Processors. RAM. Screens. They’re the way millions of people make smartphone purchasing decisions. But in recent years, another factor — fast charging — has slowly made its way to the fore. Who doesn’t want a smartphone that can charge in minutes instead of hours?
If only it were that simple. Charging standards are a complicated mix of chemistry and physics, and each have their own sets of limitations and poorly publicized incompatibilities. To make matters worse, phone-makers tend to slap confusing labels on otherwise straightforward components.
How does fast charging work?
Every smartphone has a battery, and every battery delivers power in more or less the same way. Cells consisting of two electrodes (one positive and one negative) and an electrolyte, catalyze reactions that convert compounds into new substances. Over time, ions — atoms with too few or too many electrons — form in the electrodes, driving a flow of electrons to the battery’s negative outer terminal and supplying your phone with an electric charge.
In non rechargable batteries, those chemical reactions occur only once. But in the rechargeable lithium-ion batteries that power smartphones, the reactions are “reversible.” When the battery discharges, the chemical reaction produces electricity, and when the battery recharges, the chemical reactions absorb power.
Smartphone batteries charge when a current passes through them. Greater current and higher voltages charge batteries faster, but there’s a limit to what they can take. The charge controller (IC) protects against dangerous spikes in current.
The controller chip regulates the overall flow of electricity into and out of the battery. Generally speaking, lithium-ion controllers define the current (in amps) at which the battery charges by measuring the battery’s cell current and voltage, and then adjusting the current flowing in. Some use a DC to DC converter to change the input voltage, and fancier integrated circuits adjust the resistance between the charger input and the battery terminal to ramp the current flow up or down. The amount of current the charge controller draws is generally dictated by the phone’s software.
There’s a hefty chance that your smartphone recharges via USB cable. There’s a really good reason: Besides the fact that USB cables are relatively easy to find these days, USB has a really robust, well-defined charging standard called the USB Power Delivery Specification.
The USB Implementers Forum specifies four flavors in total, one for each corresponding USB specification: USB 1.0, 2.0, 3.0, and 3.1. A typical USB 1.0 and 2.0 plug can deliver up to 5V/0.5A (2.5W).
That’s the charging rate of a typical phone, and it doesn’t amount to a lot of power. An iPhone charging at 2A over USB uses 5V x 2A = 10W. The average incandescent lightbulb, by comparison, draws about 40W of power. By default, USB 3.0 ports push 5V/0.9A (4.5W).
USB-C, the oval-shaped reversible plug on newer smartphones, is a different animal altogether. It’s technically capable of carrying the USB 2.0 spec, but most manufacturers opt for USB 3.1, which can potentially deliver a much higher voltage.
Many USB 3.1 devices take advantage of the USB Power Delivery (USB-PD) spec, which has a maximum power output of 20V/5A (100W). Smartphones don’t typically draw that much power — manufacturers commonly stick with a lower amperage (like 3A), but it’s a boon for USB-C laptops like the MacBook Pro and Google Chromebook Pixel.
Slightly complicating things is the Battery Charging Specification, which deals specifically with power drawn from a USB port for charging. The most recent spec, Rev 1.1, defines three different sources of power: Standard downstream port (SDP), charging downstream port (CDP), and dedicated charging port (DCP). CDP, the spec in modern smartphones, laptops, and other hardware, can supply up to 1.5A.
Fully compliant smartphones and chargers respect the limits of USB 2.0 and BC1.1, but not all phones and chargers are compliant. That’s why, generally speaking, smartphones always default to the lowest charging speed.
The USB specs are more like guidelines than dictum, though. Fast charging standards like Qualcomm’s Quick Charge and Samsung’s Adaptive Fast Charging exceed the USB spec’s voltage parameters, but on purpose — that’s why your phone is able to recharge in minutes, rather than hours.
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