Author Topic: USB cables used for charging  (Read 5854 times)

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Offline akis

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USB cables used for charging
« on: September 23, 2015, 06:29:47 am »
I have been using a USB doctor gadget that you insert in the USB charger and it shows the charging current. With this gadget I have seen that not all USB cables were created equal. For example my Samsung tablet charges at around 1.7A but only with a few USB cables. With other cables it drops down to 1A or even 0.3A. The same applies for my older Samsung tablet that uses a proprietory Samsung USB connector, with its own original cable it charges at 1.3A but with another one I bought on Amazon it drops down to 0.5A.

A USB cable is only 4 wires and I cannot understand how the cable would somehow "notify" the USB charger to allow more or less current. Unless there are tricks with very high frequencies involved that somehow can determine capacitive volumes between the 4 wires?
 

Online coppice

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Re: USB cables used for charging
« Reply #1 on: September 23, 2015, 06:39:09 am »
Some USB cables have microscopically thin conductors, generating enough resistance to ruin fast charging.... unless the fast charging mode involves a considerable step up in voltage. Extra resistance from dirty contacts won't help, but the super thin conductors (which might not even be copper in some cases) are the main culprit.
 

Offline slashguitar

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Re: USB cables used for charging
« Reply #2 on: September 23, 2015, 06:39:32 am »
The charging current varies because cheap cables use thin wires => high resistance.
When the voltage drop across the cable is too high the phone/tablet switches to a lower charging current in order to maintain the input voltage close to 5V
 

Offline akis

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Re: USB cables used for charging
« Reply #3 on: September 23, 2015, 09:59:11 am »
In many cases the currents required are less than 1A and I am sure we can carry one ampere even with a signle strand of less than hair thin copper. At least before it melts. There must be another explanation.
 

Offline ConKbot

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Re: USB cables used for charging
« Reply #4 on: September 23, 2015, 10:07:51 am »
 
In many cases the currents required are less than 1A and I am sure we can carry one ampere even with a signle strand of less than hair thin copper. At least before it melts. There must be another explanation.

The charging current varies because cheap cables use thin wires => high resistance.
When the voltage drop across the cable is too high the phone/tablet switches to a lower charging current in order to maintain the input voltage close to 5V

There's the explanation. Don't be surprised when your 2m self retracting usb cable can't charge a phone or tablet because it's made with tiny wires.
 

Online wraper

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Re: USB cables used for charging
« Reply #5 on: September 23, 2015, 10:11:43 am »
In many cases the currents required are less than 1A and I am sure we can carry one ampere even with a signle strand of less than hair thin copper. At least before it melts. There must be another explanation.
Yes it can carry this current, but when voltage drops from 5V to 3.5V at 1A over the cable (yes I have measured such voltage drop with crappy cables from ebay), phone detects that there is either crap cable or weak voltage source and drops the charging current.
 

Offline georges80

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Re: USB cables used for charging
« Reply #6 on: September 23, 2015, 04:21:57 pm »
The charging current is based on the cable and adapter capability.

You'll find that what Samsung in particular does vary the current drawn based on the cable - but not just due to its resistance (conductor thickness etc), but on how the ground shield is connected end for end. Some cables don't even have the shield and so the Samsung unit will charge at a much lower rate. If the shield is connected to the ground wire (the actual USB ground wire of the 4 USB wires) then it goes to a higher charging rate (based on the adapter capabilities as well).

The adapters also have the D+/D- interfaced differently (from no connections at all, to the more common pullup/pulldown of apple compatible chargers).

Buzz a few cables and you'll see that shield is connected differently (or not at all).

So, yes, if you want the fastest charging that your product is capable of doing, you'll need to choose an appropriate adapter AND cable.

cheers,
george.
 

Offline Fungus

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Re: USB cables used for charging
« Reply #7 on: September 23, 2015, 04:46:59 pm »
In many cases the currents required are less than 1A and I am sure we can carry one ampere even with a signle strand of less than hair thin copper.

Yes, but there will be a big voltage drop - you might only see 4V at the other end due to Ohm's Law.

Try passing some different currents along a thin cable. Put 5V at one end and measure the voltage across the other. The results might surprise you even at 'low' currents...
« Last Edit: September 23, 2015, 04:50:39 pm by Fungus »
 

Offline rr100

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Re: USB cables used for charging
« Reply #8 on: September 24, 2015, 08:25:54 am »
Charging over plain USB went from a very desirable and nice feature to something extremely hard to "get right" at higher currents. It isn't rocket science for consumer products to handle many amps in consumer products (think ATX power supplies for example) but the tiny cables and connectors from USB are quite hard to "get right". Especially when everybody tries to cut costs and the characteristics of a charger or cable aren't apparent to consumers.

