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The purpose of anti-surge protection on jumper cables

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Active Member
I don't see what the big deal is. Whenever you release the starter motor -- whether jump-starting another car or not -- you're instantly going from upwards of 200 amps to closer to 0 amps. Even when you turn off your headlights, or fan, or any other high current device, especially an inductive one, aren't you also getting the exact same spikes, surges, kickbacks, and dumps?!?

I understand that you're throwing in an additional component with high impedance, but I don't think a healthy car knows or cares at all if there's another car attached to its electrical system.

That said, alternator manufacturers do warn that the warranties are void if alternators are installed on a vehicle with less than a fully charged battery. I've never seen one go bad, though, with such abuse. Like Diver said, it's always a good idea on a jump-start, to let the dead battery charge for a few minutes with the other car running before attempting to start the car.

I say the cables are a joke. Automobile electrical systems are designed for such spikes and electrical noise.


Well-Known Member
Most Helpful Member
Load dump is one of the most severe disturbances to protect for, because it lasts longer than the others, as much as 0.5 seconds. The inductive spikes are much shorter, so quite small capacitors or surge suppressors can protect against them.

Load dump is where the alternator is producing a lot of current and then the load decreases suddenly, and the current produced by the alternator has nowhere to go. The alternator field current can't be reduced quickly, so the alternator continues producing too much current until the field current dies down. The power is coming from the internal combustion engine.

The starter motor can't usually induce a load dump spike, because the starter motor and the alternator are never usually operating at the same time. A starter motor will turn the engine at 200 - 300 rpm, and when the engine runs, it will tick over at around 1000 rpm. The alternator won't really do anything at 300 rpm. The amount of current taken by the starter won't affect the size of the load dump, as any starter will take more current than the alternator will produce, and it's the alternator current that causes load dump.

Load dump happens when any load is turned off when the alternator is running, but normally the battery absorbs the current. That is another reason why the starter can't cause load dump. If the starter motor turns, the battery is good enough to prevent load dump.

Individual electronic devices in cars may be resistant to load dump, and some modern cars have central load dump suppression, which limits the voltage to 33 V or so. I think that is done by using zener diodes for the main alternator rectifiers. However, a lot of cars don't have that, and many electronic devices can't survive unsuppressed load dump.

The danger is when the battery is degraded. A really flat battery will have a higher resistance than a charged one. I've seen this were the current is monitored when charging a completely flat battery at constant voltage. The current will start lower and then rise as it charges. Once the battery gets to 20% of charge or so, the current will start decreasing.

I've also seen a battery that has been left flat for some time go just about completely open circuit when charged. We had a car that had been left, the battery voltage as 4 V or so. We jump started it, left it running for 45 minutes, then turned off and measured the battery voltage. It was less than 0.5 V. Also a friend of mine started a car in similar circumstances, and while driving, signalled to another driver by flicking his headlights off and on. As he turned the headlights back on, the car lost all electrical power for 10 seconds or so. The battery had failed, and the alternator couldn't increase it's output quickly when the headlights with cold filaments needed a lot of current. It was sort of a reverse load dump, a dip not a surge, but it wouldn't have happened with a working battery, and it showed that the battery was useless.

So with a degraded battery, load dump is a real danger. That doesn't mean that a suppressor in jump leads will be any use. The problems that I can see are:-
1) The suppressor is probably too small to do much good.
2) The user won't know which end to disconnect first, as the suppressor can only do any good to a car that it is connected to.
3) Once the jump leads are disconnected from both cars, the one with the bad battery is still susceptible to load dump.

I have once seen a car module damaged by load dump. It was car from the mid 1990s. The earth strap from engine to chassis had broken, but was touching some of the time. There were multiple load dumps when running as the strap disconnected and reconnected lots of times. Eventually the surge suppressor in the engine management computer failed and went short circuit, and the 5 A fuse feeding it blew. It was fixed quite cheaply by cutting away the suppressor and putting a good one in parallel, but with a good earth strap it would never be needed.


Active Member
In the real world most damage from jump starts is caused from people hooking one side of the cables up backwards. How well does this protect against that I think would be a better discussion.


New Member
Sorry, but I still don't think there is any significant voltage spike that can damage the electronics (which already have voltage suppression) can get past the battery.
The battery isn't that slow as a 100kHz sinewave corresponds to 5μs pulses.
μs, ns, or ps spikes on the power line are unlikely to have any effect on the electronics.

