big 3 upgrade on integra, fuse box?

Assuming that you will be using a split charging system, it will have its own provision, but then Oldspark is the Guru on split charging systems.

Well I was going to just wire batt positive to positive and then negative to negative with 0 gauge wire. I was told that there should be a fuse at least 12" away from each battery. So [battery 1] -> [fuse] <---------------> [fuse] <-[battery 2] (Second battery would be in trunk). This is the way I was told to do it, and the negative just runs straight through. I'm asking because it seems like you guys don't necessarily agree with how fuses should be used.

Never more than 4" from the battery. The further away the worst case. Your info as against our experience doesn't count for much. The negative doesn't run through, it goes to a local GOOD ground. What no splitting circuit?

Yet again I see all the perverted rules, or misunderstanding of particular rules in different conditions. EG - batteries should (or shall) be fused. Some of the caveats and exceptions: Not if it is physically secure (eg telco bus bar DC supplies; car starter cables, or SHORT runs). Short runs may not be "physically secure", but the chances of a short over that small distance (or melting wire hazard) are riskable. Fuses shall (obviously) be as close to the power source (battery) as practical. Hence consideration given to mounting hardware etc. EG - better to have a secure cable to a remote mounted fusebox than riskier fittings on top of or closer to the battery.

nstillmatic wrote: I was told that there should be a fuse at least 12" away from each battery

Why? Does that person suspect a gas risk? Aren't you using approved spark proof fuses & breakers? I was once told to drain my carbies before leaving my vehicles for long periods. Why? So that they could make money cleaning the resulting corrosion out of the carby. (Else by well intentioned people to gullible or brainless to know otherwise.) The only general rule I give to such advice is to only follow the advice given by the minority - never the majority. (Generally speaking that is!) There is no "general" fuse rule with specific distance. The general rule is "as close as possible" but "without being stupid".    EG - IMO your 200A fuse does NOT meet that rule. Your last pic probably shows a greater hazard because of that fuse. Either remove it, else add insulation etc to prevent fraying. But that length is IMO too long. A shorter smaller gauge probably has less resistance - definitely if run direct with no fuse. Ignore what you were told about a second battery. Yes - you have protection (fuses or breakers) at each end of the battery hot interconnection. From there it depends on what you want. If they are to be permanently connected in parallel, then you want matching batteries - ie, from the same batch and history with both batteries located together with matched +ve & -ve bridging connections and power taken from the +ve of one and the -ve of the other. Deviate from that and the batteries become less matched. But I never recommend parallel batteries unless in constant use. Why - because if one fails, both do. Your paralleled 5-year life-rated batteries may last 5 years or more, but if one fails within a few months, they both fail. (IE - a battery comprising 2 matched "monoblocks" has twice the failure probability of the single monoblock. Except that in the parallel case, the bad battery will destroy the good battery. [Whereas in series, no damage if not being charged.] And dare I mention acid, heat and thermal runaway?) Hence they should have a isolator. I use the charge light to control plain relays. If no charge light (circuit) is available, then voltage sensing isolators can be used. But typical alternator charge-light circuits win hands-down as battery isolators, and are cheaper and infinitely scalable to boot (in general, you merely add an ordinary on-off relay of your choosing). And as with advice, be wary of the hype & bullsh with the various "smart" battery isolators, charge splitters etc. I just bagged a particular splitter's advertisement for being a pile of bullsh (subject to recall or penalties if shown here). See here (Reply #13) on mp3car if interested. PS - I forgot to detail the fuse "not on top" explosion risk in case of gassing (hydrogen gas from any lead-acid battery - especially deprecated bateries; especially wet cells, but AGMs too). But I'll omit that. I don't want to drag up the common hype about connecting the +ve terminal first (because that's where the hydrogen is) (sic).

