Hi, good detailed, real-world, questions now on the nuts and bolts that we all deal with. Battery
placement is one of the toughest. I am struggling with that issue on my boat. I will have to compromise since I can't get 100% of what I want, so I am prioritizing what I need.
First, you absolutely need to be able to get to your batteries on a routine basis. You didn't say outright but it sounds like you have wet cell batteries. You will need (must) inspect the electrolyte/water levels in each cell of each battery
often. The level will drop as they are charged. The heavier the charging
the faster it will drop. You need to be able to look down in to each cell so you can tell what the level is, so stuffing some of them, or even part of one battery over in a corner where you can't see down in to the cell will ensure you either don't fill it enough, or you will overfill it trying to make sure it doesn't go dry. If the level gets below the top of the lead plates in a cell, it will start destroying that part of the cell and lessen the capacity of the battery.
If you overfill them you will end up having overflow of the excess to the top and then to the sides and bottoms, and then in to your bilge
or wherever it can go. It is highly corrosive - which is why you want to provide at least a pan underneath all the batteries. Inevitably, even if you correctly fill each cell each time, some acid will get out and you want to contain that. So a wood-only box/pan will not do. It must be impervious to acid.
You don't need a separate battery box
for each battery at all. You can put them in to one or into two boxes/pans no problem. Boxes help to secure the batteries so they can't slide around and move too much. You also absolutely need to secure the batteries so they can't move and get loose. Loose batteries will cause the cables
on the posts to loosen over time. And you don't want the batteries to fall out if the boat capsizes (no one expects to capsize
their boat but....). So you should strap the batteries down securely or use bars across them that are secured to something strong. There is a lot of mass to a battery and if it gets moving it can hurt something, including its own case, and then pull the cable off which might result in a fire.
Some people build their own battery boxes to custom fit with a wood frame that they epoxy
at least the bottoms and some part of the sides. Others use polypropylene store-bought boxes.
One other must is to provide some air space around and between the batteries. 1/4" between is great but anything is helpful. It helps keep them cooler and helps prevent thermal runaway. Heat is not a friend of your batteries. Which is one reason to keep them outside of the engine
room but many times that is the only place they can go. You also want ventilation holes if you use boxes, especially if you use a lid on the box. Big holes - like 1" or more and several. It won't weaken the polypro boxes very much. You also need to vent the hydrogen gas that batteries produce (where the water
goes as it splits in to hydrogen and oxygen). It is explosive if concentrated.
So - put them where you can see the cell electrolyte - often. Secure them. Provide air flow. Secure the cables
close to where they are put on the posts so they can't move and loosen. You can put two batteries away from the other two if needed - just follow the above.
A benefit of AGMs is that they are sealed so you don't have to monitor
the electrolyte level and they can be mounted on their sides. But they are expensive and you should be able to find a way to mount what you have (what I would do). My previous boat had two batteries where I could not get to the far cells on them. I could only check the electrolyte level with a mirror and that was hard and error-prone. And I couldn't fill them well.
Don't use the gas-station type filler that you press down and it "auto-fills" the cells. They will over fill over time. There are automatic filler systems but I have not used them. There are also some caps that recycle the gases and let you go longer without refilling the cells. Hydrocaps is one brand. But they need more height as they are put on the top of each cell.
My new boat which I am rewiring has three large 8D AGM
batteries. They weigh 155# each. That is definitely spleen busting weight. I want to replace them with either four or six 6-volt batteries - but - I don't think I can fit them in where the others are. Like your problem. I am still thinking it out hoping that I will get a brain-spark and an obvious solution will appear. But I think I will have to compromise - either less total capacity (smaller batteries or fewer batteries) or find another location (last resort).
Regarding cable size: that depends on how much current
you need to flow through the cables, i.e. what they will be used for. And then you fuse them so they won't melt and cause a fire if you use more or short them out. You asked about 50mm cables - is that diameter or mm-squared? And the size depends on the length of the cables - positive one way added to negative back. And the temperature rating of the cables. If the cables will go through the engine
room you should use wire rated at 105 deg C. And any cable should be fine-stranded and tinned. Good marine
grade wire. It's expensive but you only do it once. And use HD heat shrink at each terminal, the kind with adhesive
lining. It keeps the corrosive salt
and acid air out of the metal inside.
You can use 50 mm2 (our 1 gauge) cable if your round trip wire route
is 15 ft or less for 3% (critical systems lowest voltage drop - radios, radar
, inverters) or 50 ft or less for any uses not critical (lights, starting engines). But what I would routinely put on 400Ah batteries (yours) is 2/0 cable (70 mm2). It is bloody expensive though. And then fuse them for the wire size - less than or equal to 250 A for the 50mm2 and 300 or 350 for the 70mm2. You can put on smaller but not bigger, and these sizes already are more than the nominal rating for the wires. I doubt you will ever need more than 200A of current
though, but that depends on what your max sustained usage would be. If you had a huge alternator
, and/or big inverter
, then you might use that much. All this is for 105 degree C rated cable.
Good luck with the placement and boxes. Not an easy task sometimes. And none of this is cheap
. I have bought about $1000US of cable/terminals for my rewiring so far. Go with smaller (less expensive) sizes where you can but stay safe.