Still say it's snake oil. ;)
I'm not disputing the possibility it may be snake oil. My gut has always said it probably is. Just saying you have not provided evidence that it is. By evidence I mean actually logging the transients in the car electrical system and analysing them. You speak of 100 kHz sine waves as corresponding to 5μs pulses.

1) Firstly, I see no evidence that we're talking about 5μs pulses. You need to measure real data.
2) Transients are pretty much never sine waves at their source. Why would they be? They are usually modelled and tested using uni-polar spikes. Standards usually describe these test wave forms by peak magnitude, a 90% rise time and the time to decay to 30% of their peak. If you know anything about Fourier then you will appreciate that a 5μs spiky pulse has a lot of energy well above 100kHz! Again, this calls for real data.
3) While a battery may have low impedance to a repetitive sine wave, this is simply not true of short transients. These transients travel along cables at close to the speed of light and any sub nano second transient coming down the cable has a physical pulse "wave" length on the cable that is shorter than the typical spacing of car battery terminals. Read up on transmission lines. So it will arrive at the battery and actually continue on into the vehicle wiring before the battery impedance can even start to draw current to clamp it. That's what these TVS devices are intended to clamp. And why it is so important to know what transients we are defending against.
4) The common TVS assemblies such as posted earlier in this thread are pretty much crap as they are clearly not designed by someone competent in this stuff. The uncut length of wire on the TVS will let transients shorter than about 0.1 nano seconds sail past as they are only about 10 times shorter than the battery terminal spacing. In other words the design only offers maybe one decade of operation as a protection device. At the very least they should have employed a surface mount TVS diode to vastly improve performance. That alone tells me the designer was not experienced and adds to my snake oil suspicions. But they remain suspicions. We need real data on the duration and magnitude of these transients. Most especially their rise times.
5) Just as these "protected" cables have been designed by someone without appropriate knowledge or experience, so too have a lot of car electronic devices. After nearly 5 decades as an electronics engineer working in commercial and Defence electronic design I have to tell you that excellence in this area is rather rare. You seem to be happy to expect everything to be safely protected. I'd love to be so blissfully naive but on that I can absolutely differ!

For the record, I drive a diesel LandCruiser. I am cautious with my communications gear when using jumpers but I have zero risk of killing my truck as far as the engine or lighting are concerned. So no, I have no protection on my cables. If you want me to give you a jump start, that's fine - but your electronics are at your risk! :-^


New Member
Thanks KISS. That is useful. Data.

From the type of example conditions discussed in that document it seems they are not primarily encountering particularly fast transients which is where the 1.5KE series devices (such as used in the Matson unit) shine so well. Think lightning induction spikes. So maybe leaving the device wires a couple of centimetres long is not as critical a blunder as I was suggesting.

Bottom line is you need to know the transients you're trying to protect against. So maybe the Matson units can protect from these slower transients more than I initially thought. And maybe they are redundant duplication if the car maker has done their homework and there are no cheap and cheerful aftermarket electronics installed. Leaning towards the snake oil conclusion. And when I look at the component costs versus the product cost (mark-up) I lean further. Someone is getting rich!


New Member
If jumper cables have a surge protector how will they ever work. Isnt the point of using jumper cable to send a huge surge like 200 amps through the cables to start the other car. I would be scared a the surge protector would not let the other car start and you will be waiting for a half hour for the other battery to charge. Then you might as well stop calling them jumper cable and call them charging cables.


Well-Known Member
Most Helpful Member
protection on jumper cables
If you really want to protect your cables:
Don't forget the sun block SPF30. (on the cables)
Rub on some essential oils.
Always carry a raincoat.
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New Member
If jumper cables have a surge protector how will they ever work. Isnt the point of using jumper cable to send a huge surge like 200 amps through the cables to start the other car. I would be scared a the surge protector would not let the other car start and you will be waiting for a half hour for the other battery to charge. Then you might as well stop calling them jumper cable and call them charging cables.
They don't protect against a current surge (amps). They protect against a brief voltage surge that can potentially (pun noted) destroy electronic devices in either vehicle.


New Member
Yeah, you'll never know definitively what it's done to help you. Have I had electronics blow up in my home before installing surge suppression on my switchboard? Yes. On several occasions. Have I had electronics blow up in my home since installing surge suppression on my switchboard? No. Was that coincidence? No way of knowing definitively. What was the cost of damage? Many times higher than the protective device! User's call. In my case I have an extensive network of computer gear and the downtime impacted my ability to earn my income so I elected to install protection. Being both an electronics engineer and licensed electrical contractor I was in a position to do the installation quite cheaply.
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