A battery relocation from the engine bay to the trunk sounds simple enough - just move it and wire it up, right? Well, the cable sizing alone can be a headache, and then you have the fuse placement and ventilation needs on top of that. Plenty of S30 owners find themselves deep in forum threads around midnight, and they try to sort through contradictory opinions about wire gauge and mounting locations. The whole project can spiral pretty fast.
Skipping the installation steps or picking parts that are too small for your setup will cause problems. A cable with an insufficient gauge will lose voltage over just 8 feet of wire, which means your starter motor will crank more slowly when the weather gets hot. Fuses need to be positioned near the positive terminal - without the right fuse protection in place, even a minor short circuit can become a full-blown vehicle fire. Battery boxes need to be sealed correctly as well because hydrogen gas can accumulate inside an enclosed trunk space, and just one stray spark in that environment is all it takes to ignite it all.
That work pays off when you get the swap done right. When you move 40 pounds from in front of the front axle all the way back behind the rear wheels, it matters in how the S30 drives - it takes that notorious front-heavy balance and brings it much closer to neutral. The turn-in gets sharper, and you'll feel a genuine improvement in rear traction as you put the power down. Your engine bay also opens up quite a bit for upgrades like headers or better brakes, and the car just feels more stable and planted through the fast sweepers.
Let's talk about the steps to safely move your S30 battery to the trunk!
Table of Contents
How Battery Position Affects Your S30
When you move around 40 to 50 pounds from the front of your S30 to the back, it can make a massive difference in handling and balance. From the factory, these cars have a weight distribution of around 54% over the front wheels and 46% over the rear. Removing that heavy lead-acid battery from the engine bay and moving it back to the trunk will get that balance much closer to an even 50/50 split.
You'll feel the biggest difference when you're taking a corner. The front end is noticeably lighter, and it wants to pivot a lot more easily as you turn the wheel. Hard braking feels more stable, too, mainly because your front tires don't have to manage as much weight trying to push forward through the turn. The whole car settles into curves with way less effort from you, which makes everything feel more responsive and controlled.
Drag racers get a genuine edge from this setup. More weight over the rear wheels means much better traction right when you need it most - at the launch off the line. All that extra weight back there lets the car take off much harder with way less wheel spin to fight through.
The S30 came from the factory with a pretty decent balance already built into the design. This battery relocation mod takes that great foundation and just pushes the way the car corners one step further than where it was. We're not trying to fix a car with big problems here - we're refining something that already works quite well to make it respond even more directly to driver input.
Pick the Right Battery and Mount Spot
After you choose to move your battery to the trunk, the next choice on your list is the battery type. Lead-acid batteries have been the standard for decades, and they work just fine for most builds without causing any problems. Lithium batteries change the equation substantially, though. Most lithium batteries weigh between 15 and 20 pounds, which translates to another 30 to 40 pounds of total weight savings compared to what you'd get with a conventional lead-acid battery. For a weight reduction build, those numbers add up fast.
After you pick your battery type, the next choice you'll need to make is where to actually mount it. The driver-side rear spare tire well is probably the most common location for this type of build, and a few reasons explain why. For one, it puts your battery out of sight and tucked away where nobody will see it, and it also puts the weight down low in the chassis for better weight distribution and balance. You get to have your trunk space free for whatever else you want to put back there. Installation is usually pretty easy too - in most cases, you can bolt a battery tray or box right into the well without doing a lot of custom work or fabrication.
Another popular option is to mount your battery on the center of the trunk floor. This moves the weight closer to the middle of your car, and depending on your build goals, that middle position could be just what you need for better weight distribution. Just remember, though, that this spot takes a lot more fabrication work since you'll need to create a custom mount. It's also going to eat into your trunk space, which means less room for storage or other equipment.
When you choose where to put it and start making permanent changes, it's worth taking a look at what else is already back there. You might still have a spare tire in place, or maybe you already installed a fuel cell that's taking up a big part of the space. In either case, your battery placement options are going to be a bit more limited. Fuel pumps and other accessories might be in the trunk as well, so you want to make sure that your parts can fit together without any problems before you start drilling holes or welding in any brackets.
Route the Cable from Trunk to Engine
Cable choice will matter quite a bit for this particular installation. Your minimum should be 4 AWG wire, which can take care of the power load over that 8-foot run from your trunk to the engine bay. That said, 2 AWG welding cable tends to be the preferred option with experienced builders, and it does a better job at minimizing voltage drop. Voltage drop can be a problem with longer cable runs. All that resistance turns into heat, and when that happens, your starter doesn't get the full power it needs to turn over the engine like it should. Welding cable is actually a great option for this type of setup because the copper strands stay nice and flexible, and the insulation holds up much better compared to what you get with standard automotive wire.
Once you have that part figured out, the next step on your list is to map out the path the cable will take through your car. This part matters more. Make sure it stays away from the exhaust system and any sharp metal edges that could eventually chafe through the insulation. The transmission tunnel is a large source of heat, and the floor pan has all these stamped ridges built into it that will wear through the wire coating if the cable is pressed against them for long enough.
