battery relocation

Drag racing: Theory suggests placement over or behind the rear axle for traction.  Placement in the bed is acceptable, as the height of the center of gravity is secondary to front-to-back location.

Road racing: Theory suggests placement near the centerline of the truck, both front-to-back and side-to-side.  For road racing, placement under the bed is more logical, as such a placement both helps to reduce the front weight bias and helps to lower the center of gravity.

The NHRA requires that any vehicle with a relocated battery must have a master disconnect switch installed on the exterior of the vehicle.  SCCA rules are reportedly similar.

The truck below is owned by Ed Chambers ("SVTED"):


While the battery placement is less-than-ideal for road racing (or hauling anything), this is fine for drag racing.  The switch and terminal placement on the roll pan looks great.  More photos of Ed's truck are here.

parts needed:

Battery cable:  I used #0 (sometimes called "1/0") welding cable, which is much more flexible and rugged than "battery cable."

The lugs are soldered, crimped, and shrink wrapped.  All exposed surfaces are covered in either TechFlex and/or corrugated tubing.

The front of the main power cable is covered with Thermo-tec heat shielding where it goes near the exhaust manifolds.

Master disconnect switch:  This is required by the NHRA (and most other sanctioning bodies) to be located at the rear of the vehicle.  The best location appears to be the rear bumper.  I chose Moroso's Super-Duty battery disconnect switch ($62 from Summit or Jeg's).  It is rated at 2,000 amps intermittent and 300 amps continuous use (stock alternator is 130 amps, Mr. Alternator makes a 200 amp version).

If you file down the locating pin, the switch fits perfectly in the trailer hitch ball hole.

The supplied "lightning bolt" decal does not appear to conform to all sanctioning body requirements.  The SCCA requires a blue decal like the one to the left.

Note:  I am not sure whether this switch will do the job alone.  If the alternator output is not shut down, the engine can keep running and fry the ECU.  Thus, there needs to be a relay installed (such as the Painless Wiring #10105) or the use of a double throw switch.  The Moroso is a double-throw switch, but it is rated for only 20A on the secondary circuit (300A continuous on the main circuit).  I am currently (no pun intended) researching this further.  See below.

Accessory terminals:  Since the terminals may no longer be accessible, accessory terminals allow access to the battery for charging, jumping, or use of 12 volt accessories.  I mounted them on an aluminum I-beam that's bolted on in place of the bumper.  They are accessible from .
Mount:  I used a drop-down hot rod box, mounted on the passenger side frame rail.  It's hard to tell where I placed it from these shots, but there is really only one place for it there, so you'll figure it out when you mock it up.  Also check out the ground cable shot below.



Other items required include some means of securing the wire to the chassis, terminal ends for securing the cable to the disconnect switch, an "on/off" decal (included with the Moroso switch), and anything needed to prevent chafing of the wires where they pass through any bulkheads.

wire size:

The factors involved in cable selection are electrical resistance versus weight and cost:

AWG Ohms per foot lb per foot
00 .0000811 .411
0 .000102 .326
1 .000129 .258
2 .000162 .205

Source: Marks' Standard Handbook for Mechanical Engineers (8th ed.).  Stranded cable.  Resistance is at 77 degrees F.  Wire resistance increases with temperature. For the normal operating range of a battery cable the resistance increases around 0.21% per degree F.

Assuming a 24-foot round trip, the voltage from a fully charged (12.6v) battery will drop as follows:

Cable Gauge Voltage Drop @ 400 AMPS Weight Of Cable Approx. Initial Voltage at Starter
00 .79 9.9 11.6
0 .98 7.8 11.4
1 1.24 6.2 11.2
2 1.56 4.9 10.8

Note that the actual voltage drop will be slightly more, as this only includes the positive line--there will also be some loss from the ground line return from through the chassis.

While relocating the battery can improve the front/rear weight distribution a full percentage point, unless you are really serious about your truck (or need the underhood space for a cooler or intercooler fluid reservoir), I don't think the time would be worth it.

switch installation:

From the NHRA rulebook (others similar):

Master cutoff switch is mandatory when battery is relocated. An electrical power cutoff switch (only one) must be installed in the REARMOST part of each vehicle and be easily accessible from the outside of the car body. The cutoff switch must be connected to the POSITIVE side of the electrical system and must stop all electrical functions.  The off position must be clearly indicated with the word OFF.  If the switch is push/pull type, push must be the action of shutting off the electrical system, pull to turn on.  Any rods of cables used to activate the switch must be minimum 1/8 inch diameter.  Plastic or keyed switches prohibited.

