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November 18, 2014


Electronics & Systems
Fast Charging
Mobile Devices
Nadim Maluf

We know and complain about the dismal charging times of modern smartphones! But why is it that these marvels take several hours to charge? That’s what we will explore today.

There are two pieces to this puzzle. The first piece relates to power delivery, and the second piece relates to maintaining the health of the battery.

Let’s start with power delivery. That’s the circuitry, cables and necessary components to deliver sufficient electric power from the wall socket to the mobile device. Specifically, this includes three key elements: i) the AC adapter, ii) the USB cable, and iii) the circuitry, integrated circuits and other passive components such as inductors, all residing inside the mobile device itself.

Presently, the AC adapters are rated 5 Watts, taking 120V from the wall and delivering an output of 1A at 5V into the smartphone. Charging faster means more power. How much more? With 3,000 mAh becoming common, we are looking at a minimum of 15 Watts.  Therefore a 15-Watt AC adapter is required, but most importantly, it has to be at a cost that is nearly equal to the earlier and smaller AC adapter.

We also need a USB cable that can take the power from the AC adapter to the device. Standard USB cables are typically rated at or near 2A. This automatically says that the higher power should be delivered at a higher voltage. This is very similar to how power is transmitted from large power stations to your house. Power leaves on high-voltage transmission lines (these are the large towers that you see in open fields or on top of hills). When the power gets close to your house, the voltage is then stepped down to the standard 120V that you are familiar with — you can see these little transformers perched on top of utility poles up or down your street.

It is a similar concept in the mobile device. The new 15-Watt adapter operates at higher voltages, typically 9V or even 12V, up from the standard 5V. Take 120V from the wall, then step it down to either 5V (the old way) or 9V or 12V. Of course, the circuitry within the smartphone itself now has to be able to take the 9V or 12V. Voila! These AC adapters, USB cables and proper internal circuitry capable of operating at the higher voltages are just getting rolled out in 2015….so the first hurdle in fast charging is solved.

The second piece of the puzzle is the health of the battery. How do we make sure that the battery will operate properly for the life of the mobile device when we dump 15 Watts of power into it? This has been the challenge of the industry — and fortunately, one of several problems that we have solved here at Qnovo.

The problem with dumping 15 Watts of charging power into the battery is that it will destroy the battery’s cycle life. This older post will explain what cycle life is. Poor cycle life manifests itself with rapid loss of battery capacity and increasing warranty returns. That’s when the consumer (you!) notices a rapid deterioration in the battery life from one day to the next, and you start b*$%!ing.

Right now, the battery manufacturers are struggling with making the cycle life specifications, albeit trying to deny it. They are promising new generations of batteries that can take the higher charging power, but in reality, they are failing to deliver the requisite performance. This highlights the industry’s idiom about “liars, liars and battery suppliers.” The failure to deliver is due to fundamental limitations about the underlying physics of charging the battery. Instead, the battery manufacturers are scaling back the battery capacity (i.e., reduce the number of mAh) in order to charge faster, but that’s not what consumers want. We want high battery capacities AND faster charging, not a choice of one or the other.

The good news is that this problem is also being solved and we expect these solutions will also be rolling out in 2015. In other words, expect to start seeing fast charging becoming increasingly common some time next year. Yet I constantly wonder, why in the world did it take this industry so long!

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