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I'm relocating the battery to the trunk and was wondering what size cable people had used. An "electrical minded" friend of mine suggested 25mm2 cable (not sure what that is in imperial), but it struggles as soon as the battery drops down in charge (mine sits for a couple of months between track days). Any ideas?

Cheers

Damn, you made me work on this one! But your friends suggestion sounds about right, maybe you could go up one size, but it seems close.

However I'm wondering, you mentioned going a couple of months without starting it, and I think that might be more the problem than the wiring. Do you actually disconnect the battery or just leave it hooked up? Is there any residuial draw like there would be with a street car, stereo, clock etc. Even the smallest drain over 2 months would be enough to drain a good battery. And if the battery isn't in great shape, 2 months would be enough to discharge it so it wouldn't start the car.

I think not disconnecting it was the problem. I usually do, but it's been in my garage rather than in the shed, so I've moved it every now and then. In the past, it's been OK as there's no electrics. However, I installed a GPS lap timer and it's always on when there's power (damn old fashioned cigarette lighter circuit!) and that's done it.

There's enough power to start it when I hooked it straight up at the front, I think there's just too much voltage drop in the long cable to crank it from the boot.

I've got a "trickle charger" now, so maybe I won't change the cables.

Cheers

Pete, I think your problem is the time between running the car, not the cable size. Hopefully the trickle charger will fix you up.

But for others thinking about battery relocation, on my first project I used welder cable. Second time I just bought a kit from a speed shop.

I have thought about it and accumulated some 6 gauge welding cable but unless you track your car (which I don't) is it really worth it ?
The rear strut brace/battery box installs look cool etc. but I would think it will be a bitch to get the battery in and out and reach for jumper cables if needed......worst being you loose cargo space for beer coolers !

thoughts ?

alotawatts wrote:I have thought about it and accumulated some 6 gauge welding cable but unless you track your car (which I don't) is it really worth it ?
The rear strut brace/battery box installs look cool etc. but I would think it will be a bitch to get the battery in and out and reach for jumper cables if needed......worst being you loose cargo space for beer coolers !

thoughts ?

Agreed, they look cool, but I wouldn't want to lose the trunk space. Regardless, I think #6 is too small, the 25MM he's running is very close to the nearly non existant #3 in AWG sizes.

I used a pos cable out of an e 36 car , works well and has the "factory look"I also used the e 36 jumper/juntion box, mounted on the firewall, makes it easy to jump off the car and to run cables
BD #112black and blue

My kit from Top End this past summer included 4 gauge cable.
I'm trusting that it's fine!
It is Anchor brand welding cable, and measures about 9mm in O.D.
bdigel, so far I don't see the e36 jumper/junction box on RealOEM.com.
Could you point me in the right direction? I think an underhood "hard" junction point would be the "right" thing to do.

I'll take pics and try to figure out how to post them this evening
BD #112 Black and blue

I have 70mm2 in my e12 with battery in trunk.

Hmmm...

I had success with an off-the-shelf ~1 metre battery cable (bought from Supercheap) running from starter motor to a cutout switch mounted in the centre console, and a standard battery cable from a 1950s Rover 90 (!) from the cutout switch to the boot (trunk) mounted battery. No problems with voltage drop, and the cutout switch (purchased from Revolution Racegear) prevents battery drain.
The Rover is a 111 inch wheelbase car with battery under rear seat. Come to think of it, my e34 Touring has the battery under the rear seat, I wonder how long that cable is...
Cheers,