elec help please .. which ground cable is best?

bkreigsr · 12-10-2006, 09:26 AM

Is there a difference in the electrical properties between the 3 cables in the picture?
My battery does not seem to hold a charge as long when I use the middle stranded+tinned+woven one. I can hold a charge fine with the bottom one (oem), but that one will not reach the neg on my optima, so I'm considering the top one (Pep Boys).
thanks in advance
Bill K

ljowdy · 12-10-2006, 09:36 AM

Most everyone skimps on the Negative cable

Personally, I go to a welding ship and buy the largest welding wire available and make my own ground and positive cable. Current must flow freely and a small ground restricts the electrons.

RoninLB · 12-10-2006, 11:41 AM

The center cable is the best design. Welding wire is second best and auto store cable the worst. 12v is direct current in auto books but it's really pulsed dc. The amount of strands affects inductance. This also applies to alternator grounds.

From a mechanical point the strap and welding wire is more flexible concerning install. If I see auto store battery ground wire on a 911 then I have an idea of what else I'll find.

I'd use 50-100% amp rating greater than factory supplied. I'd clean the tub lug, wire connectors, use dielectric, use no more than 4 connectors on the lug, and install the battery connector first because there is voltage resistance between connectors.

Your charging prob is another story.

I'm pretty sure jlowdy's electric's are beautiful.

all above a prejudicial rant.

ljowdy · 12-10-2006, 11:47 AM

Thank you for complementing my electrics???!!!! My customers think so.

bkreigsr · 12-10-2006, 04:08 PM

I think I solved the charging problem. I took the Optima to be tested and it failed. As soon as load was applies volts went down below starting ability. I was told I probably fried it when the alternator and regulator went south. Optimas turned out to be tricky devils. Mine showed 12+ volts when I tested at home, showed full charge on the charger, but would not handle a spike to the starter.

The answer on the braided, tinned ground strap surprised me. I would have thought less strands of lower gauge (read thicker)would be the hot ticket.
thanks
Bill K

RoninLB · 12-10-2006, 04:47 PM

Quote:

Originally posted by bkreigsr

Mine showed 12+ volts when I tested at home, showed full charge on the charger, but would not handle a spike to the starter.

kinda like a sponge is how the battery plates absorbs a charge. There remains a surface charge that hasn't been absorbed. Best to turn on your headlights for a few seconds and then test for v.

RoninLB · 12-10-2006, 04:52 PM

Quote:

Originally posted by bkreigsr

I would have thought less strands of lower gauge (read thicker)would be the hot ticket.

in theory voltage travels along the outside of the wire. If you buy into that then marine Anchor brand AWG and welding wire is nice.

Jim Sims · 12-10-2006, 07:31 PM

"in theory voltage travels along the outside of the wire. If you buy into that then marine Anchor brand AWG and welding wire is nice."

Not at the near DC frequencies found in the low voltage (12V) portion of the wiring system. During the time scales of these circuits the current easily penetrates through the full cross section of the ground cable even if it were a solid rod of copper. For example, at the common utility AC power frequency of around 50 Hz the skin depth in copper is still 10 mm. If low inductance were a goal in these circuits, one would not utilize body grounds through the sheet metal or a path through the transmission housing of all things (one would instead use something like a coaxial cable). I believe automotive wire is braided for flexibility and fatigue strength under vibration not for inductance or related high frequency purposes. Most grounding issues will be found to be due to poor contacting (high resistance) surfaces originating from loosening (vibration), oxidation and other corrosion. In very old braided cables that have been subject to significant vibration there is the possibility of increased resistance caused by individual wires work hardening and breaking (the ends of break separating) due to fatigue. Copper work hardening of itself will not increase copper electrical resistance enough that it would be noticeable in an automotive wiring system.

Walt Fricke · 12-10-2006, 11:12 PM

Thank you, Jim

I was cowering in fear of being slashed with Ronin's samuri sword if he saw what I have used as ground straps (or that he'd tear my car apart to find my other sins, which are many).

Especially the 12 gauge solid house wire in the track only car running from a valve cover stud to a (solid) motor mount bolt in place of the awkward to connect tranny to chassis strap (which I do use on my street/race car). If that breaks from fatigue I'll know it because I will see it (no issues after 10 years of track only use in any event, though perhaps the solid mounts themselves are ground enough back there).

