I find this a strange problem but I can't find anything online about it.

Recently I purchased a few short USB cables (20-30cm), to use in my car's USB connector to charge my phone.

Strangely an older longer USB cable (1.2m), charges the phone fine (and the icon changes to show charging). With any of the short cables in the same USB slot no charging happens.

Both the long and the short cables are USB2 compatible.

I naturally assumed the short cables were faulty but connecting them to a PC or connecting to a high capacity external battery charging is successful using all the short cables and longer cables.

Hopefully someone here is able to give me an explanation of the problem and how I can overcome this and use a short cable where it is most appropriate.


2 Answers 2


This sounds extremely odd. I seriously doubt a usb cable can be too short to charge something., What is perhaps odd, but more likely, it sounds like an extremely odd incompatibility. But I wouldn't expect that with power. But if it is an incompatibility for some strange reason then by that I mean either your car adapter is picky, or your usb cable is picky. The picky device would be the nuisance. As car chargers and usb cables are cheap, you could try some short cables from another source, and a car charger from another source.

I have had incompatibility type problems with USB-Ps2 adaptors, and I chucked a whole load 'cos they worked well in one computer but not another. I kept the lot that worked in any computer.Tthey were dependent on data and power, I always assumed they were getting power and the incompatibility was maybe data related. Perhaps it can affect power though. In which case your incompatibility is less strange but still strange. All I can suggest is getting another car charger and short cables from another source.

I can't explain the incompatibility other than to say i've had what might be a similar incompatibility related to USB, and I think incompatibility is more likely than a diagnosis that short cables won't carry the power. It looks like you've diagnosed that it's not strictly one or the other (cable or car adapter) that is all bad.

So i'd troubleshoot for an incompatibility. Though that would require buying more of the cheap things. more car chargers more short usb cables (in both cases, from other sources). You might find a charger that works with some small usb cables and you might find a particular charger tends to work less than others with cables, or a particular charger works with almost any of the cables, and similarly with the cables. So then you can keep the ones that are lenient and chuck the ones that are strict. Such is the nature of incompatibilities based on my somewhat similar experience of USB-Ps2 adaptors (though I didn't chuck out the strict computers in favor of my lenient computers! I merely kept the lenient USB-Ps2 adaptors which worked with any of my computers even the strict ones).

You could go further and test with a multimeter, or you could see if a USB voltage meter (google that for a pic) gives you any clues. But that shouldn't be necessary as you know if you're getting power to your phone based on whether it charges.

  • Hi barlop Okay, so there's no 'standard' reason that I would be experiencing this. Actually the USB port is on the SatNav/USB/CD/DVD device. I have two different short cables from different sources and I already have another couple coming, as you've already suggested. I've tested all the cables using a recharge battery so I'm having thoughts it is to do with the USB extension installed in the car. More testing is next on the agenda. At least I have a better idea that it has nothing to do with length... Cheers
    – tropolite
    Dec 16, 2012 at 14:20
  • @tropolite if you want to do a simple experiment, you could also hack open any usb cable farm7.static.flickr.com/6194/6049933558_5374cbeddc.jpg the red wire is 5v, the black wire is GND. ignore the two data wires as you don't need them. You could hack at the USB cable from your car, pick out the red/5v wire and GND wire, and tie them to the ones on your short cable. As you can probably imagine it'd make no sense for the power to just stop! If it's not happening at the connector and it's the car adapter or the car adapter is playing a role.. I'm not clear what you have in your car..
    – barlop
    Dec 16, 2012 at 14:52
  • is it ipartsuk.com/wp-content/uploads/2012/05/car-usb.jpg (so usb socket there) or i.ebayimg.com/t/… (so usb cable coming from it) ?
    – barlop
    Dec 16, 2012 at 14:52
  • @tropolite While you may or may not find out precisely what is causing it, particularly if it's electronic weirdness(though it shouldn't be that weird if it's just 5V and GND!), you will be able to isolate the problem and eliminate it with troubleshooting, then you could post your own answer, better than mine, and accept it
    – barlop
    Dec 16, 2012 at 20:11

I have had the same problem when charging a battery pack. Long cables (> 12") work fine, but short cables (~ 3") don't work. Both cables are 'USB power cables' meaning they don't have the data wires in them.

Another symptom is that the problem does not happen when I charge from a computer USB port (PC). But the problem does happen when I charge from a 120V 'power cube' (plugged into the wall). The problem is consistent across 1.0 Amp and 2.0 Amp power cubes.

Another symptom is that the problem only happens on a battery pack with an 0.8 A input rating. It does not happen in the same configurations when I charge an MP3 player, which draws less than 0.1 A.

I'm an electrical engineer, and I believe that the problem has to do with the 'output impedance' of the computer port, versus the 'output impedance' of the power cube. Output impedance is an advanced term that would take too long to explain on this post (it's covered in about the third year in engineering school). However, the computer probably has a lower output impedance than the power cube, meaning it can instantaneously push more power. What is probably happening is that the battery is demanding more current because of the low impedance short cable, and is causing the power adapter to switch off.

Impedance is a difficult thing for non-engineers to understand, and probably isn't known to the people in China who make this equipment. From a technical perspective, it's pretty 'dodgy' stuff, and of poor quality.

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