Battery Box Relocation - 65-66 Mustang Fastback

Doing an EFI swap on an early model Mustang doesn't absolutely require relocating the battery, but it does make it easier and cleans up the engine compartment.  There are 4 basic options when it comes to the battery: 
  1. Do nothing, this leaves you with a cramped area to install your breather, and makes for lots of exposed wiring under the hood.
  2. Turn the battery box sideways (using a 69-70 battery tray), this works better for the air filter but still limits the space and the wiring is still there
  3. Relocated to the battery to driver's side front apron; this fits the original wiring scheme of the Fox EEC-IV harness. It still leaves all the wiring exposed, and you have to fabricate a new battery box and support structure, as well as relocate the overflow tank.
  4. Relocate the battery to the trunk.  This is the most visually appealing and provides the most room for the air filter.  The drawbacks are that you have to run extended cables and deal with a battery in the passenger compartment.

As for the latter, there are two choices.  You can purchase a sealed battery, such as an Optima, and mount in the truck using standard mounting hardware, or you can use a standard battery with a sealed battery box vented to the outside.  For this installation, I chose the latter.  

Trunk Mounting:  In order to mount the battery box on a level surface, The entire trunk was leveled using a 3/8" plywood. (Photo 1) Underneath the plywood, 1"x1" steel angle was used in 3 rows to brace over the fuel tank. (Photo 1) The plywood is cut in two pieces and spans the entire truck, including dropoff areas.  This allowed the relocation of the spare to the drivers side.  The spare tire hold-down hook runs through the plywood and through a hole in the angle iron.  The tire is secured to the hold-down. (Photo 4)

The battery box is secured to the using 1"x1" angle iron as footings. (Photo 5)   These are bolted through the plywood, through holes in the angle iron and secured using clipnuts.  To remove the battery box, the bolts are removed and the box slid forward. Inside the box, steel straps are mounted up the interior sides of the box and attach to angle brackets which secure the battery inside the box. (Photo 2) 

Photo 1 Photo 2 Photo 3 Photo 4 Photo 5
Cable Routing:  Three wires are routed from the aft battery box (Positive, Negative, and direct power to the vehicle).  The following series of photographs show the path for the positive cable. 

The positive cable runs from the battery box, under the rear seat (Photo 2), through the passenger door well, and into the panel behind the kickboard.  From here, a hole is drilled and the cable routed behind the fender up to the splash panel (Photo 3).  The cable then runs along the top of the fender apron till it reaches the old battery tray area. (Photo 4),  Here, the cable is routed into the engine bay and down to the starter solenoid, which has been relocated down to just above the framerail. (Photo 6).  At final installation, this cable run is covered with a large rubber hose that is cut and slides over all wiring protecting it from damage.  Rubber grommets secured with silicone adhesive are used in all areas where the cable passes through a panel.  Rubber lined adel clamps are used to secure the cable along the run.  In any locations where chaffing could occur, insulation is installed (Photo 2). The ends are swaged and soldered to the end of the cable.

The negative cable runs out of the battery box, and down through a rubber grommet in the trunk floor.  The end of the cable is soldered and swaged, and secured to the rear frame rail.  A clean surface and strong bond is needed to ensure a good vehicle ground.

The third wire is optional.  I chose to run an independent 8 gage wire from the positive post of the battery along the drivers side and up behind the dash in order to feed the vehicle's main power distribution system.  This replaced the original Black/Yellow wire that drew  vehicle power from the + post of the starter solenoid.   The reason this wire was added was to shorten the total wire length to the main fuse panel, and to avoid any spikes or voltage drops from picking up the power at the solenoid.  

Photo 1 Photo 2 Photo 3 Photo 4 Photo 5 Photo 6
The independent power cable routes to a new fuse panel, located behind the glove box door.  This panel feeds the EFI relays, and provides power to the vehicle's original fuse box.  

In Photo 6, the battery cable attaches to the positive side of the solenoid.  Also attached to the positive side are:

  • Power leads for the electric fan and headlight relays
  • Lead from the 175A MegaFuse (charging cable from the alternator)
  • Power lead to the starter.

This vehicle is using a late model PMGR starter, and has a solenoid located on the starter itself.  The positive cable routes to the starter, and a secondary cable also routes to the starter's solenoid.