be big enough to handle the current requirements of both batteries.
If you add a third battery, and the fuse post will support it, you can use a separate cable for the positive lead. If you try to cheat the distance and connect it to an existing battery terminal, you'll have to increase the size of the wire going from that battery to the fuse.
Basically, if you're going to connect 3 batteries to one fuse point, the battery closest to the fuse will have the largest wire size (and highest current passing through it), the next battery will have a smaller wire as it's carrying the current of two batteries, the last battery only has to carry the current load it's rated for.
The way my boat has the negative cables set up is they all link with each other and one then attaches to the main ground on the engine. Is this okay even if they are in separate banks?
That's one way of doing it, if the runs are short between batteries and the wiring is up to snuff. It sounds like they wanted to save some money and linked them all together. If that's so, then I'd check that the wire going to the main ground is big enough. If it's the same size as the one connecting the negative terminals together, I'd be a bit concerned. Voltage and current go out the positive terminal and back the negative, so I'd want the wiring to be identical in size.
There is some discussion of "star" grounds where each battery has a dedicated wire to a common ground point. This is supposed to help level out current loads.
Speaking of grounds, it's also highly suggested that all grounds for all loads go the the same ground point. This will help prevent ground loops which can be difficult to diagnose and fix. If you absolutely have to have another ground point then you should run the largest possible wire between the ships ground and this new ground. You want to minimize the voltage drops in the wiring as much as possible.
Going back to my original set up with both house batteries each having their own connection to the fuse, do I get any less power or capacity than if they are correctly wired in series?
I think you mean parallel if your description is correct. Not enough to worry about I think. Depending on the wire size and load, there can be an additional drop in voltage due to the resistance of the wire especially in high current loads. That's why most folks go for the biggest wire the opening, and wallet, will tolerate.
One last stupid one, the distance between the new third house battery and the original two house batteries is about 4-5' and will require a cable of about 8-9'. I am planning on going with #1 cable. It seems to follow that the negative cables that tie all the batteries together and to the main ground should also be #1. Am I on the right page?
Thank you in advance, Bill
I don't know the current capacity of the house or 3rd batteries but if they're the same, I'd probably go one size larger than the original house bank for both the positive and negative terminals.