How USB Charging Works

USB Connector Types

The tech world has finally coalesced around a charging standard, after years of proprietary adapters and ugly wall wart power supplies. Well, sort of: We’re already seeing some fragmentation in terms of the new USB-C connector, which could eventually replace USB, as well as what is thankfully turning out to be a short-lived obsession Samsung had with larger USB Micro-B connectors for its Galaxy line. But aside from that, and with the obvious exception of Apple’s Lightning connector, micro USB has destroyed the industry’s penchant for custom ports.

There are now four USB specifications — USB 1.0, 2.0, 3.0, and 3.1 — in addition to the new USB-C connector. We’ll point out where they significantly differ, but for the most part, we’ll focus on USB 3.0, as it’s the most common. 

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 smart phone, tablet, or camera is the device. Power always flows from the host to the device, but data can flow in both directions.

Okay, now the numbers. A USB socket has four pins, 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). USB 3.1 bumps throughput to 10Gbps in what is called Super Speed+ mode, bringing it roughly equivalent with first-generation Thunderbolt. It also supports power draw of 1.5A and 3A over the 5V bus.

USB-C is a different connector entirely. First, it’s universal; you can put it in either way and it will work, unlike with USB. It’s also capable of twice the theoretical throughput of USB 3.0, and can output more power. Apple is joining USB-C with USB 3.1 on its new Mac Book, and so is Google with the new Chrome book Pixel. But there can also be older-style USB ports that support the 3.1 standard.

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 may have 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 of 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.