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View Poll Results: Best Cable Length vs. Charge Rate
1.5 ft or less 2 66.67%
3 ft 0 0%
6 ft 0 0%
10 ft 0 0%
greater than 10 ft 1 33.33%
Voters: 3. You may not vote on this poll

[Q] USB Cable Length and Charge Rate?

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bmather9
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Default [Q] USB Cable Length and Charge Rate?

So I'm looking to purchase a slew of new chargers for myself and my wife for at home, office in the car, etc... She has a Galaxy S4 and Ipad3; I have Galaxy S3 and Galaxy Note 10.1 2014. I'm at least somewhat aware of the charger requirements and plan to supply at least 2 amps to each device in general.

My real confusion is in the USB cable length. It seems most stock cables are roughly 3ft long and from what I can tell, going with a longer cable increases resistance, causes a voltage drop and lowers the current/charge rate. Of course a higher gauge cable will help decrease this resistance, but I'm already planning ot go with 24AWG cables.

So my question is, how long of a USB cable can I use before it has a noticable effect on my charge rate? If anything over 3ft significantly slows it down, then I'll probably just use AC extension cables instead of longer USB cables when necessary. What cable length vs charge rate do you find acceptable?

Thanks in advance!
 
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I have a 10 foot cord that I got from mobstub and I noticed a significant increase in the time it takes to charge. I'm not sure how much degraded it is but if there's an app or something that can show me that I'll tell you the stats.

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mjculross
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Default low voltage / low current

I don't have any empirical data to back it up, but at 5VDC & 2A or less (typically), I would not expect much of a drop in charge voltage/current due to length (10 ft or less). That's just a gut feel from building lots of cables & electronic assemblies for the fun of it !!

Good luck & have fun !!

Mark J Culross
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-thorian-
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Quote:
Originally Posted by mjculross View Post
I don't have any empirical data to back it up, but at 5VDC & 2A or less (typically), I would not expect much of a drop in charge voltage/current due to length (10 ft or less). That's just a gut feel from building lots of cables & electronic assemblies for the fun of it !!
KD5RXT
If your cables are thin - don't even expect to deliver 2000mA on them. The thickness is the key here.
 
TimeTomorrow
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Quote:
Originally Posted by bmather9 View Post
So I'm looking to purchase a slew of new chargers for myself and my wife for at home, office in the car, etc... She has a Galaxy S4 and Ipad3; I have Galaxy S3 and Galaxy Note 10.1 2014. I'm at least somewhat aware of the charger requirements and plan to supply at least 2 amps to each device in general.

My real confusion is in the USB cable length. It seems most stock cables are roughly 3ft long and from what I can tell, going with a longer cable increases resistance, causes a voltage drop and lowers the current/charge rate. Of course a higher gauge cable will help decrease this resistance, but I'm already planning ot go with 24AWG cables.

So my question is, how long of a USB cable can I use before it has a noticable effect on my charge rate? If anything over 3ft significantly slows it down, then I'll probably just use AC extension cables instead of longer USB cables when necessary. What cable length vs charge rate do you find acceptable?

Thanks in advance!
At the voltages, gauges, and distances we are talking here, you are way over thinking this. It will certainly be fine at least up until the USB data transmission cable limit of 5 meters.
 
bmather9
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http://forum.xda-developers.com/show....php?t=2545497

According to the results from this other thread the wire length really seems to make a drastic difference.

I've heard quite a few people say that voltage drop will be minimal, but also heard of people being unable to charge their ipad 3 with a 10 ft cable.

I'd really like to use longer cables since they are generally more convenient to use the phone while plugged in, but the charge rate is also very important. So I'd like to get a feel as to what I'd be sacrificing by using longer cables.

I'm certainly overthinking this; I'm an engineer...that's what I do
Regardless, I'm planning to purchase quite a few cables and figured I should do so with some intelligence.
 
bmather9
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So with 2 votes so far for "1.5 ft or less" does that mean that people are really using even shorter cables to get better charge rates?
 
mjculross
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(Last edited by mjculross; 1st January 2014 at 03:56 PM.) Reason: included additional information
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Default wire size vs voltage drop

OK, in place of my "gut feel" in my earlier post, here's just one website that allows you enter the parameters of your cable (size in AWG gauge, voltage, length & current) & then calculates the theoretical voltage drop through your cable: http://www.powerstream.com/Wire_Size.htm (scroll down to the bottom to find the calculator). For example, according to their calculations, a three foot cable using 24 gauge wires carrying 2A current would impart a little over a 0.3VDC drop. If your charger is supplying 5VDC at the source, then only 4.7VDC would make it to your phone for use in charging at the full 2A rate. Contrast this with a ten foot cable of the same size under the same conditions which suffers a little more than a 1VDC drop, resulting in only 4VDC being available at your phone at the full 2A rate for charging.

However, I would not expect the phone to continue to try to draw 2A of current under these conditions (particularly the 10 foot cable), else charging may not take place at all if/when the voltage is too low. Instead, I would expect that the charging circuit on the phone would diminish its current draw (to something LESS than 2A) in an attempt to keep the voltage closer to the desired 5VDC (or whatever the spec'd minimum is to charge the specific battery, assuming that the charger itself is putting out a nearly constant amount of power, somewhere near to its rated number of watts).

It's very likely because of this reduction in current that your overall charging rate is reduced (or to put it another way, your overall charging time is increased) on lesser size cables, etc.

YMMV . . .

Good luck & have fun !!

Mark J Culross
KD5RXT
 
bmather9
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Quote:
Originally Posted by mjculross View Post
OK, in place of my "gut feel" in my earlier post, here's just one website that allows you enter the parameters of your cable (size in AWG gauge, voltage, length & current) & then calculates the theoretical voltage drop through your cable: (scroll down to the bottom to find the calculator). For example, according to their calculations, a three foot cable using 24 gauge wires carrying 2A current would impart a little over a 0.3VDC drop. If your charger is supplying 5VDC at the source, then only 4.7VDC would make it to your phone for use in charging at the full 2A rate. Contrast this with a ten foot cable of the same size under the same conditions which suffers a little more than a 1VDC drop, resulting in only 4VDC being available at your phone at the full 2A rate for charging.

However, I would not expect the phone to continue to try to draw 2A of current under these conditions (particularly the 10 foot cable), else charging may not take place at all if/when the voltage is too low. Instead, I would expect that the charging circuit on the phone would diminish its current draw (to something LESS than 2A) in an attempt to keep the voltage closer to the desired 5VDC (or whatever the spec'd minimum is to charge the specific battery, assuming that the charger itself is putting out a nearly constant amount of power, somewhere near to its rated number of watts).

It's very likely because of this reduction in current that your overall charging rate is reduced (or to put it another way, your overall charging time is increased) on lesser size cables, etc.

YMMV . . .

Good luck & have fun !!

Mark J Culross
KD5RXT
Thanks for that explanation. It seems that even a 6 ft USB cable will significantly slow charging, and that a 10 ft even more so to the point that it may not even charge sometimes. So its looking like 3ft USB cables with AC extensions where necessary is the way to go. Maybe I'll try some 1.5 ft as well, but not sure how practical they will be for using the devices while plugged in, even with the AC extension.

If anyone has another opinion please voice it.

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