Jumper Leads

All of my vehicles have a set of jumper leads, for that day when you accidently leave the lights on, or someone else that needs a bit of spare energy.

The leads are usually purchased from the local retailer "where everyone gets a bargin" and are less than $20 and in a bag so that they keep the car clean.


I'm well aware that the insulation looks like it's designed for 1,000,00 volts, but bigger size always means better ... right

I know they aren't high quality but the other day I found out they were a little high in the resistance department.




Low Battery

I have a vehicle that gets very little use over the month.

It's an older 4WD that we have for winter sports, and towing the trailer or caravan.


Because of the age, when we purchased it there was no remote door opening, so I added one.

This unit doubles as an alarm, and well it really needs to go on a diet.


The unit is small enough, but the current consumption is bit high, and after a few weeks the battery is too low to start it.


Out came the jumper leads, a spare battery and ..... well suffice to say it wasn't going to happen.

At first I thought it was the battery, so I grabbed another, and same again.




I was a bit puzzled as I knew both batteries had plenty of charge, but despite a few minutes of trying, it wasn't going to start.

I know that these cheap jumper leads have more insulation than wire, but I wasn't ready for the quite warm clips at each end of the leads.





High Resistance

We've discussed how the current through a resistor results in a voltage.

Voltage x Current equals Watts, which produces heat across the resistance, so for the clips to get hot, there had to be resistance.


I checked the grip end was making good contact and a bit more checking showed the heat was closer to the junction of the cable.


The photo shows why there was a high resistance



Rather than properly crimping the wire, they have simply folded it underneath and crimped the insulation, and hoped that the wire makes contact.






The first job was to extract the wire from the clip.

A bit of prising and a pair of pliers helped open up the bent parts.



I measured the wire as 4mm diameter, and found some crimp lugs that the wire would fit into.


My smaller crimp tool is the old style where it forces a piece into the lug, while the newer version are hexagonal and squeeze the whole lug.

I decided that two punches would be the idea and regardless it was better than the original method.

The alternative is soldering, but this creates a hard transistion point, and when heated the joint can change and then the resistance increases.



After they were crimped, a quick brush with a wire brush made sure that any oxidation on the mating surface was removed.


I had to drill a 5mm hole in each clip, and then a 5mm screw, washer, lock washer and nut completed the task of fastening the lug to the clip.


This shows how little wire there is in the cable, and the marks from the origianl crimp method.





At this stage the battery is still charged, so there is no need to use these.

I did think of some other methods to prove the difference, but we're talking milli-ohms at each end.


This raised a few questions about testing something that is designed for 100-200 Amps.


I had thought of a load, and passing current through them.

Rather than try to measure the current, it would be better to measure the voltage across the load, and the leads.


Trying to compare it before and after, relies on being able to duplicate the connection of the clip to the source and load.

Since these are not bolted, then duplicating it is difficult.




So there were some technical challenges to both methods of proving the effectiveness of the fix.

Since I hadn't tested them prior to the fix, the final testing would need to be to measure the resistance.


In the end I have decided that my fix has to better than it was before, and when I next have a flat battery, I'll check how well they work.





I'm interested in hearing some ideas of how you might test these, given that they are not high quality, and we are trying to prove the termination issue has been fixed.