Not all USB cables are created equal!

Posted: May 14, 2014 in General, Other Products, Raspberry Pi

In recent months I’ve had issues charging my phone, it became apparent that some chargers were more effective than others, and it seemed that some of my USB cables were better than others.

The setup

To put this to the test I’m going to use a USB Voltage and Current monitor I purchased for measuring the power requirements of the Raspberry Pi.  If you search around this can cost a few dollars (

4.94V being supplied.

4.94V being supplied.

2014-05-11 20.40.24

The connected device is drawing 1.07A

The charger is an HP Touch Pad charger which is rated at 5.3V @2.0A  (unlike a lot of chargers it seems more than able to supply that).  This makes this a (P=VxA) 10.6W supply.

HP Touch Pad charger - a very solid USB charger.

HP Touch Pad charger – a very solid USB charger.

I also connected the monitor to the charger via a decent USB extension cable, making it easier for testing.

Test Items

To test out the cables I purposely used three types of USB devices:

1. A USB Battery Pack – I don’t think this uses any data lines for the charging input, so it should provide a good test of a dumb charging cable, it is also designed to draw as much power as it can to charge most effectively.  At the time of the test the pack was at around 25% charged, so it should draw a reasonable current during testing.

2. Samsung Phone – This represents a typical power hungry smart-phone, currently charged at around 45% it should demand a reasonable charging current.  As I understand it, smart phones often make use of the data lines to determine the charging rates supported by the charger and allow charging cut-off when complete – I’m not really that familiar with the process and the features of this aspect to charging but had wondered if my charging issues were related to that.

3. The Raspberry Pi – This particular setup was fully loaded with composite video output, keyboard, 3.5″ LCD-Screen and power hungry Wifi dongle.  Powering the Raspberry Pi is a great test since it is sensitive to under voltage (caused by low current supplies) and only the power pins of the connector are connected (so there is no question of interaction with the data-lines of the USB cable).

 The Cables


#01,#02,#03: These are unnamed cables which appear to be identical to each other, they look like they were designed to be a copy of the other HTC cables I have (the only difference other than the HTC label is that the material inside the USB plug is white, where the “genuine” HTC cable is black.

#4: A samsung USB cable (with the markings U1-APCBU10BBE-KD1Z410TS B), I think this cable came with my phone, although not sure.

#05 / #07: The HTC cables (at least they have a HTC label but that may not be “genuine”), are almost identical to the first three (except the USB material colour).

#06: No idea what this cable is from, looks quite thin and a little short.

#08 / #09: The cables which came with the battery pack, a long and short cable which have a two pole radial connection on for fitting different ends (therefore will only provide power connection).

#10: A flat pink ribbon cable noodle from Pimoroni (picked up at a Raspberry Pi event)

The Readings

Trying out each cable results in an interesting range of readings, some very low/unusable and some rather high too…

Remember all the readings are with an additional USB extension cable, so the voltage and current readings will be much lower than if they were connected directly to the charger.

Name  Internal Colour Battery Pack (25%) Phone (45%) Raspberry Pi
01 Unnamed 1 (HTC) W 5.05V 0.43A 5.15V 0.04A 5.04V 0.63A*
02 Unnamed 2 (HTC) W 5.04V 0.41A 5.15V 0.04A 5.04V 0.63A*
03 Unnamed 3 (HTC) W 5.05V 0.43A 5.17V 0.04A 5.04V 0.63A*
04 Samsung U1-APCBU10BBE-KD1Z410TS B W 4.85V 2.09A 4.87V 1.70A 4.93V 1.17A
05 HTC B 4.74V 2.31A 4.87V 1.8A 4.95V 1.07A
06 Unnamed 4 W 4.92V 1.56A 4.95V 1.09A 4.85V 1.17A
07 HTC Broken? B 5.16V 0.00A 5.16V 0.00A 5.16V 0.00A*
08 Battery Long Cable W 4.87V 1.53A** 5.05V 0.48A** 4.95V 1.17A**
09 Battery Short Cable W 4.73V 3.1A 4.87V 1.87A 4.97V 1.07A
10 Pimoroni Cable 4.89V 1.48A 4.96V 0.85A 4.93V 1.09A

*Fails to run! **Bad connection

The first three cables (#01,#02 and #03) have some serious problems supplying current to anything, with insufficient current to properly run the Raspberry Pi, it highlights that the USB cable is just as important as having a good power supply.  Each of these fail totally to charge my phone, which can be a serious problem since it indicates on the phone that it is charging but the battery will drain just as quick (if not quicker if your settings are set for performance when charging) – suspect these cables are wired incorrectly (the micro USB has a 5th pin used to indicate the cable type).

