Battery Ground Strap Spec for Optima Install, Please

MotoSook · 01:13 PM

This is for an Interstate Battery Optima Red Top, model 34.

I did a search and the only info I found is from a Wil Ferch post..but not definitive enough for my thick skull.

I know I'll need a 9 inch or so long strap and a custom plate to mount the Red Top.

I asked our electrical designer, and he said a number 6 or 4 cable will be adequate, 95 or 125 AMP capacity respectively (if memory is good). Having a cable rated for a higher current flow should not hurt, right?

So my question is what is the amp rating of the factory ground strap. I have a place that will make a strap on request, but they want a spec.

Anyone?

tops911 · 05-15-2003, 01:26 PM

I just used a bigger cable, you know bigger is better. So I found the right lenght but got the biggest gauge cable to be on the safe side. I did not realy answer your question, just telling what I did.

RoninLB · 05-15-2003, 01:31 PM

Quote:

Originally posted by tops911

I just used a bigger cable, you know bigger is better.

Ditto

Mike Feinstein · Mike Feinstein's Garage

I went with 2 guage. Factory is 4 I believe.

Jim Sims · 07:30 PM

Quote:

Originally posted by Souk
Having a cable rated for a higher current flow should not hurt, right?

As long as you don't mind adding unneeded weight. I don't think you'll gain much by exceeding the factory grounding cable gage. Far more important for minimum voltage drop and increased reliability (given the short length of the strap) are clean, tight and protected (dielectric grease) connections. Jim

Wil Ferch · 05-16-2003, 08:16 AM

Hi:
I'll bite....
Aftermarket is usually 4 gauge....if you look long and hard, you'll find a larger diameter ( 0/1 gauge) that I used for my Optima install, and more closely mimics the factory metric diameter. I think you need at least a 9" length, but I recall using something like 14-15 " and had it loop over the top of the battery in a very sanitary and professional looking "J" hoop. The "woven thread" type cables are usually equivalent to 4 gauge, so i staye away from that, even though it bends and contorts easily enough.
--- Wil Ferch

s_wilwerding · s_wilwerding's Garage

My dad had an old ground cable from his 914 - it works perfectly!

RoninLB · 05-16-2003, 09:34 AM

Quote:

Originally posted by Wil Ferch

The "woven thread" type cables are usually equivalent to 4 gauge,

Wil.. I don't have the hard info close. but the woven ground cable if a more efficent ground.. marine stores have various sizes.

Wil Ferch · 05-16-2003, 10:43 AM

RoninLB:
When it comes to electrical things.. and RoninLB speaks...I listen....

So...how is it more efficient? I would think cross-sectional area ( thickness) is the determining factor. The weave, I thought, simply made it easier to handle, lay-down, or bend.

Wil Ferch

RoninLB · 12:30 PM

Quote:

Originally posted by Wil Ferch

So...how is it more efficient?

I couldn't find a textbook answer..
But a ground strap induces less inductance into the circut.. or I could say the circut is cleaner.. and the more complicated the whole system the more critical it is that the voltage stays clean.. the DME, IMO, is the most sensitive to this.. audio is another sensitive area.. and these computer fuel/ignition systems is another electrical complicated area, possibly.. Warren could give a clearer description, for sure.. voltage drop at various chassis ground points makes the topic more fun.. and because the alternator's alternating current frequency impedance is very low, cleaning up a problem circut can be a PIA.. LOL.. I could spend the bucks on an EFI if I wanted.. but I want carbs, LOL......Ron

Bill Verburg · Bill Verburg's Garage

There are 2 commonly used methods for assigning wire size

AWG
SAE

4AWG is good for 136A continuous, 2AWG is good for 178A continuous, either in lengths up to 4'

Since current carrying capability is directly proportional to surface area, stranded has the advantage inthis area. In addition stranded is less affected by mechanical vibration.

There are several different classes of stranded wire. Type 3 is the most flexible and has 5 - 10 times as much Cu as type 2.

Another consideration is corrosion, the use of fully tinned wire and fully tinned ends is optimum in this regard. If tinned ends are not available dip the raw ends in solder.

