NC miata trunk battery relocation - alternative solution (2024)

zfreak280

Join Date: Jun 2015

Location: shalimar, fl

Posts: 39

This is my first thread, so please forgive me if I screw up locating it and uploading images.

I plan on installing a turbo this summer and the turbo pipes were going to require relocating the battery. There are several solutions out there. Moto-east has a simple kit that requires buying a bracket from them, buying a 15 lb battery or smaller, and installing it sideways where the old battery was. Trackdog has a great kit for those looking to free up a lot of space, which requires locating an 11 lb battery to where the windshield wiper reservoir is. Begi and v8roadster have decent kits that move the stock battery to the trunk.

However, nobody offers a nice in-between solution. The moto-east and Trackdog kit both leave you with tiny batteries that have almost no reserve. God help you if you leave accidentally leave your interior light on over night. The Begi and v8roadster leave you with very little trunk space and zero weight savings. I didn't like any of the solutions because they either sacrificed precious battery reserve or a massive amount of trunk space. So I came up with a solution that uses the space behind the driver rear wheel well, but minimizes trunk space usage.

This is where the Begi kit installs the battery, but the battery can't go all the way up in the space. The NB batteries could fit in this space, but it is going to be rubbing up against the rear tail lights and fuel line cover. The biggest battery that I could find that would fit in this space is the Deka ETX-30L, which is a 20 lb AGM battery manufactured by East Penn. Braille, Duracell, Big Crank, and others all use this same exact battery in their AGM battery product lines. Since it is AGM, it practically has the same CCA and reserve as the larger and heavier wet cell batteries for the NC miata.

I put SAE terminals from Odyssey on the battery for a cleaner looking install and quick disconnecting. You can just as easily bolt your wires directly to the terminals if you want to save a few bucks.

I made the tray out of the cheap steel using a few pieces of wood, clamps, a hammer, and a drill. Simplest metal work you can do.

The tray can easily be bolted through the floor since there is nothing below this space to interfere with the install. I personally choose to put down a piece of rubber in the bottom of the tray to keep the battery from sliding around in the tray. I bought new J hooks to install the battery, but I think you can re-use the stock J-hooks for this. The stock hold down works perfect for this solution.

The ground wire can be bolted anywhere to the trunk floor. I do not recommend grounding anywhere else and I also recommend keeping the ground wire to 24 inches or less of 2-4 gauge ground cable, which can be found with terminal ends already installed at any automotive store. Make sure you sand down the bolting location to the raw steel to prevent poor grounding.

The power wiring was the hardest part. I used 12' for 2 gauge power wire for the positive side and ran the cable alongside the stock cable harness. I do not have a picture of it, but the power wire enters the engine bay through a hole punched into the wiring harness gasket on the firewall. I can post pictures of the installed 2 gauge cable if any of you want to see how to route the wire.

I cut 3' of wire off the stock power wire and spliced it into the 2 gauge slicer kit. You don't have to trim the stock power wire as much as I did. I was worried about voltage drop over such a long distance, so I got carried away trimming the cable.

Once I got the wiring done. I just needed to clean up the old ground wire in the engine bay (yes, I do recommend keeping the engine ground wire) and cutting the trunk panels to fit around the battery.

I did this install a couple of months ago and I am please to say that the conversion has been fantastic. There is no discernible voltage drop over the stock battery location, the battery hasn't budged a millimeter, I dropped 6 lbs without losing any reserve, I added 20 lbs to the trunk and removed 20 lbs off the front (which will help offset the weight of the turbo on the front), and I didn't lose any useful trunk space.

In closing, I hope this conversion can be found useful for those who are looking to move their battery, but don't want to lose battery capacity or an appreciable amount of trunk space.