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Mazdaspeed Protege battery relocation


A little background: in order to fit the IC core for the water-air IC setup I'm planning, I decided to relocate the battery to the trunk. After a little bit of planning and putzing around, I finished the project up last night. Here's a material list and a basic walkthrough...

-Sealed battery (i.e. Optima)
-25' of 0 gauge welding cable (I got mine here)
-Group 24 marine battery box (about $12 at an auto parts store; if you don't have a sealed battery, you will need a box that is sealed to the interior of the car, and vented out of the car somehow)
-150 amp circuit breaker (I used this one)
-Distribution block, preferably with both fused and unfused outputs (This is what I used)
-0 gauge ring terminals with both 5/16" and 3/8" holes.
-Battery terminals with ring terminal connections (Like this)
-electrical solder

Tools Needed
-Various metric sockets (specifically 10,11,12,14,17mm)
-Propane torch
-Large linesman pliars (or something specifically for cutting heavy cable)
-Bench-mounted vice
-Utility knife

First thing I did was pull up the carpet in the trunk look for a suitable pass-through for the main battery cable. I decided after nosing around the car for 30 minutes or so, that routing the cable under the car would be less hassle than going under the carpet and drilling a hole in the firewall. Right at the back of the spare tire area, there's a giant rubber grommet--perfect! I drilled a 1/2" hole through that, then pulled about 4 feet of the 0 gauge up through from the bottom; enough to locate the battery anywhere in the trunk.

From the top, and from the bottom

I decided to run the cable up along the drivers' side of the car. I went up over the rear crossmember, above the lip of the gas tank, and then along the brake lines, securing with zip-ties as often as necessary to ensure the cable doesn't sag. There are probably better ways to secure the cable, but didn't feel like drilling holes into the floorpan if I didn't have to.

Pics, going from the rear of the car to the front

Finally, I pulled the cable up along the firewall, making sure it was going to be clear of the axles and suspension. Sorry, no good pics except for where the cable ended up in the engine bay

Now that the power cable was in place, I decided to locate the ground wire. If you pull away the trim in the trunk, the bracket that holds the seats to the frame offers a nice heavy secure bolt to ground to. I used a wire brush to clean up the metal around the bolt and on the bracket itself. A ring terminal with a 3/8" hole was perfect.

I guess this would be a good place to point out why I listed solder and a propane torch in the tools/materials. Main reason is that securely crimping those large terminals is a PITA without the proper tool (which I don't have). An alternate method I read about requires that the wire attachment point be closed at the end, like this as opposed to open like this. Once you've stripped the end of the wire, you put the ring terminal in a vise with the wire opening pointing straight up, and fill the wire opening with bits of solder (like literally fill it). Then you torch the terminal until all the solder melts and jam the end of the wire in there. If done properly, it makes a damn good mechanical and electrical connection. Anyways, back to the walkthrough....

I then went back to the engine bay, removed the strut bar, unhooked the battery, removed it, and removed the battery tray

The distribution block I bought has three fused outputs and one unfused output. Given that the stock battery connection has a cable running to the fuse box, and a larger cable that I assume runs back to the starter/alternator, I ultimately decided it was fine (and simpler) to hook both of those to the unfused output. For lack of a good idea on where to permanently mount the distro block, I just zip-tied it to the fusebox lid.

Now back to the trunk to mount the circuit breaker and locate the battery box. I decided mounting the circuit breaker to the battery box was as good of an idea as any. I just drilled a couple 3/8" holes and used some spare machine screws to mount it.

The circuit breaker has 5/16" studs, so that's where the ring terminals with smaller holes come in handy. The main power cable gets attached to one of the terminals on the breaker, and then I cut a short piece of cable to run from the battery terminal to the breaker. Note the battery terminals I linked to use a 3/8" ring terminal

All I had left now was to mount the battery box. The one I bought just uses a couple of brackets that screw into the frame, and then a strap that holds the box to the brackets. Not an ideal solution, but it holds everything down tightly. I was deciding between two locations. Behind the passenger sidewheelwell, or up against the rear passenger side seat. Against the passenger seat probably gives a better weight distribution, but it would completely block off any storage with the seat folded down. So I put it in the nook behind the wheel well.

As a final test, I hooked up the negative terminal and turned the key. She fired right up! I was a little concerned that a 150 amp breaker wouldn't handle the current from the starter, but I saw a walkthrough on a subie, where only a 100 amp breaker was used, so I should be okay. The real test will be on a cold morning in the winter when the starter sees it's maximum load.

Here's my filthy dirty engine bay, sans battery! Oh, and a box the approximate size and location of the l-a IC core I'll be ordering soon

I've got a couple things I want to clean up, but they are lower priority now that everything is functional. Main thing is that I want to replace the main power and ground cables in the engine bay with something a bit beefier. I also think I want to eventually replace the battery box with something that uses a standard tie-down.
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Wow. Well done. I may contract your services for myself. My cable will be going in the car though, since I already know where to route the cable in my car.


is the word, beotch
Nice write-up Jay! Did you use anything to protect the insulation on the 0 gauge wire where it comes through to the trunk, so the plastic to metal contact doesn't rub through?...silicone caulk or a rubber grommet or anything?


Well, the pass-through in the trunk is a giant rubber grommet. Where it comes out on the underside of the car, that whole raised lip is all rubber, so I'm not concerned about that spot much.

I roughly checked to ensure that there were no obvious sharp surfaces that could rub through the insulation, but now that you mention it, I may pick up some plastic loom for the few areas where it's in contact with metal surfaces.


is the word, beotch
Couldn't hurt when dealing with something that could potentially cause some serious problems with the vehicle were it to rub through. Some loom taped in place so it won't slide around on you would probably work great, yeah.


SUDO Make me a SAMCH
The only two things i would add would be:

1) As already mentioned a wire loom would be a wise idea when running the cable outside especially as it makes the corners down from the trunk and up into the engine bay from the floorpan.

2) Not sure if it was mentioned, but if you are not using a sealed battery like an optima you need to make sure you get a sealed / vented battery box and have the vent tube installed to vent outside the car.

Looks great though and awesome writeup!


Ahh, thanks for reminding me about the loom... I still need to get some.

And +1 to the need for either a sealed battery, or a vented box. I will add that point to the write-up


SUDO Make me a SAMCH
One other thing I came across while researching this myself, If you are going to track the car and you do this I 'believe' you need an externally operated disconnect switch to pass tech inspection. Some just remove a tail light and insert the handle for said switch while at the track, but it's something to consider if you are planning to go.

Note: This is for 1/4 mile stuffs, I do not know if its required for Auto-X, hotlapping on road courses, etc.


Yes, you are correct about that. A lot of the battery relocation research I did talked about wiring in safety circuits and disconnects for track use.

I don't have any real plans to hit the track, but thanks for the reminder... It likely wouldn't have crossed my mind otherwise ;)

ZoomZoom Diva

New Member
One other thing I came across while researching this myself, If you are going to track the car and you do this I 'believe' you need an externally operated disconnect switch to pass tech inspection. Some just remove a tail light and insert the handle for said switch while at the track, but it's something to consider if you are planning to go.

Note: This is for 1/4 mile stuffs, I do not know if its required for Auto-X, hotlapping on road courses, etc.
As far as I know, it's only needed if your drag time is below a certain level and not needed for Auto-X or road courses (though track rules can vary). I know of plenty of Contour folk who have trunk batteries without outside cutoffs who track their cars.


I know for Autox, it must be secured properly (no zip ties or bungee cords, etc), but I'm not sure on a specific location or cutoff being required.