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How to charge your smartphone and other USB devices as quickly as possible

USB charging is shrouded in mystery — which is a bit ironic, given that almost all of us rely on USB charging of our smartphones and tablets on a daily basis. Why, for example, does an iPhone recharge quickly when you plug it into your MacBook Pro, but slowly when you plug it into your PC, or a non-Apple wall charger? Why does your smartphone charge quickly to start with, but slow down as it fills up? In this story we will answer these questions and more as we investigate the optimum methods of charging your USB devices as quickly as possible. We will also be taking a look at a new device, called the Practical Meter, which finally gives us insight into the arcane world of USB charging.

How USB charging works

To begin with, you should read our story on how USB charging works, which outlines USB 1, 2, and 3′s specifications for charging, why some sockets and wall chargers output more power than others, and whether it’s possible to blow up your device by using a different charger (it isn’t). What that story doesn’t cover, though, is the variation between devices — why iPhones charge differently from Androids — and why two seemingly identical USB 2.0 (or 3.0) ports charge your devices differently.

Why your smartphone charges slowly

In general, there are two key factors that affect how quickly your device charges over USB: The maximum amperage of the USB socket or wall charger, and how your device negotiates power delivery from the socket/charger.

For a start, every charger or socket has a max amperage — usually between 500 milliamps (mA) and 1.5 amps (A). In general, unless it says otherwise, your laptop and PC are probably outfitted with 500mA sockets (unless they’re USB 3.0, in which case they’re probably 900mA). The max amperage of a wall charger varies, but can usually be found written somewhere on the charger — generally it’ll be between 0.5A and 2.1A. The max amperage dictates one thing only: The max theoretical charging rate of your device.

It is through negotiation that your device actually decides how much power it wants to draw. When you plug your device into a charging socket or wall charger, the USB controller in your device talks to the USB controller attached to the socket/wall charger — or, if no controller is available, the device reads the voltage flowing across the four USB pins. From this data, your device decides how much amperage to draw, and thus how quickly it will recharge.

The question is, how does your device decide how much power to draw?

Unfortunately for us, this is a complex area that is full of proprietary information that can only be ascertained experimentally. In short, there is a USB power charging spec that many devices stick to, but there are also definite differences between classes of devices, such as iOS and Android. Your iPhone, for example, will only recharge at its max speed if the right driver is in place for power negotiation — otherwise it will fall back to slow charging. Likewise, it can be quite hard to coax an Android device to charge quickly from a laptop or PC that also wants to initiate a data connection (which reduces the max draw over the power lines).

Another factor is that most devices, if they’re equipped with a lithium-ion battery, reduce their power draw as the battery reaches capacity, to preserve the longevity of the battery.

In short, getting your smartphone or tablet to charge quickly is something of an arcane art. If you have an iPhone or iPad, be sure to charge it with your Apple PC or use the Apple-provided wall wart. If you have an Android device, be sure to install any manufacturer-provided drivers and use the manufacturer-provided wall charger (or the official “fast charger,” if there is one).

Practical Meter, diagram showing LEDs = watts

The other, intriguing option is to use the Practical Meter, a small (Kickstarter) dongle that shows you how much power your USB device is drawing — and thus how quickly it will recharge. Plug the Practical Meter into a USB socket or AC adapter, plug your USB device into the meter, and marvel as the LEDs light up. Each LED equates to one watt, with five watts being the maximum that most that smartphones will take.

The first thing you’ll notice with the Practical Meter is that almost every USB socket on your desktop or laptop will only give you two or three bars — which equates to 500 mA, the bottom end of the USB charging spec, and a very long charge indeed. Five bars is equal to 1A (5 watts @ 5 volts), which you can draw from a dedicated USB charging port on your PC (but these are rarely labeled) or with the right drivers. Generally, though, to get the fastest charge, you will need a wall charger or third-party connectors that trick your USB device into drawing the full five watts. Fortunately the Practical Meter also comes with those third-party connectors.

The short answer, then, if you want to charge your smartphone or tablet as quickly as possible, is to get a Practical Meter and some third-party connectors — for $20. Plus, once you have a Practical Meter, you can use it for all sorts of other cool purposes, such as measuring the power draw of your USB keyboard, or how productive your solar panels are.