I plan on doing this battery relocation to my 00 Civic to the driver side fender like pictured below. Have a few questions though.

1. What would be the best wire gauge size from the + positive battery post to alternator and starter ?

2.What would be the best wire gauge size from the + positive battery post to the fuse box ?

3. In the 2nd pic below, I see straight connections from the + positive battery post to fuse box, starter and alternator. Do I need to add any fuses in between for any of the connections ?

 

4 gauge wire for those should be plenty.
No fuses should be necessary.

Someone correct me if they feel otherwise but that's what I'd use.

 

I would add a fuse if you have any through penetrations (hole & grommet) where you could short to ground.
If you want to track that setup you will likely be required to add a cutoff switch since that is not the stock battery location. Might want to check your rulebook for your local track/sanctioning body

 

No penetrations through firewall or anything like that. Everything is staying in the engine bay. Maybe at the longest, 6 feet of wire from the + battery post to the fuse box. Its not a track car, just turbo B-series weekend warrior.

 

I'd run a fuse simply due to the additional length of hot wire you are adding and the fact that you are adding it outside of the crash structure. It won't hurt anything, make space for it right on top of or as near the battery as practicable. Use a breaker if you really want to get fancy.

Any particular reason you're adding the battery to the left side and not the right? It's adding a lot of extra cable run and increasing the LHS weight bias.

 

It’s a turbo car, so the cold pipes are in the way. Unfortunately I can’t do back door on both ends. Originally I was going to put the battery in the trunk , but that seems like a hassle more than anything. Im trying to keep things simple, so this seems like the better set up to me.

 

I’m just gonna put it in the trunk. I’m thinking of upgrading from the Civic battery (51R) to the Accord (47) battery. Besides being physically bigger and having more CCA’s, is there any other advantage or disadvantage to this ??

 

If you run it to the trunk then I'd go 2 gauge from the battery to wherever you put your post, and add some loom to it to make sure it doesn't rub/short anywhere. From the post you can run 4 gauge to your accessories and fuse box. And in this case now you'd definitely want a circuit breaker between the battery and the post, ideally placed somewhere fairly close to the battery. I'm using a 100A breaker in mine.

I doubt you'll see any difference changing the battery, personally I'd stick with the 51R, there are plenty of tray options available for the 51R.

 

I got this diagram from another post from a few years ago. This is what I'm going to follow. The old diagram had 100 AMP breaker, but after some reading it said to go bigger. So I'm going with the 200 AMP breaker just to be on the safe side. Also price difference isn't much. Where do you have your battery mounted ? I was thinking in the center of the trunk, but I also see people mounting it on the rear trunk passenger side. Luckily the previous owner of my car had a sound system. They had a custom trunk tray made out of wood installed.

 

I have a hatchback and was able to just barely fit the entire 51R battery + tray under the rear spare tire cover, so it's completely obscured in the interior.

200AMP breaker is overkill, go with the 100AMP as that will do it's job better, you want the breaker to flip before you start blowing fuses or shorting out other things. The diagram looks good and is exactly how I have mine setup.

 

With no inline fuses I run 25mm2(oversized 4awg) cable from battery to one pole on a kill switch, and from the same pole I run 25mm cable to the fuse box. From the other pole I run 25mm2 cable to the starter. By doing this I can remove the key from the kill switch which will disconnect power to the starter without disconnecting power to all other systems. It will be 5 years like this without problems.