The image to the right shows the brilliant idea from Joe Messana ("RTKILLA") of hiding the master disconnect switch behind the license plate.  Perfect!  Just remove the license plate at the track.  This will also keep punks from flipping the switch when you are parked.

I am copying this idea, mounting the switch behind a flip-down license plate in my roll pan.


I still haven't completely figured out how the wire the thing up properly.  The alternator cutoff has me stumped.  From Sal at PowerSurge Performance:

You cannot simply hook your positive battery cable to the cut off switch. The truck will continue to run if you disconnect the battery. You must also tap into the power to the PCM relay to be cut off as well, from the same switch, so the truck will stop running when the switch is used.

From Johnny Lightning:

1.  Run a # 1 cable from the ON side of the switch up to the original battery cable that hooks to the solenoid on the fire wall.

2. Unhook the alternator cable from the solenoid on the fire wall and hook to a #2 wire and ran it back to the battery side of the switch.

3.  Take a #1 wire and run from the battery positive side to the battery side of the switch same side as the alternator wire.

4.  Run a #2 wire from the negative side of the battery to a good ground on the frame (make sure you clean the paint etc. away before connecting the ground to the frame!)

5.  Up front I just cleaned a spot on the frame of the truck and I connected the ground wire to the frame.  Keep in mind I didn't cut ANY factory wires because I might want to put it back to stock again one day.

To the right is a copy of the starting and charging system wiring diagram from the Haynes manual (click on image for larger version).

This is the approach I am taking.  The legend is as follows:

alternator kill relay = Painless Performance High-Amp Alternator Relay (part # 50105, shown below)

Martin's diagram to the right is another representation of the same approach (we actually developed it together, along with some assistance from Moroso).

Martin's diagram detail of the master disconnect.




The Moroso switch has two small terminals and two large ones.  The small circuit is used to provide triggering power to the alternator relay.  It must be used with a switched source -- otherwise it will drain your battery when the truck is sitting.

The alternator relay is spliced into the existing alternator-to-starter solenoid cable.  The photo to the left shows Martin's neat mounting idea.








Copper cable lugs are used on the 1- and 2-guage cable.  Strip the insulation from the wire.  Place the lug in a vise, cable end up.  Using a propane torch, heat the copper lug and fill with electrical solder until the lug is almost full.  Keeping heat on the lug, insert the stripped end of the welding cable into the copper lug. Continue applying heat until the solder wicks up into the cable.  Although the lugs can be crimped, this method guarantees a superior electrical connection that can't pull loose.  Allow to cool. Cover with heat shrink tubing.

For the main connections to the existing battery cable, I used marine terminals, bolted to the right side fender well.  They will be covered by an aluminum plate.

Martin's approach is much more artistic.


I wanted a nice, solid main ground, so I made up a cable and connected it to a large bolt just to the right (passenger side) of the bottom of the radiator.  If you print this picture out and take it to the truck, you will find it.  Note that the intercooler tank has been moved out of the way in this photo.
For the grounds at the battery box, I made two cables.  For the battery box to fit where I installed it, the main frame-to-bed ground cable had to be removed.

This shot shows how I routed the main cable under the cab.


This shot shows the zip tie stays that I used to run the cable under horizontal surfaces.



The master disconnect hides behind a flip-down license plate in my rol pan.  I was extra careful with the wiring back to the disconnect.  In addition to lots of clamps, I covered the cables with corrugated tubing to prevent chafing.  The cables are required to pass in/out/over various places on the frame.  Rubbing through the wiring is a concern.

Here is my last issue -- what to do with the little wires shown below.  If the relay is placed between the alternator and the solenoid lugs, it would not stop the power coming from the alternator through these little wires.  According to the charging system wiring diagram above, it appears that this is the light green/red wire which goes to the charging system indicator.  Could this feed back to the PCM?

note on weight:

Relocation of the battery can impose a substantial weight penalty.  Take the already heavy battery and add 10 pounds of cable and a 2-pound mount, and the weight of the other parts, and the weight penalty could be as much as 15 pounds.  This penalty can be offset with the substitution of a lightweight racing battery, such as an Optima, Odyssey,  or Holley Annihilator.  The net weight may actually be less after substitution of one of the above batteries.

back to weight page

08/21/2007 Tim Skelton