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USB chargers - variations in performance

I have just bought an interesting gadget, a USB power voltammeter, to compare the actual performance of various USB chargers at home and in the cockpit, and to diagnose spurious cases of unusually slow charging. Here are some first observations.

1. Voltage variations. The USB standard says 5.00±0.25 V, and output voltages do indeed vary all over this interval. My best charger could pull 2.5 A at 5.20-5.25 V.

2. Cable resistance. I tried connecting my new tablet to its charger with three different cables.

  • a thick short cable that came with the tablet: charge current 1.8 A
  • the same cable + a thick 5-metre USB extension cable: charge current 0.6 A
  • a thinner cable I usually have with me when travelling: charge current 1.5 A

3. Differential resistance of the charger. Some (but not all) chargers have a negative differential resistance at very low loads: e.g. 4.76 V with no load, 5.15 V at 0.05 A, 5.10 V at 0.5 A.

4. Actual maximum current vs. rated maximum current. Mostly what it says on the tin, no shameless exaggerations found yet. I hope I have already evicted all 500 mA-rated chargers from my home anyway.

LKBU (near Prague), Czech Republic

Very interesting. I use an extension cable, although not that long… I imagine I’m seeing a substantial reduction in charging current based on your data. It’s enough however to keep up with a mini iPad running Foreflight.

Hope you’re feeling better after your accident!

Well done on your recovery!

Your findings tend to confirm what I have found.

The biggest thing by far is that a LOT of consumer products won’t charge (at all, or usefully i.e. while they are being used) from “5V” – which in reality is around 5.00V (because 5.00 is dead easy to achieve nowadays) – because by the time it has passed through a 1m-2m cable, at the fashionable 1A or even more fashionable 2A, there is not a lot left at the device you are trying to charge. I have not measured it but perhaps some devices are down to 4.5V. And then it is really bad design of the device to not charge from 4.5V…

The EU may have mandated the micro-USB charging connector but they didn’t know everybody was going to take the p1ss out of them, by needing 5.25V (which most USB chargers don’t do) and thick USB cables (the best of which don’t have the two data wires).

Even my S6 phone charges very slowly at 5.00V and with a normal short (0.5m) USB cable. The Lenovo T2 tables doesn’t charge at all at 5.00V.

So the only sure way is to specially buy a 5.25V charger, and use thick cables which will possibly not work for data.

The idea of a negative output resistance is a great one, but it would really work only if the charger came with the output cable permanently fixed i.e. its resistance was known.

I am having to rebuild the multi-output power supply in the plane because it doesn’t really charge anything. It is exactly 5V, and if the T2 tablet is running it only just maintains it, and the S6 phone takes hours to charge by perhaps 1/4.

Administrator
Shoreham EGKA, United Kingdom

Oh yes, one more point: in configuration (2) voltage could be measured on both ends of the 5 m extension cable. This is what I got:
Charge current: 0.59 A
Upstream voltage: 5.16 V
Downstream voltage: 4.65 V
A 0.51 V drop on 0.59 A = 0.86 Ω, or 0.086 Ω/m in each wire. That’s a medium-thick (4 mm OD) cable, 2 × 28AWG + 2 × 24AWG.

Last Edited by Ultranomad at 04 Jul 22:47
LKBU (near Prague), Czech Republic

I looked up some wire resistance.

Standard 7/0.1 wire – the type likely to be used in a USB cable – is 0.3 ohm per metre. So, there and back, 1m cable, is 0.6 ohm. At 2A that’s a 1.2V drop! That’s a 25% loss of power, or 25% longer to charge, right away.

We can be charitable and say that one of the two conductors is the shield and let’s say that’s much less than 0.3, but that still leaves you with a 0.6V drop.

But a lot of Chinese shielded cables don’t use the shield for anything, because connecting to the shield can be messy and connecting the shield to one of the pins on a tiny connector is even more messy. Unless the cable is thus specified (I buy a lot of custom made cables from China) the shield will just terminate to the connector shield, not (in the USB case) be used for the ground wire.

Administrator
Shoreham EGKA, United Kingdom

Slightly off topic but I’ll try http://www.tp-link.com/en/products/details/cat-4735_TL-PB10400.html to see how many times it can charge iPhone or iPad from 0 to 100% when full. The idea is to use it as inflight charger for mobile devices instead of lighter socket (which I don’t have in DA42).

LDZA LDVA, Croatia

I have that same power bank ( amongst others ) and use it instead of a lighter socket when sitting in the back seat. Works fine for that and maintains the charge on the iPad which is what I want.

EHLE / Lelystad, Netherlands, Netherlands
Administrator
Shoreham EGKA, United Kingdom

Emir,

(Sorry, can’t quote your post, and don’t feel like retying, but you know what I’m refering to!)

Unless your iPad is an iPad 1 or iPad 2, it will not charge it from 0-100%. The hi-res displays in the iPad3, brought a significant jump in battery capacity. You’ll need something with 15000mah to fully charge an iPad 3 from 0-100%. I presume iPad4+ require even more.

EIWT Weston, Ireland

I use a couple of different USB power monitors when testing my chargers, I can calibrate them to ensure they are correct during any given series of runs.

The quality of the USB cable is important too, as mentioned above. Worst I’ve seen are the clone Apple lighting cables, as they fake the chip in the connector, which causes all sorts of interesting issues. Generally look for 20/24 cables if you can which are 20awg for the power and 24awg for the data pins, you can get them from Amazon for £5-£10.

There are more USB charging details up on the website. (www.charge4.co.uk)

Last Edited by Harkwood at 08 Jul 11:09
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