How Does USB Charging Work?

How Does USB Charging Work

5 years ago, there was a whole market of wildly customized port chargers and proprietary connectors for smartphones, tablets, MP3 players, and many other gadgets.

Manufacturers commonly developed their own ports for their devices and as a result, severely limited file and media sharing amongst different gadgets. Since then, micro USB has made unbelievable progress towards the standardization of these products – all but completely eliminating the competition.

Nowadays, you could charge your smartphone at any computer, Charge your tablet anywhere, and download photos from a DSLR camera directly onto your HDTV, all thanks to this standardized USB connector.

Since then, another major issue has taken its place: Power. Not all USB chargers, connectors, and cables are exactly the same. You’ve probably noticed that some wall chargers are stronger than others. Sometimes you’ll find that on USB port on your laptop is a bit stronger than the others. Some desktop computers, even when they’re powered off, allow you to charge USB devices through its USB ports.

This is what I’d like to call, “organized chaos”. Yes, there actually is a method to all this madness, but first let me explain to you how USB power actually functions.

There are currently 4 USB specifications (standardizations). They are USB 1.0, USB 1.1, USB 2.0, and USB 3.0. As of 2012, USB 2.0 is the most widely used variant implemented on digital devices, so we’ll be focusing on that one. But we’ll also quickly cover how USB 1.0, USB 1.1 and USB 3.0 significantly different.

In any USB network, there is one host and one device. Most of the time, your computer is considered the host, and your smartphone, tablet, MP3 player, camera, or other gadget is the device. Power flows in the direction from the host to the device, but data can flow in both directions.

A USB socket has four pins and a USB cable has four wires. The inner pins carry data (D+ and D-), and the outside pins provide a 5 volt power supply. There are three kinds of USB ports dictated by the current (milliamps or mA) specification: a standard downstream port, a charging downstream port, and a dedicated charging port. The first two are found on your computer, and the third is found on wall chargers.

In the USB 1.0, USB 1.1 and USB 2.0 specifications, a standard downstream port is able to deliver up to 500mA. While in USB 3.0, it moves up to 900mA. The charging downstream and dedicated charging ports provide up to 1500mA.

The USB specification also allows for a “sleep and charge” port, which is how some USB ports on a powered down PC can still remain active. You’ve probably already noticed this on your desktop, where there is always some power flowing through the motherboard, but some laptops are also capable of sleep and charge.

Now the USB specification may dictate these rules, but in reality there are plenty of USB chargers that do not go by these specifications. Apple’s iPad charger, for example, provides 2.1A at 5V, Amazon’s Kindle Fire charger outputs 1.8A, and car chargers usually output anything from 1A to 2.1A.

As a result, there is a huge difference in power between regular USB 2.0 ports rated at 500mA and dedicated charging ports which can range all the way up to 2100mA. This begs the simple question: If you take a gadget with came with a 900mA wall charger, and plug it directly into a 2100mA iPad charger, will there be a nuclear explosion that will wipe out all humanity??

Well, no. You can plug any USB device into any USB cable and into any USB port, and nothing bad will happen. In fact, using a more powerful charger should actually speed up battery charging. So if anything, it’s a good thing.

Keep in mind however, that the age of your USB device does play an important role in this, dictating both how fast it can be charged, and whether it can be charged using a wall charger at all. Back in 2007, the USB Implementers Forum (the group behind USB specifications) released the Battery Charging Specification, which standardized faster ways of charging USB devices, either by pumping more amps through your computer’s USB ports, or simply by using a wall charger. Quickly thereafter, USB devices that implemented this specification were released in the market.

So if you’ve got any modern USB device – which basically included any modern smartphone, tablet, eBook reader, or camera, then you should be able to plug into a high amp USB port and enjoy the benefit of faster charging. But if for any reason you’re still carrying around an old device, it probably will not work with USB ports that have the Battery Charging Specification. It might only work with old fashioned, original 500mA USB 1.0, USB 1.1 and USB 2.0 computer ports. In some extreme cases for devices used from the nineties, you can only charge your devices on computers with specific drivers installed.

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Vincent Clarke

Vincent Clarke is the Universal Serial Bus (USB) Guru for When he's not writing tutorials and catching up on the latest USB news, Vincent is busy preparing his next blog post and answering USB questions from his readers and subscribers.

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