Relocating a battery to the trunk requires a rigid mount, heavy-gauge cable routed through protected body panels, and—for sanctioned racing—an external cutoff switch and proper fire barrier.
Moving the battery to the trunk is a common weight-transfer mod for drag cars and a clean solution when an engine bay swap leaves no room for the factory battery. The job isn’t hard, but it has more safety requirements than most DIYers expect. These are the mounting rules, wiring steps, and common mistakes that decide whether the install is race-legal or a hazard.
What the NHRA Rules Require for a Trunk Battery
If the car will run at a sanctioned drag strip, the battery relocation must follow the NHRA rulebook. The battery cannot be relocated into the passenger compartment, period. In the trunk, you need either a rear firewall or an approved sealed battery box.
The firewall must be.024-inch steel or.032-inch aluminum, separating the battery from the driver and passenger area. The alternative is a sealed box made of.024-inch steel,.032-inch aluminum, or an NHRA-accepted poly box, and that box must be vented outside the body.
- Mounting: a minimum of two 3/8-inch-diameter bolts securing the battery to frame or frame structure.
- Cutoff switch: an electrical power cutoff mounted on the rearmost part of the vehicle, accessible from outside the body, wired on the positive side, and capable of stopping all electrical functions including ignition.
- Grounding: the negative cable connects to a clean, paint-free chassis point in the trunk.
These rules are written for competition use. Street legality varies by jurisdiction and inspection standards, so check local requirements if the car is daily-driven.
The Step-by-Step Installation Process
The core procedure is consistent across kits: disconnect the old battery, remove it, mount the tray in the trunk, route the cable, and secure everything. Two manufacturer guides detail the exact order.
Step 1: Prep the Trunk
Disconnect the vehicle’s battery first—always. Then remove the OEM battery and clean out the trunk area. One install guide has you pull the trunk carpet, deck, and styrofoam insert to expose bare sheet metal before drilling tray holes.
Step 2: Check What’s Behind the Metal
Before drilling a single hole, inspect the underside for gas lines, brake lines, or other critical components. A drill through a fuel line turns a simple mod into a fire. Another guide explicitly warns against mounting the battery over the gas tank or spare tire well.
Step 3: Mount the Tray or Box
Position the tray, mark the holes, and drill. One kit instructs bolting the battery box down with supplied washers and nuts; a second guide calls for a minimum of four 5/16-inch bolts for the tray. Either way, the mount must be rigid—a loose battery in the trunk is the most common failure this mod sees.
Step 4: Route the Positive Cable
The main power cable runs from the trunk to the engine bay through the body in protected conduit or loom. Where it passes through a firewall or floor, a rubber grommet is mandatory to stop the sheath from chafing against sharp metal edges. Anchor the cable at intervals so it cannot shift, and keep it clear of moving parts.
Step 5: Ground and Finish
Connect the negative cable to a clean, paint-free chassis or frame point in the trunk. Some guides also call for a separate engine-block-to-chassis ground strap to keep the electrical system continuous. Install the external cutoff switch per the NHRA configuration if the car will race.
Common Mistakes That Ruin a Battery Relocation
The failures here are predictable because they are visible in almost every bad install. Skip the grommet, and the cable eventually grounds out on the body. Mount over the gas tank, and a crash turns the trunk into a fire source.
- Leaving the battery loose in the trunk instead of bolting the tray down.
- Drilling blind without checking for fuel or brake lines below the sheet metal.
- Omitting a grommet where the cable passes through the firewall or floor.
- Skipping the cutoff switch on a car that will run at a sanctioned event.
- Using an unvented box when no metal firewall separates the trunk from the cabin.
One more safety note: never work on battery cables while they are connected, and double-check that the positive terminal is fully isolated from anything metal in the trunk. If you’re choosing a kit, our tested battery relocation kit roundup covers the mounts and boxes that actually hold up.
When done right, the install takes an afternoon and transforms weight distribution without compromising safety. When done wrong, it is a fire hazard with a racing disqualification attached. Follow the bolt sizes, the firewall rule, and the cable protection, and the car is both safer and faster.
FAQs
Do I need a cutoff switch for a street car?
Only if the car competes at a sanctioned event. NHRA rules require the external cutoff switch for relocated batteries at the track, but most street-vehicle inspections do not demand one. Local jurisdiction rules vary, so confirm with your state’s inspection standards before relying on it for daily use.
Can I keep the factory battery tray in the trunk?
The factory tray is designed for the engine bay’s vibration profile and mount points, not trunk sheet metal. Replacement trays and boxes use larger bolts and rigid framing to handle crash loads. Keeping the stock tray in the trunk usually means it is not secured well enough to satisfy NHRA rules.
Is a sealed battery box enough protection without a firewall?
Yes, if the box meets NHRA material standards and is vented outside the body. The rulebook accepts a sealed.024-inch steel,.032-inch aluminum, or NHRA-approved poly box as an alternative to the metal firewall, but the venting requirement is not optional—it must exit the car completely.
References & Sources
- NHRA Rulebook. “General Regulations: Battery Relocation and Electrical Systems.” Details firewall, mounting, and cutoff switch requirements for relocated batteries.