This is a photo of two automotive battery cables. On the left-hand cable there is a blue wire that goes between the red (positive) and black (negative). It obviously does not directly short the read and black terminals. Why is that blue wire there and what does it do?

 

Keeps the clamps together for convenience?

 

Well...that made me think...

 

Keeps the clamps together for convenience?

I like that answer, but personally I'd find it rather inconvenient to limit how far the cables can be separated since when jumping I always make the positive connection first on the battery then make the negative connection to bare metal, most commonly on the engine block if I can get a convenient spot. Having that "convenience" would be rather inconvenient.

As to the answer to this question: I don't know.

Would recommend testing for continuity between red and black, then reverse the test leads. Perhaps there's a sacrificial component with a diode to warn you of a reversed connection. But that's just a guess.

 

I finally received a photo of what that blue wire is connected to. Looks like a Metal Oxide Varistor, which sounds like it could serve a purpose yet not have an effect on normal operation of the jumper cables.


Mystery solved (the hard way : -)

 

Nice that the person taking the photos reserves the interesting part of the puzzle for a follow up photo.

 

The photos were taken at my requests about one week apart. The request to open up the cable was made after this thread did not firmly answer the question. Not everybody is schemer.

 

I finally received a photo of what that blue wire is connected to. Looks like a Metal Oxide Varistor, which sounds like it could serve a purpose yet not have an effect on normal operation of the jumper cables.

View attachment 275677
Mystery solved (the hard way : -)

OK, so what is the purpose?

 

Though I cannot imagine what circumstance would create the situation (jump starting a car?) the MOV would clamp the voltage across the jumper cable to whatever the MOV is rated at. Just guessing from the markings that is 20 volts.

 

Oh yeah, I can see that maintaining/clamping a battery/alternator source from another vehicle, or boost cart!
The source shouldn't be greater than 14.5v and that MOV is not going to save the boosted vehicle, should it go higher..

 

The only thing I can think of is that, perhaps, the reasoning is that there might be a short transient when the cable is connected due to the arc that is often created. But I can't help but wonder if it is either wishful thinking or was a marketing gimmick. Would be nice to see the original product packaging.

 

I suspect it maybe someone who thought it a good idea or just being a little over-optimistic.

 

When I saw the first image it looked like the blue wire was a direct short. Curious and having nothing to offer I started watching the thread. Ah OK now I see the component in there. I have to go with Max as to good idea but overly optimistic if the idea was to cap voltage.

Ron

 

I have seen a set of cables that had a red and green LED to let the user know if the polarity is correct. And I have come across, abandoned in a snowbank in a snowy parking lot, a set of aluminum wire jumper cables that appeared to have been melted through by having been connected backwards. The cables were about #10 aluminum wire so it must have been a few amps flowing to fuse it and open the circuit.

 

I have seen a set of cables that had a red and green LED to let the user know if the polarity is correct. And I have come across, abandoned in a snowbank in a snowy parking lot, a set of aluminum wire jumper cables that appeared to have been melted through by having been connected backwards. The cables were about #10 aluminum wire so it must have been a few amps flowing to fuse it and open the circuit.

That doesn't really explain the existence of the MOV in the OP?

 

The MOV could be provided to protect against the transient when the starter motor is switched off. That would raise the sell price while costing very little. And it would never fail because that spike is absorbed by the battery.

 

The MOV could be provided to protect against the transient when the starter motor is switched off. That would raise the sell price while costing very little. And it would never fail because that spike is absorbed by the battery.

Good luck with that.
What happens when the starter motor switches off normally with regular starting, . i.e. with out boosting?

 

Good luck with that.
What happens when the starter motor switches off normally with regular starting, . i.e. with out boosting?

Either there must be a little blue varistor on the car as well or the varistor on the cable isn't needed.

 

My point, which both missed, was that the varistor is not needed. It has no effect and nothing happens. But a little knowledge can be dangerous, and selling protection against a non-existent danger is easy money. THAT is the point that I was making.

 

I have a vestige pole in my yard that keeps evil spirits away. It works. I haven't been bothered by evil spirits in a long long long time.