Okay, this is what I'm getting from what you're saying: - Get rid of 200a fuse, run alt straight to battery - (Obviously) shorten the wiring distance wherever possible - Don't use any fuses whatsoever for wiring 2 batteries (?) I appreciate your indepth responses, but it can be hard to decipher exactly what you think I should be doing. So I should be wiring both batteries together straight through without any fuses? I read your post on mp3car, what type of "plain" relay are you suggesting? I was looking at this: http://www.amazon.com/PAC-PAC200-Battery-Isolator-Relay/dp/B000CEBXRS That was merely an option though, I read that it's not necessary to use a relay at all so I wasn't dead set on adding one yet (if you have any input on that). Also, to howie: the reason why I said ground straight through is because the best ground is from the negative battery terminal; not all cars have a good ground resistance so the battery is generally a safer bet.

BTW, the 2 batteries that I will be wiring together are a yellowtop under the hood (as you can see) and a Kinetik HC2000 in the trunk. What is the best way to accomplish this?

I think both Howard and I suggested getting rid of that fuse. The main reason is that - in YOUR CASE - it is a hazard. Normally fuses are desirable, but I explained above when they are not needed or not preferred. I addition to that, I argue that they are not required between the battery and the alternator (other than the normal "cable protection from the power source" reasons. For your case, I am worried that you think I said to connect wire two batteries together WITHOUT fuses. How do you get that idea if you are placing one battery in the trunk? (Others please chime in if you too get that impression. Not that I have yet re-read my post.) Also can you name any cars that do not have a good ground resistance? What examples and situations have you witnessed? Yes - "plain relay" means heavy contacts (a switch) actuated by a solenoid. There is no other (complex) circuitry - no sensing, no delays, etc.

nstillmatic wrote: it can be hard to decipher exactly what you think I should be doing

That is because you have not defined what your are trying to accomplish. Until then, I can only give general answers to common situations. My general advice is to keep the standard cranking battery under the hood. Then a remote deep cycle or cranking battery (eg, in the boot) depending on what performance is desired.

Well this is for car audio and other minor electronics. What I have right now is a high output alternator installed + Big 3 done w/ 0 gauge wiring. The alt is connected to a deepcycle yellowtop battery under the hood with a 200a fuse as you can see. After reading all of your responses, I will be taking out the fuse and cut down the length of any wire if possible. The next step I'm planning on taking is installing a Kinetik HC2000 as an auxiliary battery in the trunk. I'll fuse both ends as close as possible to each positive terminal and then go with that PAC200 isolator in the middle I suppose. As for negative, I guess I'll just try to find the best spot I can find for ground around the trunk area. Does that sound about right? My only question I have now is what size fuse should I put on each end? (Obviously my choice of 200a between the alt and batt+ was not correct)

I don't know if your choice of 200A between the battery and alternator was incorrect. I just reckon its fitment was incorrect. As to inter-battery protection, the fuses are to protect the cable and isolator. EG - the PAC200 is 200A rated, hence a 200A fuse maximum. But if our cable is 150A, then 150A maximum. If you only need 100A, then 100A will do. I don't know whether the PAC200 is the right size - that depends on the drain at the remote end. IE - the inter-battery link should handle the maximum (average) current demand PLUS the recharge current for the remote battery. But yes, you are now getting the gist of it.... (Yay!) Now, how do you intend to control the relay? (IE - the battery isolator.) And maybe you prefer putting the yellow-top in the trunk and using an ordinary cranking battery for the engine - unless you have a reason for a yellow-top or AGM cranker, or money is no issue.

The cable I'm using is 250A maximum, so I'm thinking 200A at each end because of the isolator. I was going to control the relay with the 12v ignition wire, or do you have a better approach? And I already had a yellowtop before I decided to do any of these electrical upgrades. It's not quite as big as I wanted to use for my auxiliary so I opted to leave it for the engine (even though it's a little unnecessary). Also, just out of curiousity, if the max amperage of the wire exceeds whatever your alternator is able to put out do you even need any fuses? (Sorry if that's a dumb question)