What is "BC 1.2 port"?

Question

This story about a new Thunderbolt 3 dock from Promise Technology shows one of the two USB 3.0 ports labeled “USB 3.0 BC1.2”.

Some searches show results for "BC 1.2 port" but I've not yet seen a definition or explanation.

Answer 1

Battery Charging spec

BC 1.2 is an abbreviation for (USB) Battery Charging (revision) 1.2. I'm no USB specs specialist so I can link some abstracts/files only:

• USB Battery Charging (Wikipedia)
• USB Battery Charging Revision 1.2 (PDF)
• Overview of USB Battery Charging Revision 1.2 and the Important Role of Adapter Emulators
• Overview of USB Battery Charging Revision 1.2 and the Important Role of Charger Detectors

Basically it's a USB 3.0 port with some extra features to detect devices that can make use of charge currents beyond the standard 900mA.

Answer 2

USB-BC stands for "USB Battery Charging" and it is an optional enhancement to USB ports allowing them to provide power to charge mobile devices, with or without a data connection. In a sea of multiple standards for doing this, USB-BC is probably the most widely implemented and the most simple to implement, but it is fairly modest and its maximum power level - 5 to 7.5 Watts - is low by today's standard, relegating it to a "lowest common denominator" of charging standards.

USB ports were originally only intended for data and for a small amount of power, enough to power a low power device like a keyboard. It was assumed that more power hungry devices like printers and hard drives would supply their own power. At the time, USB wasn't designed with the portability of small laptops or mobile devices in mind.

USB ports can initially guarantee that they can supply only 0.1A of power to devices (0.5W). This isn't very much, but if a device needs more, it can request more by sending a request on the data lines as it starts up. The maximum a standard USB port can provide is 0.5A for USB 2 and 0.9A for USB 3. With a working voltage of 5V that translates to 2.5W of power and 4.5W of power respectively, and even that is not guaranteed to be granted, as multiple devices sharing a host may need to divide this power between them. The host keeps track of how much power each device has guaranteed to it.

This is not a great situation for devices that just want some power, and don't want to have to implement a whole serial communication on the data lines just to request it. And even if the request works, it's still not much power.

The alternative to this is for the port to support one or more special charging or power delivery modes, implemented as extensions to USB. These are commonly called "fast charging", but that name fails to convey that there are several different standards that are not compatible.

Notable standards include:

• USB Battery Charging (USB-BC) (currently at version 1.2) - this is an official standard that provides for up to 1.5A on the port, and can be provided with or without a data connection. It is typically implemented with a current limiter at the supply end and in practice the maximum current drawn under normal circumstances is nominally 1A (5 Watts).

  These are by far the most common charging standard for dedicated USB chargers that don't otherwise specify a particular charging standard. What's more, most of the more exotic fast charging standards can be easily made to be backwards compatible with USB-BC, meaning that your device will still charge at the standard USB Battery Charging rate as a fallback.

  Note that there are multiple modes of USB-BC - including modes in which the port is for charging only, and modes in which it can also use data at the same time. The "Dedicated Charging Port" mode makes USB-BC very simple to implement in a charger device, with no need to be able to do any serial communication on the data lines, reducing costs.

• Apple fast charging (12W) - A proprietary Apple extension to the above, allowing up to 2.4A (12W). This is backwards compatible with USB-BC, and Apple devices like tablets can use it to charge a bit faster. It is not possible to use data at the same time.

  You'll often find cheap power supplies that claim to be able to supply 2.4A at 5V, and this is why: they support Apple's 12W fast charging standard, which is pretty simple to implement on a dedicated charging port, and is backwards compatible. If you are implementing USB-BC and you know your power supply can provide the 2.4A, you may as well make it also compatible with this standard. Non-Apple devices will usually just use it as a regular USB-BC supply. In fact, it is so common for USB-BC supplies to implement Apple's standard that some people mistakenly conflate the two, thinking that USB-BC has a 2.4 Amp (12 Watt) mode. It doesn't, it's just two different standards similar enough that chargers can easily support both. (To technical people, the USB-BC dedicated port just needs to short D+ and D- to convey this to the device, while Apple's standard requires putting +2.7V on both D+ and D- - it's easy to do both at once.)

