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Old 06-03-2008, 03:40 PM   #1
420WRX
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Default Common chassis ground for dual battery setup?

Take a look at this diagram, am I crazy to skip a proper ground wire run and using a chassis ground for dual batteries?
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Old 06-03-2008, 04:28 PM   #2
420WRX
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Oh yeah I had to add an 80amp inline fuse for the run between the isolator and the rear/2nd battery. Plus I had to find room for at least 1farad of caps between the main sub amp and the rear battery.

Last edited by 420WRX; 06-03-2008 at 04:49 PM. Reason: Updated the pic..
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Old 06-03-2008, 04:38 PM   #3
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gauge of ground wire: 4 gauge

gauge of chassis: 3000 pounds of metal

easy choice! a well done chassis ground is always better than a dedicated wire run of any gauge. Just curious as to why 2 gauge to the main battery, but only 4 gauge to the secondary? Wire requirements go up with distance, it would make more sense to run 4 gauge to the main, and 2 to the secondary. The isolator->main cable won't ever really have much current on it, the big draw is starting the vehicle (no alternator running then!), after that is just a couple amps to keep everything running.

I'd also ground your secondary battery with a 2 gauge since you will have that on hand anyway. Having a larger ground cable never hurts. And speaking of grounds, the alternator is already bolted to the engine, no real need to add a cable. I WOULD use a 2 gauge cable to ground the block to the chassis though, but I'm assuming you were probably going to do that anyway.
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Old 06-03-2008, 04:57 PM   #4
420WRX
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I was given a 24 foot run of marine grade oil/water proof 4ga that is better than the 18ft run of 6ga welding cable I had purchased before I got crazy and added a huge sub-amp. Good wire gets pricey so I'd like to use the 4ga if it's going to be sufficient.

As you mentioned, the front already uses 2ga from the battery because of the starter draw on the engine. The link to the alternator is probably 6ga stock, I'll likely swap that for a 4ga wire run when I add the isolator if I have leftover cable.

And yes, there will be upgraded engine to chassis grounds as part of this install.
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Old 06-04-2008, 09:27 AM   #5
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Quote:
Originally Posted by 420WRX View Post
I was given a 24 foot run of marine grade oil/water proof 4ga that is better than the 18ft run of 6ga welding cable I had purchased before I got crazy and added a huge sub-amp. Good wire gets pricey so I'd like to use the 4ga if it's going to be sufficient.
yep, understandable, I'd do the same myself if it was free!

Quote:
The link to the alternator is probably 6ga stock, I'll likely swap that for a 4ga wire run when I add the isolator if I have leftover cable.
I haven't checked, but I'm guessing it is more like 12 gauge. (just checked the math, 12 gauge would probably suffice for the stock alt, they might have used 8 gauge, but certainly no larger)
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Old 06-05-2008, 06:41 PM   #6
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Okay this weekend looks like the 'date' to do this..

Anyone ever run 4ga wire through a 2004 WRX firewall? Is there a good spot to do it, or a bad spot to drill (ick!)?
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Old 06-05-2008, 07:29 PM   #7
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I'm assuming the drivers side rubber plug exists on '04... look right above your gas pedal. Pop that plug out and then drill/cut a hole in the plug itself, no need to punch a new hole in the metal.
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Old 06-06-2008, 04:38 AM   #8
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Is it this plug?
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Old 06-06-2008, 09:22 AM   #9
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yep, looks like it. takes a little bit of fishing to find the cable on the other side, helps to remove the washer tank though.
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