How USB Charging Works, or How to Avoid Blowing Up Your Phone

The tech world has finally coalesced around a charging standard, after years of proprietary adapters and ugly wall-wart power supplies. USB-C is in the process of replacing them. But, in a brilliant example of getting exactly what we asked for and not at all what we wanted, USB-C has solved the wall-wart problem by transplanting it. Instead of running around with a collection of manufacturer-specific wall plugs, customers with multiple USB-C devices have to run around with a manufacturer-specific or genuinely compatible third-party cable.

Ten to 15 years ago, you always had to make sure you had the correct power supply for each of your gadgets. Now you have to make sure you have the right cable for your gadget. Part of the reason for this problem is another feature that was buffed in USB-C to make it more effective and flexible: USB power delivery.

Not all USB chargers, connectors, and cables are born equal. You’ve probably 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. It turns out there’s a method to all this madness — but first, we have to explain how USB power actually works.

New Specifications

There are now six USB specifications — USB 1.0, 2.0, 3.0, 3.1, 3.2, and USB4. USB 1.0 is old enough that you’ll virtually never see it, so USB2-USB4 covers the spectrum for most people. Separately from that, there’s USB-C, which is a physical connection standard that devices can use. In a USB network, there is one host and one device. In almost every case, your PC is the host, and your smartphone, tablet, or camera is the device. Power always flows from the host to the device, although data can flow in both directions, such as when you copy files back and forth between your computer and your phone. The descriptions below apply to all versions of USB currently in use, including USB4, 3.0, 3.1, 3.2, and their various sub-standards (1×2, 2×2, etc).

What’s New About USB4?

USB4 requires USB-C, while previous standards merely made it an option. Support for USB Power Delivery is now mandatory and the standard now allows for DisplayPort and PCI Express tunneling. Nominal data rates on USB4 are 20Gbps and 40Gbps. Up to DisplayPort 2.0 is supported via alt mode, DP1.4 is supported via tunneling.

As for how USB4 performance compares against other USB standards, we’re genuinely sorry, but that’s a bit confusing at the moment.

Unlike previous generations of USB, USB4 is based directly on the Thunderbolt 3 specification. This does not present any kind of problem for AMD or other companies that use USB4. To the end-user, the switch to Thunderbolt should be seamless and unnoticeable. USB4 devices are not required to interoperate with Thunderbolt 3 products, but Thunderbolt 4 devices must also support USB4.

USB4 doesn’t always add maximum top-end bandwidth, but it should make it easier to hit higher performance figures. It moves us towards USB-C as a universal standard and it offers some additional protocol support and tunneling features that weren’t available earlier. It’s not going to be revolutionary, but USB4 Gen 3×2 devices will be significantly faster than what has shipped before.

USB 3.2 and Earlier

A regular USB 1.0 or 2.0 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. USB 3.0 ports add an additional row of five pins, so USB 3.0-compatible cables have nine wires. In terms of actual current (milliamps or mA), there are three kinds of USB ports 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); with USB 3.0, it moves up to 900mA (0.9A). The charging downstream and dedicated charging ports provide up to 1,500mA (1.5A). USB 3.1 bumps throughput to 10Gbps in what’s called SuperSpeed+ mode, bringing it roughly equivalent to first-generation Thunderbolt. It also supports a power draw of 1.5A and 3A over the 5V bus. USB 3.2 does not change these aspects of the standard.

USB-C is a different connector entirely. It’s universal; you can put it in either way and it will work, unlike with USB, and like Apple’s Lightning connector. USB-C is also capable of twice the theoretical throughput of USB 3.0 and can output more power. Apple joined USB-C with USB 3.1 back in 2015 with its 12-inch MacBook and new MacBook Pros, and phones soon followed. 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 there are plenty of USB chargers that don’t conform to 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 1.8A; and many 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 ports rated at 500mA, and dedicated charging ports, which range all the way up to 3,000mA. This leads to an important question: If you take a phone which came with a 900mA wall charger, and plug it into a 2,100mA iPad charger, as an example, 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 explode — 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. Way back 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, you should be able to plug into a high-amperage USB port and enjoy faster charging. If you have an older product, 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 (much older) cases, USB devices can only be charged by computers with specific drivers installed, but this is now going back nearly two decades.

What About USB-C?

USB-C is a special case. While you won’t blow up your device from plugging in the wrong charger, you can blow up your phone, Nintendo Switch, or other device by using the wrong USB-C cable. How do you know what the right USB-C cable is? Sometimes — and this is the ugly truth — you can’t. In the past, there’ve been spreadsheets dedicated to recording good versus bad cables, but the projects seem to have fallen by the wayside and are now outdated. If you are buying a replacement USB-C cable for your manufacturer-provided cable, we recommend buying from the OEM or an authorized, third-party manufacturer. We covered the initial issues with USB-C in more detail in this article.

What if you already have a third-party cable or don’t have any option but to use one? Even if it works, slow charging speed could be a problem. This is preferable, in the sense that it won’t actually damage any of your hardware. In many cases, your device will only charge at minimum or near-minimum speed as opposed to the fast charging options now supported by many devices. This chart from Android Authority shows how charge speed varies depending on which device you use.

Keep in mind, however, that the consequences can be worse than just slow charging. There are cases of people blowing up devices with the wrong type of cable, and other instances where one device may flatly refuse to work with another device’s USB-C cable for unknown reasons. The assumption, in this case, is that the device is refusing to connect through a given cable to protect itself.

Quick Charging and Other Notes

Many new phones offer some kind of quick-charge capability — often with variable names, the most common being Qualcomm’s Quick Charge that works with Snapdragon-powered phones. The standard evolved quickly and is moving to version 5 at the time of this writing. For quick charging to work, usually you’ll need to use either the power adapter that came with the phone or an appropriately labeled third-party adapter. Otherwise, it will still take several hours to juice up. Quick Charge seems to be fading out of the market with the advent of USB Power Delivery (USB PD), but both capabilities are still supported.

There are a few other things to be aware of. While PCs can have two kinds of USB ports — standard downstream or charging downstream — OEMs haven’t always labeled 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 is 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. Most vendors now put a small lightning icon above the proper charging port on laptops, and in some cases, those ports can even stay on when the lid is closed. You may have to check your motherboard or system documentation to determine which USB ports provide the full capabilities you want. A system might only support charging, video playback, or Ethernet over a single USB-C port, even if it has two to four USB-C ports in total.

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