Between chargers that brag about multiple 2+A outputs and have only at best a 1.5A capability, cables with huge resistance and devices that can take even 3A there are enough things to go wrong and multiply your charging time (keep in mind there are not only big tablets with USB charging but even x86 tablets, yes Atom based but in the end a quad core PC with decent graphics - for video decoding not games).

Some manufacturers already try to compensate the voltage drop by increasing the output voltage (5.1-5.2V are almost the norm)  - some go ahead and increase it based on current, some kind of "poor man 4-wire measurement without the 4 wires" - of course they can increase it too much or too little if the cable rezistance is wildly different compared with what they have - but still it is much simpler than "real" 4-wire. Frankly I'm quite sure a charger that gives about let's say 5.7V won't hurt anything and it should compensate the drop from most cables or cable/connector combinations.

On the bright side most today's devices (unlike some early USB-charged devices) will still charge even if the voltage they see drops - they'll back off and use less current maybe but still charge - slower. This is probably why there is no consumer push, every combination works even if maybe slower.
 

Offline rs20

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Re: USB cables used for charging
« Reply #9 on: September 24, 2015, 11:22:35 am »
The easy answer, as already stated, is that the cables are cheap and thin, the resistance is therefore high, there is voltage drop, and mobile phones behaving according to the MPPT standard accordingly reducing charging current.

The hard part: where do you buy good, thick cables from?
 

Offline cowana

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Re: USB cables used for charging
« Reply #10 on: September 24, 2015, 11:33:22 am »
The easy answer, as already stated, is that the cables are cheap and thin, the resistance is therefore high, there is voltage drop, and mobile phones behaving according to the MPPT standard accordingly reducing charging current.

I recently cut a cheapy ebay cable in half - while it was a decently sized thick cable, it was almost entirely PVC - the actual copper strands were tiny (and in the order of 2ohms/meter each). No way charging at any significant current could happen with that! Even with a 50cm cable, 0.5A at 2ohms round trip works out to 1v voltage drop and thus a 20% drop at the device...

The hard part: where do you buy good, thick cables from?

PortaPow 20AWG Fast Charge USB Cables (they also do a sync version too).  The only cable seller I could find that actually states the conductor size - with one of these, my phone charges several times faster than with 'brandless' cables. Not the cheapest, but good quality connectors and very little voltage drop over the length of the cable - highly recommended.

Andy
« Last Edit: September 24, 2015, 11:35:47 am by cowana »
 

Offline rr100

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Re: USB cables used for charging
« Reply #11 on: September 24, 2015, 01:07:44 pm »
Interesting cables from PortaPow - I was thinking, what a dumb idea to have a power cable that isn't also a sync cable ... but very often this can be in fact a good firewall feature if you use random ports to charge your phone from! Well, if you are hunting for power on a regular base (a lot on the move without your own car for example) you probably already have a "USB powerbank" and I assume most won't connect the data pins between computer and phone if you put it in between. Probably it would also act as some kind of overvoltage protection anyway (some actually brag about that and I've been able to test (not on purpose) the Xiaomi 10000 with 12V on the microusb charging port - indeed it just shuts down - they also claim reverse polarity protection).

Apart from that we probably need to have some list with decent chargers, maybe some that give anyway something over 5.5V (maybe have some voltage boost up to 5.8-6V proportional with the current?).

And why not have it together and have some decent (let's say 2.5-3A) charger with a fixed cable with 4-wire arrangement so whatever is dropped on the wire it is always compensated and you have something like 5.1-5.2V at the tip of the connector. Is there such thing?
 

Online Monkeh

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Re: USB cables used for charging
« Reply #12 on: September 24, 2015, 01:24:03 pm »
The hard part: where do you buy good, thick cables from?

PortaPow 20AWG Fast Charge USB Cables (they also do a sync version too).  The only cable seller I could find that actually states the conductor size - with one of these, my phone charges several times faster than with 'brandless' cables. Not the cheapest, but good quality connectors and very little voltage drop over the length of the cable - highly recommended.

Andy

Pricy.

http://www.amazon.co.uk/dp/B00NAN3O20

I use a set of these: http://www.amazon.co.uk/dp/B00WMCIH90
« Last Edit: September 24, 2015, 01:26:39 pm by Monkeh »
 

Online edpalmer42

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Re: USB cables used for charging
« Reply #13 on: September 24, 2015, 05:15:56 pm »
I recently bought an Asus T100 tablet.  The only way to charge the stupid thing is through the micro usb connector with the included cable that is very short.  I went looking through my stash of USB cables and was surprised to see that a lot of them state right on the cable what the wire guages are.  I've seen values from AWG 24 to AWG 28.  I've even seen a couple that tell you that there's one pair of each guage.

On some of the cables you may have to look *really* closely to see the labelling.

Ed
 

Offline DanielS

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Re: USB cables used for charging
« Reply #14 on: September 24, 2015, 09:37:46 pm »
I've even seen a couple that tell you that there's one pair of each guage.
Cables with different wire gauges for power and data are common enough when you shop specifically for higher current cables - there is no point in wasting copper on data wires that get little to no benefit from heavier gauges than #28 over a 10' USB cable run.
 

