By: Glenn Wilkinson
Senior Consultant, Sensor Systems

15th February 2017

They live behind cupboards, skulk under the bed, lurk at the back of desks and hide in car glove boxes. They lay in wait across the world. Coiled and ready to unleash their potentially deadly power in an instant. And we wouldn’t have it any other way.

The now ubiquitous USB charger lead has increasingly littered our lives for nigh on 10 years now. Adopted by portable device manufacturers near unanimously as soon as their handheld marvels had a need for a power and wired data connection. So given how common the USB charger is – it’s surprisingly misunderstood.

Stay with me now – it might stop you getting bitten…

Start looking into the subject and you’ll find forums, reviews and help boards littered with common complaints, comments and questions (sad face emoji’s removed to avoid repetition);

“I bought a 2 amp charger, but my phone doesn’t charge any faster.”
“I left it connected overnight and it was still flat in the morning.”
“Will this phone charger work on my tablet?”
“My Galaxy charges faster on my PC than with this supply.”
“I get a ‘charger unrecognised’ message when I connect it.”

So what’s going on?

First off, your charger doesn’t determine how much power it is going to supply – your device decides what it is going to consume. It’s a self-protection mechanism. The batteries you are charging need a limited charge current to prevent overheating and maximise cell lifetime, so your device contains a charge circuit which implements this limit. Go and buy the beefiest charger you want, but it will only charge your handheld quicker if the device allows it.

So if the charging smarts are in your device, why can it perform differently with some chargers, and not at all with others, even though they have sufficient power capability?

We could start digging into history here, pointing fingers and quoting standards – but let’s not go there. Simplistically put; somewhere between standards being insufficient or optional, and manufacturers implementing in a manner to suit their own needs, we end up with the ‘mish-mash’ of solutions we have now amalgamated over time.

Strictly a USB-compliant host port can support a number of power modes (USB2.0 high/low, BC, PD for those curious). The host USB port and device need to indicate to each other across the USB data lines and agree the most suitable mode to operate in.

Some devices ignore any form of convention completely and just draw a fixed current. This is relatively common in legacy smartphones and is usually around 500mA, not unintentionally the same as the original USB2.0 high power spec.

Others create their own hybrid language solutions, predominantly by bias voltages on data lines to indicate whether faster charge modes beyond USB low or high power current limits can be used.

It has to be said that things are getting better with the widespread adoption of the USB battery charging spec (BC1.2), and particularly the simplicity of the direct charge port (DCP). But the delay in it being published means there are still a huge number of devices out there with their own niche charging characteristics and languages.

Why am I telling you all this? Because it’s another one of those hidden little hurdles that us engineers have to overcome. Plextek recently created an award winning commercial device featuring USB charging ports. The brief required those ports to support charging of any common smart device at a rate equal to the manufacturer’s charger. This means speaking every language, and knowing which to use at any particular time.

Of course, the main fun of this is in the testing. After a good day swamped with more tech than a Gadget Show giveaway, and kind co-operation of a very nervous man at PC World (after some negotiation) – job done, a truly multilingual solution.

So hopefully you may have gained some small appreciation of the engineering behind something the vast majority of people take for granted – and there may just be fewer sad faces on those internet message boards.

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