So I would use type 3 tinned wire in 4AWG. That's what we use in our million + $, private yachts and smaller commercial vessels. this stuff is available at most Marine supply houses like West Marine

RoninLB · 05-16-2003, 04:37 PM

Quote:

Originally posted by Bill Verburg

So I would use type 3 tinned wire in 4AWG. That's what we use in our million + $, private yachts and smaller commercial vessels. this stuff is available at most Marine supply houses like West Marine

Hey Bill... I'm not an expert on marine electrics.. especially since the start of marine engine computers, which would drive me crazy.. but could you confirm that a 46ft. Viking w/ twin 3406 Cats uses 4AWG wire.. without the tower it should cost around $900k.. I kinda have a different prospective on this stuff.. but I've been out of the circut for awhile, LOL...............Ron

Bill Verburg · Bill Verburg's Garage

The problem w/ boats is that you generally have such long runs that you need much larger than usual wiring(we need to run 12AWG for a 12v 15W Tricolor/Anchor light on a 60' masthead) and often have 200+ amp busses and they are over designed. What I quoted was the ABYA/Lloyds/USCG spec for the wire. 4AWG 13/32" OD 21.15mm² .24 k/ft. In a 10' round trip 120A 12v circuit will have ~3% voltage drop. Wire run in engine spaces is downgraded 15% due to heat and wire is not bundled. Each installation is unique and will have it's own spec. Bigger installations often use 24v,36v or 48v to decrease the voltage losses.

I have to look it up, but a recent boat out the door was a 120' schooner with twin 300hp cats each w/ 2x210A alternators on each engine and a 15kw gen set which had a bus that ran the length of the boat. It used 4/0 for the DC circuit because of the length of the runs(220' round trip), Almost the full length of the boat, engines aft, gen set mid ships, and batteries scattered where ever they would fit in the bilges from far forward to behind the engines, ~100 x 6v batteries wired for 48volts house current and an inverter for 110/220v AC.

In any event for a 9" one way 12V run 4AWG is more than sufficient for our 100A batteries. its so cheap for such a short run that 2AWG can be used if you want some extra headroom.

Wil Ferch · 05-17-2003, 03:28 AM

Guys:
I think we're going over the deep end...Let me dig out my notes on the install....

--English 4 ga is approx 21 mm2 cross section ( Euro designation "25" in mm2)
--English 2 ga is approx 33.6 mm2 = "35"
-- English 1 ga is approx 42.4 mm2 = no direct Euro equivalent
--English 1/0 ga is approx 53.5 mm2 = "50"

So...I measure what came on the car originally. It was twisted, stranded copper at about 7-8 mm diameter...making it close to 40-50 mm2 in cross section.

From the available units in car parts stores, you could choose either 4 ga ( ~25 mm2) ...or 1 ga ( ~ 42 mm2)..

---So I chose the fatter 1 ga...totally insulated with tinned ends and proper end connections for battery post hook up and a blade/hole end to attach to the chassis. The one I bought was 15" long, but I believe a 9-10" would work. Matched or exceeded OEM diameter. No need to make this more complicated...it's the major trunk ground wire from the battery that carries a lot of current, and it's located in the forward portion of the car far away from delicate DME's , etc...

---Wil Ferch

pwd72s · 05-17-2003, 08:59 AM

Will should have no problems with this one...right guys?

jriera · 05-17-2003, 09:06 AM

If I recall crrectly I used the 1ga, picture attached

Sorry for the mess... one of this days I have to organize that mess of cables

lateapex911 · 05-17-2003, 09:42 AM

Souk-

Not sure about your year, but where are you mounting the batt? On my '73, I mounted my Optima in the right batt box, on it's side, with the pos post end protruding about 2". I used a braided cable because of it's inherent flexibility, it's better construction, and after years in high end audio, I had confidence in it's electrical characteristics. Plus, it just seemed "German"!

I think the advantage of the batt is it's small size, but higher cranking power (than the two small lead acid batts it replaced), the fact that it will never mess up the inside of my 911, and the ability to put weight where I wanted it. A total win-win-win.

My ground clamps right to the post above the box. I fabbed up some mods to the stock clamp, which mounts to the same post. If interested, I'll try post a picture, but I'm sure tons of guys here have done this a thousand times.....!

PS....thanks again for the t shirt...I'll wear it for the first time on June 3rd in a test day for my race car at Lime Rock!

911pcars · 05-17-2003, 10:00 AM

I sense we're obsessing too much about a battery ground cable - not that it's unimportant. After all, it does complete the circuit path for current flow.

Is there some electrical problem we can help with or is this a "nice-to-know" discussion on the intricacies of wire gauge sizes and voltage drop? Nothing wrong with that either.

Sherwood Lee
http://members.rennlist.org/911pcars