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Discussion Starter #1
A few simple questions

How did Alfa (or how would a modern electrical engineer) route the B+ cable of the trunk mounted battery? Bringing power to the starter seems the easiest from a routing perspective. This would leave the alternator to starter wiring unaltered.

How is the engine block grounding handled? No change - leave the grounding strap in place and then run the battery ground to the chassis in the trunk?

I'm planning to use a 1/0 battery cable for the B+
 

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But Mad North-Northwest
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As in the cars that came with the trunk mounted battery, battery positive goes to the starter. Starter pulls a LOT of amps and you want the heaviest gauge cable going direct to the starter. Then you connect from there to the alternator (or to the junction box and then to the alternator).

Yes, as long as your engine grounding strap is in good shape you can just ground battery negative to chassis anywhere in the trunk.
 

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I concur with Tom.
Both of my Alfas have trunk mounted batteries. It is a better environment than under the hood and as 105/115 cars are light at the rear it improves the weight distribution.
 

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Discussion Starter #4
Thanks guys,that helps.

I mapped out a route for the B+ thru the cabin, sort of the mirror image of the bundle that goes from the fuse box to the trunk, thru the wheel well, thru the cabin, along the floor and then thru the firewall with a grommet, coming out not so far from the starter. I doubt ill get the 1/0 tucked under the stainless sill cap however, so i anticipate hiding it under the carpet. Sounds feasible?
 

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I have owned a couple of GTV's. I think that the cable went under the car, possibly exiting through a hole in the front of the trunk and following the fuel line into the engine bay.
 

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Discussion Starter #6
Do we know how much current the starter pulls? I believe the factory starter is ~.7HP = ~525w = ~40amps @ 13.5v? Is my math right here? Is there some "peak amps" on first cranking that differs from the operating amps the motor pulls once up to speed?

If I use a 50 amp load for the sake of argument that would suggest that the factory 4AWG ~5' cable, 60 amp max, has .12 volt drop.

To keep the drop close to that number when the cable is ~15' it seems I'd use 1/0 AWG?
 

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Trained (ex)Professional, , 1953-2018 RIP,
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Do we know how much current the starter pulls? I believe the factory starter is ~.7HP = ~525w = ~40amps @ 13.5v?
Unfortunately, that formula can not be used to calculate current draw of the starter (nor do I know if there is a formula).
However, the tech specs in the S3 Spider workshop manual for the 1.1HP starter show that the solenoid alone can pull 40 amps. Running current draw can reach 290 amps and locked armature draw can top 500 amps.
 

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But Mad North-Northwest
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Yeah, I'm sure the starter pulls a metric buttload of amps. Just see if somebody knows what gauge the cars with the stock rear-mounted battery used and go with that.

And just a warning: wherever you run the cable, be extra sure to add GOOD grommets/insulation/whatever to places where it passes through body openings. If the insulation rubs through on the mail positive cable you're going to have a REALLY bad day.
 

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I have a dc clamp on ammeter, 0 to 100A. It goes over range when I energize the starter, so I can tell you that it is over 100 amps.
 

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Welding Cable - Remy Battery

Battery Wiring

Battery hot wire is 2/0 (two-ought) welding cable, about 15-feet long. Welding cable has a tough heat-resistant jacket, and is extremely flexible for its size. This cable runs directly to the starter relay battery post on the starter. The lug at the starter, since it is exposed to water and dirt, is soldered on. The lug is close fitted to the welding cable. I flow rosin core solder into the lug, totally flooding the connection with solder. To keep the cable's insulation from melting during soldering, I wrap the cable insulation with a wet rag.

 
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