Next we have some averagely performing cables, #06 (another unnamed one), #08 (battery long cable) and #10 (Pimoroni cable), each of these provided a reasonable 1.5A to the battery pack when charging, but between 1.1A and 480mA for the phone.

Finally we have our star performers, #5 (the HTC labelled cable) and #9 (the battery short cable), providing 2.3A to 3.1A to the battery pack and 1.8A-1.87A to the phone.  It is interesting that there is such a marked difference between #08 and #9 which are identical other than the length (the longer cable is about twice as long as the short one).

I must admit that I find the thought of 3.1A getting pulled into the battery pack a little alarming, since the micro USB plug is only specified to support up to 2A [1]…so it is pushing the limits a little.  Also charging any battery at that rate will cause a lot of heat, it probably isn’t too good for the battery in the longer term (the charging circuit should provide some protection here though since there should be thermal cut-outs/throttling when things get too hot).  To be fair though, this cable was supplied with the battery, so I would hope it is suitable for the battery pack.

Why are Amps so important?

There is a whole host of standards which relate to USB charging, and so many exceptions to the rules that in reality it is all quite a big mess, you will find a number of things will change how effectively your device will charge, the cable is just one link in the chain.  This is why I’ve focused on the actual current which is getting to the device, which directly affects the rate at which a device will charge (or in the case of the Raspberry Pi if you will get issues when it draws more current than is available – resulting in errors).

The power they can provide (Voltage x Current) the faster the battery and phone will charge and the more power which is available for the Raspberry Pi.  With power hungry devices, such as tablets and large phones, the cables may in fact provide less current than the device needs when you are using it – so you are only slowing down the battery discharge (at best) rather than re-charging the device.

Imagine you spend the afternoon playing on your phone or tablet with it plugged in and when you come to take it away somewhere else…if your cable is poor the battery could be almost dead!

Charging time

Assuming you have your device on when you charge it, the important information you’ll need to know is how much power does it use just to run and how much is actually charging the battery.

Fortunately this is easy to work out by letting your device fully charge and observe the drop in current it draws from the USB cable. The Smart phone I have is quite power hungry (although tablets will probably be worse), it appears to use about 430mA with the screen off (data/phone service enabled).

So even for the average performing cables, this leaves a very small amount of current to perform any charging (#06 will have 660mA, #10 will have 420mA and #08 will have just 50mA!).  With the screen on, the power draw jumps to around 700mA and performing CPU work such as playing 3D graphical games or using the GPS and data, it is clear that it soon gets to the point where the device may not even charge.  Compare this to out better performing cables, which provide up to 1.8A, we can expect that we have at least 1A of current available for charging in most situations.  By having the screen off we can expect the device to charge around 24% faster with a good cable and with an average one this can be 60% (for #06), 280% (for #10 – yes it will take almost 3 times as long) or it may never charge! (for #08).

Of course the fastest way to charge a phone is to remove the background draw of the device, by switching it off the phone isn’t using that 430mA or similar and that can be put to use directly charging the battery.  If your power supply or cable isn’t very good then this can make a huge difference.

Remember, the standard for a computer USB2 port is just 500mA*** (and USB1 this was just 140mA), therefore compared to modern devices there is very little left over to charge your device if it is running (some additional information on this is available here –

***Fortunately, most modern computers and laptops allow this limit to be exceeded, by providing charging downstream enabled ports (however this may only be selected ports and often they are not labelled) either.

Selecting a good cable?

While most of my cables are cheap ones, there is not always a correlation of price verses quality, spending a fortune on a cable may increase your chance of a good one, but it is no guarantee it will out-perform a cheaper one.

Branding is usually a good indicator of quality, but in many cases you pay a lot for the brand and sometimes the branding isn’t even genuine.  An expensive cable will either be exceptional quality, or a rip-off (con).

I think the best advice here is to select somewhere with good customer service and will let you test a cable and return it if it is not up to the job.   A good shop should be interested in the quality of the products they provide and will be keen to see how they perform against alternative items.  To help with this ensure you can demonstrate a like-for-like comparison so you can prove if a particular cable is inferior or not.

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