The cable needs to run through the interior of your car, and the whole point is to have it tucked away behind the panels where it won't get anywhere near moving parts. A lot of installers like to run it along the rocker panel first and then bring it up through the firewall from there. You can also run it under the carpet right down the center of the vehicle. In either case, what matters most is that you protect the wire from any damage and make sure it's secured in place properly. A cable that's left loose will bounce around as you drive, and all that vibration will eventually wear right through the protective jacket. You need grommets wherever the cable passes through the metal. You should add split loom or heat sleeve for extra protection against the heat near the engine or exhaust.
Install the Fuses and Ground Connections
That thick positive cable that runs back to your battery carries a massive amount of power at all times. If that cable ever gets damaged or shorts out against the metal body of your car, you need something in place that can stop all that electricity right then and there before it causes real damage. A high-amperage fuse (or sometimes a circuit breaker) takes care of this scenario and protects the rest of your electrical system.
You should mount this protection within 12 inches of the positive battery terminal. The exact placement actually matters quite a bit, and what matters is what it protects. This location covers nearly the entire length of the cable as it runs from the hood way back to the trunk. If anything goes wrong anywhere along that cable run, the fuse will blow in an instant and cut the power off. If you don't have it in the right place, a fire could start before anyone realizes there's even a problem.
For the negative cable, ground it to some clean bare metal near where the battery sits in the trunk. You'll have to sand away the paint in a small area until you can see the fresh metal underneath - that's what gives you a strong connection. This creates a ground point at the rear of the car, and your electrical system will depend on that connection.
You should run a dedicated ground cable forward to the engine block and chassis as well. This extra ground cable helps to balance out the electrical load and stops ground loops, which are responsible for all kinds of electrical gremlins in your system. Plenty of installers skip it, and then they spend hours trying to figure out why the car suddenly has weird charging problems.
Rear-end collisions are a real concern if you move your battery to the back of your vehicle. The battery ends up right in the crash zone where another car will make contact. If you don't have a fuse installed up front near the original battery location, a collision could damage that positive cable and create a live wire that just throws sparks around back there. The fuse is what prevents this dangerous situation.
The wiring phase will test your patience a bit, and this part just takes some time. Go through each connection and verify that everything is tight and locked down before you move on to the rest of the installation process.
How the Battery Box Keeps You Safe
The vent tube actually won't take that much effort to install. You'll run a small hose from the box outlet down to somewhere underneath the car, or out through the trunk floor if that makes more sense for your setup. The main point of this tube is to give any hydrogen gas that builds up inside the battery a path to escape to the outside air. This stops it from accumulating inside your trunk, where it could eventually turn into a real safety hazard.
On a very hot summer day, your trunk can become an oven pretty fast, and when it does, the battery will start to off-gas hydrogen. If that hydrogen doesn't have a way to escape, it just builds up as a small pocket of flammable gas with nowhere to go. A small spark from a loose connection or a bit of static electricity is all it would take to ignite it, and at that point, you would have a big problem on your hands.
A sealed and vented box gives you two layers of protection, and the two of them really matter to keep you safe. Any battery acid stays contained inside the box if you ever have a leak or spill issue. It's sealed - you sure don't want it to leak everywhere in your car. The vented part is what takes care of the gas problem automatically - it routes any fumes that build up straight out of your vehicle, so you never have to worry about what might accumulate behind the back seat over time. The installation is pretty simple too, and you can get it all done in under an hour as long as you have the right box and the tubing ready to go.
Build Your Dream Car
Relocating a battery to the trunk takes planning and a weekend of work. You'll have to plan out the cable routing, pick the correct fuse rating for your setup and build a ventilated box to house the battery safely. You can knock this out over a weekend with tools you already have in the garage. The hardest part is usually that firewall hole (you probably don't want to drill through your car). You'll also have to make sure that the cables stay protected and look clean as they run from the front to the back. Once everything is securely bolted down and your connections are tight, you get plenty of extra room under the hood. The weight distribution also gets better by just enough that you feel it when you're leaning hard on the car through corners.
I can't stress this enough - this isn't a project where you want to cut corners or rush through the main steps. You might have some cable in the garage that looks like it would do the job, or maybe you want to skip the main fuse to save a few dollars, or maybe skip the battery box entirely. Any one of these shortcuts can take what should be a safe upgrade and turn it into a legitimate safety hazard. A short circuit on an unprotected 2/0 cable can start a fire in seconds, and the hydrogen gas that accumulates in a closed trunk is extremely dangerous. The upside is that if you do this correctly, it doesn't add much to your budget or timeline, and all that extra planning you do up front will pay you dividends for the entire life of your car.
Since we're already on the subject of upgrades for your 240Z, 260Z or 280Z, at Skillard, we make a whole line of custom parts designed specifically for the Datsun S30 chassis. Whether you want a cleaner engine bay, better handling or a more modern interior feel, you'll find aluminum door cards, center consoles, bumpers, spoilers and plenty of other parts that'll help you get there. Check us out at Skillard.com to browse the full catalog and see what fits your build.