I guess I thought that because like currents repel each other, surface current had something to do with how effective a conductor was, so more strands = more surface = better. That, for instance, is why sitting in your car is pretty safe in a lightning storm. That 6" of rubber under you has nothing to do with it at all, since the bolt has no trouble traveling a half a mile or so through the air. It is the Faraday cage you are sitting within that provides the protection (another reason not to ride a motorcycle - riders do occasionally get killed by lightning). But I see there is something called charge penetration that, given time, overcomes this repulsion. And in DC circuits it has plenty of time.

If I were to put my oscilliscope on my car's voltage with the engine running, what would I see (I have an old heavy scope, but not in my garage)? I thought the battery would make it a nice flat line, but does the three pole or whatever it is alternator impose some wiggling over that? Varies with RPM?

Walt

Jim Sims · 08:54 AM

All current flow starts on the surface of a conductor but will move inward from the surface, the effective depth (skin depth) depending upon the time scale (frequency), electrical conductivity and magnetic permeability of the wire. Metal car bodies and aircraft fuselages provide some lightening protection due to being "Faraday cages" (current flows on the outside surfaces during most of the rather quick lightening discharge) but one will note there is usually a qualifying statement to avoid contact with an interior metallic surface of the vehicle during a lightening storm.
If a live high voltage utility power cable falls on your metal car and the current penetrates through the paint there would be no place to safely contact the metal body. Yes, there will be an AC ripple component generated by the alternator and it's frequency will depend on alternator (engine speed) and number of poles the alternator has but again at typical engine speeds this isn't much of factor in deciding wire stranding - at 5kHz the skin depth in copper is still 1 mm (~.040").

randywebb · 11:39 AM

Yup.
This quote:
"in theory voltage travels along the outside of the wire. If you buy into that then marine Anchor brand AWG and welding wire is nice."
is either confusing current flow (voltage, per se, doesn't really travel, BTW) with the situation in a Faraday cage...

or maybe, it is conflating effects at radio frequencies "skin effect" with those like Jim says at low or no frequencies in our cars.

This used to be a common comment by audiophiles in high-end 'stereo' gear. Unless the amp is sending a lot of RF down the cables (which is bad for other reasons) there is nothing to be concerned about. Humans are not able to hear above 20kHz and only very young kids can even hear that high.

Bottom line - I like the cable with the biggest flat area on it to contact the car body.

randywebb · 12-11-2006, 11:37 AM

"Ronin's samuri sword"

That's what I used to think too! But he isn't a masterless, wandering 'killer' [samurai] (or maybe he is, but that isn't the source of the sig.)

it's really Ron in LB (Long Beach).

Wil Ferch · 11:53 AM

I'd go for the top design but I would insist on 0/1 gauge instead of the commonly found 4 gauge stuff......

- Wil

more--> Battery Ground Strap Spec for Optima Install, Please

bkreigsr · 12-11-2006, 02:23 PM

good discussion Wil. Thanks

general question: is there any advantage/disadvantage to the jacket in this application?
oem was bare thru the 80's no?
alt. and trans straps are also bare....must be a reason.

thanks all
Bill K

Bill Verburg · Bill Verburg's Garage

For a ground, insulation serves no purpose. The middle one is best because of largest surface area and full tin which prevents corrosion, even surface corrosion can be an issue.

We only use fully tinned wire in marine applications for this reason(the insurance and regulatory agencies also insist).

randywebb · 12-11-2006, 03:49 PM

useful & interesting, Bill...

Now -- why is it braided?

PS:
Here is some 'stuff' (I hesitate to say info) on Litz wire - another HiFi Fallacy -- not that it isn't useful in RF amps.

http://home.earthlink.net/~jimlux/hv/litz.htm

Bill Verburg · Bill Verburg's Garage

Quote:

Now -- why is it braided?

Increases the surface area, side affect it's more flexible and easier to work w/

RoninLB · 12-15-2006, 12:46 PM

Quote:

Originally posted by Jim Sims

Not at the near DC frequencies found in the low voltage (12V) portion of the wiring system. During the time scales of these circuits the current easily penetrates through the full cross section of the ground cable even if it were a solid rod of copper.

thx J

I was completely wrong on this one.

It took a while to sink in till I found out the ground nightmare Pierce makes may have over 30 computer grounds to the chassis without any ground straps.