View Full Version : Charing phones, usb cable length and battery packs

23-06-2013, 01:35 PM
OK so discussing on G+, some people are trying to tell others in a community discussion you should use a *real* short USB cable when charging coz otherwise it "loses" much of its charging potential down the cable.

I call hogwash.

I know you'll lose a *little* but I don't think its enough to worry about using a 10cm cable vs a 1.5m Micro USB cable... You'll lose more off a crappy thing cable vs a thicker one, yeah?


23-06-2013, 03:33 PM
I agree with you Chill. You are dealing with miliamps when charging phones even if the cable was very long and there was some loss it would only mean the phone would take longer to charge and that would only be in minuites. The phone knows when the battery is fully charged and stops the charging process. Cheers Wayne

23-06-2013, 03:58 PM
The only reason I'd use a short cable over a longer one is to avoid mess... and that's it. I've never worried about losing power.

It might be an issue with cables a few (hundred) kilometres long though. :p

The Error Guy
23-06-2013, 04:48 PM
Load of shite. Any energy loss would be minimal and the amount of current flowing through the wires is so small a loss would barely be noticed.

500mA at 5v can be handled by even the cheapest and nastiest cable. Things won't change at 1A either if you have fast charge USB ports. 500mA is the max rated draw anyway, when charging the phone boosts through charge from around 10% through to 80%, drawing about 450mA (~90% of the max supplied amps) but when it gets close to 100% the current draw will be reduced and the battery charges slower. This pattern is the fastest and most life prolonging way to charge Lith ion batteries.

So over a normal 4hr charge, approx 2 hours are spent charging at full current and the remaining 2 hours are spend charging at reduced current. Over the course of two hours in the max current phase, I really doubt any loss of current due to resistance will be noticed (you should feel enlightened now ;) )

Short story long, The current is too small and time space too short for anything to be noticed. It would be more fun watching different brands of paint dry than trying to notice the difference in charge times.

23-06-2013, 06:23 PM
Also if that was true the cables would be getting warm and that would be poor design by the manufacturers given they know exactly how much current to allow for.
When calculating the diameter of cable required for any given load you factor in the distance - if I know that it's a given the manufacturers do.

23-06-2013, 06:32 PM
What if you're charging your phone while using it? Hypothetically? Still roughly the same?

23-06-2013, 06:50 PM
I only use half metre extension cords for this very reason chill, although to get the 5 metres I need requires 10 of them.

24-06-2013, 12:22 AM
What if you're charging your phone while using it? Hypothetically? Still roughly the same?

short version - it'll probably just slow the charging down a little.

:) - well 500mA is 500mA - A port won't supply any more just because you do something with the phone. Although the .5A limit is also hypothetical so actually it might, many motherboards will supply more if the load will draw it. I believe it's due to the extra circuitry required to have individual current limits on ports, it's more likely that the limit across a few ports will be combined so that 2 front ports can supply 1A in total shared between them or 4 might be limited to 2A total and some devices might be able to get more than the .5A limit from some ports. Also some smart phones and tablets can sense when connected to a high power port and draw more current - usually only with dedicated chargers though not PC ports.

24-06-2013, 12:47 AM
Lol I use a 2 metre usb cord to charge my galaxy s4

24-06-2013, 11:34 AM
may be some truth to it

I know portable USB HD's can be VERY fussy about even 1/2 meter USB extention cables
They may not initialise if you use some USB extention cables . Some extention cables were OK, others not.

24-06-2013, 11:55 AM
Yeah that's true, but the USB HDD's usually draw a lot more power though don't they? And yes in my experience it's more the thinner cheaper cables, rather than the actual length itself?

24-06-2013, 12:38 PM
Thinner cables would affect current as they have more resistance, which could lead to a hard drive not spinning up. :)

24-06-2013, 01:23 PM
I just breifly tested this, the loss over a 2m usb cable was incredibly minimal 1-2ma

24-06-2013, 01:35 PM
I just breifly tested this, the loss over a 2m usb cable was incredibly minimal 1-2ma

You get voltage drop, NOT a current loss from resistance.

Copper wire has a incredibly low resistance, so give the low currents & short cables involved, Its hard to image cable crappy enough to cause issues, but yet there are.