Quick and dirty Micro USB power test jig and results

[Balaur]

Hello to all,

Puzzled by some strange slow charging behavior, I've built a quick setup to test my Micro USB cables.

I've started from a SMD Micro USB female connector I had laying around (I like to use Micro USB for all my 5V creations):



Then, on a piece of standard copper clad board



I've soldered the micro USB connector short-circuiting all the pins (basically a loop-back configuration):



Then, I've connected the micro USB cable on this



and a standard USB female connector on the other end of the cable.

Using a home-brew 4 wire resistance measurement scheme, I've been able to measure the resistance of the power path (i.e USB connector to cable VBUS contact > VBUS wire > cable to micro USB VBUS > board > micro USB to cable GND > GND wire > cable to USB connector contact).

Assuming that the USB <> cable contact has 20 mOhms and that the micro USB <> cable has 30 mOhms (values from the specs), we can compute the linear resistance of the cable and take a guess at the wire AWG and compare to the expectations (assuming that the wire is copper).

Results are as follows:

Brand and Product	Resistance (2-way)	Length	Stated AWG	Resistance (1-way)	Cable R	Resistance (mOhm/m)	Estimated AWG	Voltage Drop (0.5A)	Voltage Drop (1A)	Voltage Drop (1.5A)
Lenmar Undead Power	0.276	0.81		0.138	0.088	109	24-25	0.14	0.28	0.41
ASUS Nexus7	0.332	0.97		0.166	0.116	120	24-25	0.17	0.33	0.50
Nokia	0.412	1.18		0.206	0.156	132	26	0.21	0.41	0.62
Tenergy PowerBank	0.66	1.81		0.33	0.28	155	26-27	0.33	0.66	0.99
Sony	0.454	1		0.227	0.177	177	27-28	0.23	0.45	0.68
Sansa Clip Fuze	0.188	0.24	28	0.094	0.044	183	27-28	0.09	0.19	0.28
Sony	0.468	1		0.234	0.184	184	27-28	0.23	0.47	0.70
DX (HTC)	0.912	1.22	28	0.456	0.406	333	30	0.46	0.91	1.37
DX (HTC)	.947	1.22	28	0.4735	0.4235	347	30	0.47	0.95	1.42

A few comments:

Most cables (including the 2 to 10 USD cables I've found in Frys, DealExtreme) are marked as 28 AWG (power and data). That's fair, but the ones I've got dont behave as expected. Please note that these cables have very good reviews. Sorry, but not my experience. The estimated AWG points to something like 30. These cables are only able to charge my Lenmar Undead Helix at 700-800 mA, while on a good cable, it quickly goes to 1.5 A.

The cable that came with the Lenmar Undead Helix is unexpectedly good, even if not branded.

The branded cable (i.e. the manufacturer logo/name is on the cable) that came with my Nexus 7 (2012) is very good as well, allowing high current charging on the provided charger.

The Nokia and Tenergy cables are better than expected. The Sony ones provided by my Xperia smartphones are OK.

As I'm a bit of a power nut, I've ordered a few male micro USB connectors and build dedicated charging cables directly connected to a good 5V source.

PS: I've searched for lower AWG cables, but I haven't been able to actually buy them in a brick and mortar store and the online sources are not that plenty.

Best regards,
Dan

[SeanB]

Nice test and the expected results are that the cables mostly are not to spec. Generally the thicker ones are more likely to have the proper sized conductors, though YMMV on the cheap ones.

Not a fan of those connectors though, they are way to easy to break on phones.

[BravoV]

Had the same curiosity about this too, and you're now came with this report, great job Dan, thanks a lot ! 

Same experiences here, its just I'm too lazy to do a thorough tests like you, so far my own experiences between these cables, the one that came originally with my Samsung phone is the best. Its thick and quite stiff to handle, and sort of hard to roll up and keep straightening it self.