Amazing strides have been made in recent years towards standardized chargers and connectors for smartphones, tablets, MP3 players, and many other gadget species. In just five years — with the obvious exception of Apple’s iOS connector — Micro-USB has single-handedly destroyed the industry’s penchant for wildly customized and proprietary connectors. Today, you can charge your phone at your friend’s house, plug your Kindle into any computer, and download photos from a digital camera directly to your TV, all thanks to a standardized connector.
In its place, though, another problem has arisen: USB power. Not all USB chargers, connectors, and cables are born equal. You might’ve noticed that some wall chargers are stronger than others. Sometimes, one USB socket on a laptop is seemingly more powerful than the other. On some desktop PCs, even when they’re turned off, you can charge your smartphone via a USB socket.
Believe it or not, there’s a method in all this madness — but first we have to explain how USB power actually works.
There are three USB specifications — USB 1.0, 2.0, and 3.0 — but we’ll be focusing on USB 2.0, as it’s by far the most common variant. We’ll point out where 1.0 and 3.0 are significantly different. The other important fact is that in any USB network, there is one host and one device. In almost every case, your PC is the host, and your smartphone/tablet/camera is the device. Power always flows from the host to the device, but data can flow in both directions.
OK, now the numbers. A USB socket has four pins and and a USB cable has four wires. The inside pins carry data (D+ and D-), and the outside pins provide a 5-volt power supply. In terms of actual current (milliamps or mA), there are three kinds of USB port dictated by the current specs: a standard downstream port, a charging downstream port, and a dedicated charging port. The first two can be found on your computer (and should be labeled as such), and the third kind applies to “dumb” wall chargers. In the USB 1.0 and 2.0 specs, a standard downstream port is capable of delivering up to 500mA (0.5A); in USB 3.0, it moves up to 900mA (0.9A). The charging downstream and dedicated charging ports provide up to 1500mA (1.5A).
The USB spec also allows for a “sleep-and-charge” port, which is where the USB ports on a powered-down computer remain active. You might’ve noticed this on your desktop PC, where there’s always some power flowing through the motherboard, but some laptops are also capable of sleep-and-charge.
Now, this is what the spec dictates, but in actual fact there are plenty of USB chargers that break these specs — mostly the wall-wart variety. Apple’s iPad charger, for example, provides 2.1A at 5V; Amazon’s Kindle Fire charger outputs 1.8; and car chargers can output anything from 1A to 2.1A.
Can I blow up my USB device?
There is a huge variance, then, between normal USB 2.0 ports rated at 500mA and dedicated charging ports which range all the way up to 2100mA. This leads to a rather important question: If you take a smartphone which came with a 900mA wall charger, and plug it into a 2100mA iPad charger, will it blow up?
In short, no: You can plug any USB device into any USB cable and into any USB port, and nothing will blow up — and in fact, using a more powerful charger should speed up battery charging.
The longer answer is that the age of your device plays an important role, dictating both how fast it can be charged, and whether it can be charged using a wall charger at all. In 2007, the USB Implementers Forum released the Battery Charging Specification, which standardized faster ways of charging USB devices, either by pumping more amps through your PC’s USB ports, or by using a wall charger. Shortly thereafter, USB devices that implemented this spec started to arrive.
If you have a modern USB device — really, almost any smartphone, tablet, e-book reader, or camera — you should be able to plug into a high-amperage USB port and enjoy faster charging. If you have an older device, however, it probably won’t work with USB ports that employ the Battery Charging Specification; it might only work with old school, original (500mA) USB 1.0 and 2.0 PC ports. In some (older) cases, USB devices can only be charged by computers with specific drivers installed.
Finally, a quick word about a few foibles of USB charging. For a start, while PCs can have two kinds of USB port — standard downstream or charging downstream — OEMs rarely seem to label them as such. As a result, you might have a device that charges from one port on your laptop, but not from the other. This might be a trait of older computers, as there doesn’t seem to be a reason why standard downstream ports would be used, when high-amperage charging ports are available.
In a similar vein, some external devices — hard drives and optical drives, most notably — require more power than a USB port can provide, which is why they include a two-USB-port Y-cable, or an external AC power adapter.
If you have any anecdotes to share about power-over-USB, or corrections for our math, please leave a comment.