depami

Sean, I assume you meant you cut the shrink off the old one.

To me it doesn't look that bad. Is the circled portion chafing or heat damage?

It would be interesting to measure the voltage drop across it while cranking engine.

dr bob

FWIW, I went through a bit of diagnostic work looking at voltage drop in both the positive cable and the ground strap. No values jumped out. I replaced the positive cable based on what I misread as a significant drop. Turned out that the ground strap was the real problem even after it diagnosed ok.

danglerb

Some stuff its easier to replace than diagnose.

Landseer

The bat ground strap fits that category, now.
Many Rennlist data points.
I think its a central issue for the yellow S4 in Britain, too.
But the mechanic in charge of that car doesn't see it that way.

depami

Good point. I'll get some on order.

Alan

It is chemical damage - this area is corroded (oxidized) and usually not just on the outer surface.

The failure mechanism is interesting - the voltage across the ground strap can go from a few hundred millivolts to full battery voltage modulated only by physical flexing of the braid (such as you do when you remove & replace a battery - you can easily see how this leads to misdiagnosis....). This is not what you'd expect.


You can imagine scenarios

I changed the battery and all was good for a few days.... then

or I took out the battery and tested it - it was fine then when I put it back in
everything worked fine - for a while... repeat...

It can be very confusing and very counterintuitive - but very real across many cars...

=> Suspend all disbelief and replace anything suspect..

Alan

ROG100

Observations - interior lights seem brighter.
On tick over there was always the odd miss beat - smooth then a hiccup. That has gone.
The corrosion was deep into the braid on the old strap – the picture does not show this well.

ROG100

I have some spare straps if any local 928 owners want one.

Lizard928

Rog, add 2 to my order if you can!

depami

Thank you Alan, best explanation I’ve heard yet.

Presumably a large part of this “chemical damage” is caused by vapors from the battery and could be greatly reduce by proper external venting of the battery?

Also, this would indicate that the sheath over the cable is important to minimize vapor penetrating the cable and accelerating corrosion?

And, I would assume the engine block ground should NOT have a sheath thus providing the opportunity for moisture to evaporate rather than collect and cause corrosion?

I am done “testing” these cables. They will be replaced.

Landseer

Water induced corrosion for most I've seen, water collecting in the trunk on top of the ground bolt.


Charles Payne's was the only one I've seen inside the battery box.

depami

Maybe, but the corrosion of which Sean and Roger speak appears to be within the box.

Anyone else got an opinion on most common location/type of corrosion?

Landseer

One of many.

This car would cut out randomly.

One day it was running well. I parked for 5 minutes to pick up a kid. Couldn't move the seat. No power to car (3V or so, measured later).

It had just finally given up.

Alan

I've yet to see one in this state at the battery post end where it was likely to have been water induced - impossible to be sure - but cars showed no other signs of water damage. All stock straps I have ever examined (e.g. where the owner was willing to cut the cover open) have this oxidation to at least some degree...

I'm pretty sure its a battery acid vapor issue. Venting should help - but my car has had the battery properly vented for at least half its life and still had strap issues....?

Alan

Landseer

I've never concentrated on the battery end.
Maybe they were bad too.
Slightly different failure mechanism.

On the other hand, out here where it rains we have a lot of cars leaking at quarter windows, on to the (warped) inner quarters, and it collects right on the ground point if the car is a tad lower in back. Then it runs into the spare tire well (many of these cars us bottom feeders deal with are sans rear carpet) and often into the battery box because the seals are often bad or full of rocks and dirt.