• Other proprietary fast charging standards like Qualcomm QC 1.0-3.0, and more. The more complex of these introduced special signaling on the USB 1.1/2.0 data lines to negotiate a particular voltage and current. It was not normally possible to use data at the same time.

  Qualcomm QC 4.0 is implemented on top of USB-PD (which is described below), requiring USB-C cables and connectors but achieving the impressive feat of backwards compatibility with USB-PD, USB-BC and previous Qualcomm QC versions.

  Some manufacturers rebrand proprietary fast charging standards, for example Samsung's "Fast Charging" is Qualcomm QC 2.0, and Samsung's "Super Fast Charging" is Qualcomm QC 4.0.

• USB Power Delivery (USB-PD). This is a new official standard, to supplement USB-BC (it doesn't have to replace it, and power supplies may continue to allow both, on the same port). Unlike USB-BC and the above standards, it requires features in USB Type C connectors and cables, so it's electrically not compatible with earlier USB cables or sockets.

  USB-PD can support considerably more power than USB-BC, which allows it to be used on even higher power consuming devices such as laptop computers. It can allow up to 144 Watts over a standard USB-C cable or 240 Watts over special USB-C cables that rated for 5 Amps.

  USB-PD can happen simultaneously with data transfer, and is even quite independent of it: it doesn't require the "data" pins to negotiate power delivery but dedicated "CC" pins instead, the power itself is delivered over separate pins to the regular 5V supply pins, and power delivery can happen in either direction regardless of the data connection.

Answer 3

The TLDR on this is that sometime in the last 10 years the minimum spec on powered USB-A ports became 1.5 A, and maximum became 3 A, but 2.4 A in practice. USB-BC is increasingly becoming a redundant a meaningless standard. Being replaced by USB 3.x and USB-PD.

From the beginning of USB people discovered that was trivial to get 5 V of power from USB ports by just connecting to the V+ and GND. This was being used for more than it's intended purpose of providing power to parallel port adapters and the like. To this end it became customary to allow 100 mA or 500 mA on systems are were "asleep" to allow keyboard wakeup, or all too often run novelty Christmas lights.

What the USB groups came up with was the USB-BC standard for providing power to devices even though they might not he making a data connection connection to the host. For USB 2.0 devices this had the benefit of increasing power to a device to the max of what USB 3.x allows. This meant USB-DC devices might enjoy 1.5 to 2.4 amp power on the same old USB-A ports.

A USB device that is plugged it is supposed to limit itself to 100 mA (150 mA with USB 3.x) until "enumerated" by the host. After enumeration come a possibility to ask for more power in larger increments which may or may not be allowed because of a hub or other devices taking power. Depending on the spec followed this might mean 900 mA or 1.5 amps allow.

This changed at some point with USB-BC. USB-BC allows for two power levels for unenumerated or enumerated devices, 5 watts or 12 watts. There's older pre-USB-BC that allowed for levels in between, like 9 or 10 watts but these fell out of disuse. There's non-USB standards that will provide 18 volts, meaning non-USB compliant devices to use this power, otherwise it's just another 5 watt worthless supply. It looks like USB-BC is getting slowly replaced by the more popular parts of USB-BC being incorporated into USB 2.0, USB 3.x, USB4, and USB-PD. The biggest "jump" being USB devices can expect 1.5A power without all the previous power query steps from USB before.

Most any USB-A "wall wart" made to charge or power a device will be rated for 5 W or 12 watt. Most any UAB-A port on an PC will be rate for 7.5 W (1.5 A) and rate to 15 watt (3 A) on a first come first served basis)

Such "odd ball" chargers you find will provide only 5 watts to anything else it is plugged into. USB 2.0 and USB 3.0 ports on computers have been able to provide 1.5 amps for some time now, at least ten years given how my Apple SuperDrive draws 1.5 amps from a USB 2.0 port and it was made in 2010.

Thunderbolt requires 15 watts, or 5 volt @ 2.4 amp out after the USB-C to USB-A adapter takes it's bit of power. Us that instead of the USB-BC port, if one is not enough.