Online Monkeh

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Re: USB cables used for charging
« Reply #15 on: September 24, 2015, 09:40:14 pm »
I've even seen a couple that tell you that there's one pair of each guage.
Cables with different wire gauges for power and data are common enough when you shop specifically for higher current cables - there is no point in wasting copper on data wires that get little to no benefit from heavier gauges than #28 over a 10' USB cable run.

Always been common anyway. 24/28 is pretty common, I've got a few which have 22AWG.
 

Offline ralphd

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Re: USB cables used for charging
« Reply #16 on: September 25, 2015, 01:36:42 am »
The easy answer, as already stated, is that the cables are cheap and thin, the resistance is therefore high, there is voltage drop, and mobile phones behaving according to the MPPT standard accordingly reducing charging current.

The hard part: where do you buy good, thick cables from?
Sometimes I make my own.  7-10c per end for the connectors, and some 24awg wire from a spool of CAT5e.
Unthinking respect for authority is the greatest enemy of truth. Einstein
 

Offline rr100

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Re: USB cables used for charging
« Reply #17 on: September 25, 2015, 01:32:35 pm »
I recently bought an Asus T100 tablet.  The only way to charge the stupid thing is through the micro usb connector

Yea, I have a similar stupid thing from HP where the first charger was DOA and the replacement died after about 6 months. Despite the poor reliability I was somehow hoping they went for a 4-wire cable with remote sensing however they went only for 18AWG wire (x2) cable (yes, 18!). BUT they put only 2 cables inside and left the data pins out (not even shorted out somehow directly near the connector) so you can't charge a regular phone via this (in theory) very good USB charger!!!

I managed to open it, maybe I'll even try to fix it even if it won't be that easy to put it safely together in the end. If anything it looks decently overengineered and I can bet it actually gave up because of some protection that tripped when it shouldn't. Either that or the huge amount of white foam (well actually not foam, more like some harder hot glue) they put inside killed it for real (there's a TO-220 component almost completely encased in this white gunk).

By the way the tiny (under 1A) kindle charger (and I mean the 2nd gen or so small kindle without backlight, no Fire, etc) works just fine with the tablet, thank you very much...
 

Offline Warhawk

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Re: USB cables used for charging
« Reply #18 on: September 25, 2015, 09:44:16 pm »
I am happy with Nokia USB cables. I use them with various chargers and no problem whatsoever. Micro USB is CA-101 or CA-101D. Mini USB is DKE-2. I also like the way you can pack them up for trips. You can buy original cables super cheap in sale.

Tip: collect all your cheap usb cables (from ebay, dx, etc.) and throw them to a recycle bin. Keep only branded cables. It saves your time and nerves in future  8)

Offline SeanB

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Re: USB cables used for charging
« Reply #19 on: September 26, 2015, 06:48:24 am »
Samsung U6 cable, supplied with many Samsung smartphones, is also pretty good. I find the charge rate with my phone is 0.46A on USB, and I should look to see what it draws on the little wall wart, but have never used it.
 

Offline akis

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Re: USB cables used for charging
« Reply #20 on: September 26, 2015, 09:48:34 am »
The trick is to find long cables that can carry the current without dropping voltage. I bought these at 2m long  and they are fantastic carrying 1.7A no problems.

http://www.amazon.co.uk/gp/product/B010XWJBRW?psc=1&redirect=true&ref_=oh_aui_detailpage_o01_s00
 

Offline rr100

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Re: USB cables used for charging
« Reply #21 on: September 29, 2015, 11:03:55 am »
- 20AWG and not that expensive. Good find!
- 0.46A is nothing special
- Nokia cables are hit and miss if you don't know how old they are. Keep in mind they're coming from the times when Nokia phones wouldn't charge over USB but only over the round connector (that was quite a PITA but actually has a surprisingly nice and useful range of accepted voltages, look up the pdf "Nokia 2-mm DC Charging Interface Specification v1.2", this could solve easily the trouble we're having nowadays with USB). And they kept the same indicative for the cable so you don't know precisely how good it is. I have the smaller one but while it seemed a good idea at first it is a huge pain and almost impossible to use. If you have a very simple setup like a laptop on a table and the phone near it then it's fine but otherwise you just can't use it, it doesn't bend easily if you want to use a powerbank "back-to-back" with the phone, you can't find a place for the phone if you want to charge from mains, etc
- don't throw out the "no-name" cables. At a minimum cut them and see what's inside :-). You can also reuse both ends for various projects that don't need that much current (or no current at all if you want only data). Or just keep them as backups in the office, in the glove box, on your shelf to hand out to people you know won't return them, etc - most work even if slowly. Also many devices aren't so sensitive to voltage drops so you can delegate the "no-name" cables to that job if they work well.
 


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