Recently, I had to figure out all about battery charging via USB. No, not the new iPhone with its USB C connector. I had another device that was specified to be “compatible with Power Delivery 2.0”. What? I thought I could read the specs and should know what’s what.

Today, USB is a very – and I mean boukou boukou boukou1 – complex system. Consider the humble beginnings. The original Universal Serial Bus was just that. Two signal lines, plus two power lines. Four pin connectors.

A USB connection can:

  • transmit and receive data
  • charge a battery or provide power in general
  • serve as “tunnel” for so called alternate modes such as Thunderbolt

The data transmission speeds have gone from a measly few Mbits/sec to dozens Gbits/sec. Full-duplex as from USB 3.0, requiring four data lines. Power delivery can go up to 240 Watts. With USB C even in either direction. To browse the specs is interesting, not least as the newer, later versions always encompass the older ones. That is, there’s some compatibility, for devices and cables, of course falling back onto the less capable functionality of the older specs. This compatibility leads to lots of complexity in the newer USB versions. The specification for just USB connectors and cables is over 400 pages. Not including the data transmission or other protocols, or the specs for charging and whatnot. The Power Delivery specification is over 800 pages. All USB specs together literally encompass thousands of pages.

USB A and B

Regarding the cables, before we had USB C connectors, you could, to some degree, tell their capabilities looking at the “B” side connector. The “A” side, ie. the one that goes into the computer or charger, looks the same for all USB versions before and including 3.x, and only if you know what you are looking for you might see the additional contacts required for the higher speed full-duplex data connection.2 The “B” side is more telling. Check out the corresponding table on Wikipedia. Then again, you cannot assume that a USB A to Micro-B cable even supports USB 2.0. There are manufacturers that save a few micro-cents on the cable to charge a, say, phone by only putting the two power wires, but not the two data wires (D+ and D-). You’ll only discover this when trying to use that cable to offload some data from the phone.

usb tester

You often need to try different cables, and see if something changes. Or you use a USB cable tester. I am confronted with this issue so often with all the donated stuff we get at the Charity Centre that I got one of these. This way, I quickly know what’s “in there”, and don’t need to try different cables in case something does not work. It could also be the device that’s faulty, I never know, since we receive items in all states from good condition to just recyclable. Also, it could also just be the contacts in the device that need some cleaning. Salty air here. So starting with a known good cable is a Good Thing.™


Enter USB C, and things get more messy. Looking at a USB C to C cable tells you nothing. It could be anything. Same for a USB A to C cable. From the most basic charging cable with just two power wires to USB 2.0 to highest USB43 speed data connections, or even Thunderbolt, everything is possible. Some cables have markings, but most have not. If memory serves, the labels only have to be on the original packaging. Very useful if you have a box of cables in front of you. Without a tester, or trial by swapping different cables, you’re lost.

The USB C connector per se is basically a nice design. It can finally plugged-in in either orientation. And the delicate springy parts are still in the connector, not the device. USB had always got this right, contrary to, say, Apple’s Lightning connector.

Power Delivery

Going back to the battery charging issue mentioned at the top. There are the official standard specifications by the USB organisation – most prominently the Power Delivery (PD) one. Of course there are already different versions. Nearly gone are the simple 5 volts USB chargers, the more powerful of which can provide different power levels, such as 2 ampères at 5 V (10 Watts). The device to be charged and the charger can “agree” on a level of charging current based on simple pull-up and pull-down resistors on the CC wire. If the cable even provides this connection.

Power Delivery, however, can also work using higher voltages, such as 9 or 15 or 20 or more volts. This allows to use smaller charging currents for the same wattage, resulting in skinnier cables. Which are both easier to use as well as cheaper to manufacture. However, now both sides have to actively negotiate the appropriate voltage via exchanged messages, following a defined protocol, stepping through a state machine. There’s a lot going on in that simple looking cable, as well as the chargers and power consuming devices. As said, the specs are over 800 pages. Chargers should fall back to a safe voltage of 5 V in case that negotiation fails, but franchement, can you trust a cheap non-name device to do that? I would never use any odd brand of charger that is sold as “PD compatible” to charge my phone. Apply 20 volts to an iPhone, and – poof.

Of course, there are also “fast” or “quick” charging methods that are not standard. Qualcomm has them, also Samsung, Huawei, Apple. Probably a few more that I forget at the moment. Watch out using the corresponding chargers for other devices than the ones they came with. Think: poof.4

usb tester

At the end, I have not read all the specifications. Too many variations and possibilities to be a real practical help when dealing with lots of different chargers and cables. I hope I never have implement a USB device of any sort. For my practical uses I have learnt to use a few chargers of which I know that they work reliably and without “interesting” surprises, and use a metering device to check what’s going on.5 Different USB cables can also trigger different charging modes, even with the same charger and device to be charged. Depending on how a specific cable is wired, it can even change when you reverse the orientation of the USB C plug. The fun never ceases.

USB is powerful,6 but messy on a practical user level.

  1. “very very very” ↩︎

  2. Don’t trust the “colour coding” of the piece of plastic inside the “A” connector. It’s not reliable. Some cheap cable vendors will use colours that should be used for “faster” cables on the USB 2.0 ones. ↩︎

  3. Yes, these are the correct names. No space between “USB” and “4”. The naming of the standards does not help. There’s also USB 3.1 Gen2. Or USB 3.2 Gen 2x2. Le sigh↩︎

  4. Using a fixed 5 volts charger should be safe for all use cases. Watch out for anything that can change the voltage. It should be written on the case, even usually you need a loupe to read the specs. ↩︎

  5. The same metering device can also be used to test the power banks we receive, in particular the remaining maximal capacity they retain. Very useful. ↩︎

  6. For some reason my iPhone didn’t charge last night, and was at about 20% charge this morning. Using the metering thingie I could check how the charging started at 5 volts, then ramped up to 9 volts, to get back to 5 volts at about 85% to finish the charge. I got some 95% within less than one hour. Probably at the cost of the overall battery life, due to the fast charging. But it got the job nicely done of getting me a useful device for the day. ↩︎