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#4756722 - 05/14/18 09:35 AM Battery relocation wiring
Spetz Offline

Registered: 01/23/14
Posts: 426
Loc: Australia
Hi everyone,

I want to move my batter to the trunk.

Can the wiring be as simple as basically extending the +ive wiring and then grounding the battery in the trunk somewhere?

Can I use whatever gauge the +ive wiring at the battery terminal is?

Would the negative wire in the engine bay just be deleted or do I need to ground that as well?

The car is used for track so power load is low and I am doing this for weight distribution reasons
2003 Mitsubishi Diamante 6G74 3.5L. 5 Speed auto. 155,000 miles. Shell Helix Ultra 5W40 at 6k OCI

#4756776 - 05/14/18 10:16 AM Re: Battery relocation wiring [Re: Spetz]
VeryNoisyPoet Offline

Registered: 12/16/17
Posts: 349
Loc: Massachusetts
You will want to go up a few sizes on the wire gauge for the positive, since long run of wire = more resistance = less power to starter. Perhaps a 1 AWG (42mm^2) or larger. Use heavy duty terminals that crimp to the wire and attach with a big bolt.

Ground the battery in the trunk. Use the shortest cable you can get away with, though this doesn't need to be as huge as the positive cable. Size it the same as the original battery negative cable. Select a beefy bolt and use fine grit sandpaper to strip the contact area down to bare metal for good connection. You can delete the stretch of negative wire that used to connect directly to the battery up front, but make sure that the engine still has a proper ground wire connected to the frame.

Get enough plastic split loom to protect the positive cable all the way up through the car. This stuff is cheap on amazon.
Follow the factory wire harness if you can, using grommets and cable ties to hold the wire firmly in place so the it can't rub on anything sharp. There's no fuse and if that wire shorts to the frame, it will ruin your day.

Consider a battery disconnect switch that can quickly be used to kill power to the whole car in the event of a wreck or fire.

Get a properly sized battery box that bolts to the floor so there's no way for the battery to come loose. Drill a small hole in the trunk floor and get a tube for the battery vent so any gases or acid boil over is directed safely outside the car.
1998 V70 GLT / 192k miles [10w-30 Valvoline Syn w/ Maxlife Tech + Mann W917 filter]

"Use yah blinkah!"

"Be nice. The alternative involves an air horn and 7000 lumens of high beam."

#4756924 - 05/14/18 12:30 PM Re: Battery relocation wiring [Re: Spetz]
JHZR2 Offline

Registered: 12/14/02
Posts: 41975
Loc: New Jersey
Id get a clamp on meter and get a read for what is max cranking current.

Multiply that by 2.

Find a cable that gives you optimally <5%, but absolutely less than 10% voltage drop for its length at that current.

Ground to a well-welded body point near the battery.

You're probably talking 0-0000 ga cable, depending upon length of run, engine size, etc.

What's the rating of your starter? Another calculation would be starter power rating divided by 10V to give current; then add a bit of extra current as margin, and then calculate voltage drop on the line.

IIRC my 1991 BMW 318i had 00 for the run from battery to starter, and 0 for the body ground in the trunk.

#4757203 - 05/14/18 06:13 PM Re: Battery relocation wiring [Re: Spetz]
Donald Offline

Registered: 03/21/04
Posts: 20672
Loc: Upstate NY
Some people buy welding cable to use. I would not suggest you do it. Wiring chafe. Voltage drop. Venting.

I suppose it will stay cooler. But a wiring issue with no fuse on the main wires going to the starter.

Let me know when the vehicle burns from an electrical fire.

If the car manufacturer did it they would route it and protect it properly.
2015 Subaru Forester 2.5 engine/CVT
2015 Ford F250 w/Powerstroke
2016 Subaru Crosstrek CVT (wife's)

#4757598 - 05/15/18 07:31 AM Re: Battery relocation wiring [Re: Spetz]
Spetz Offline

Registered: 01/23/14
Posts: 426
Loc: Australia
Thanks everyone.

Should I add a big fuse right at the battery in case it shorts between the battery and the front of the car?

Also do I need to vent it as the battery is a sealed type AGM battery?
2003 Mitsubishi Diamante 6G74 3.5L. 5 Speed auto. 155,000 miles. Shell Helix Ultra 5W40 at 6k OCI

#4773014 - 05/31/18 07:39 AM Re: Battery relocation wiring [Re: Spetz]
SHOZ Offline

Registered: 06/28/03
Posts: 5873
Loc: Illinois
I've put a battery in the trunk on two cars. I just use THHW (water resistant) building wire. 3ga currently on my Gen Coupe, 2 gauge on my old Taurus V8. Starter draw is only 40 amps continuous on the Gen Coupe.

I use the split loom and snake it through the sill channel to the back seat.

I have a 150A 32V fuse at the battery. I also add a ground buss like used in a circuit breaker panel. The main ground I run back to the transmission. This worked noticeably better than the seat belt anchor bolt I was using as the starter spins faster grounded to the aluminum trans bolted to the aluminum engine block.

This picture does not show the ground running from the ground bus to the transmission.

2008 Hyundai Accent 1.6L 5 sp manual hatchback
2010 Hyundai Genesis Coupe Track 2L Turbo 6sp manual