USB - power and charging
It seems like USB only accidentally became a standard for charging things,
which is why the history of transferring more power through it is interesting.
USB3 and USB-C as a plug made things more capable -- but also more negotiated and more varied,
so harder to understand and to explain.
Attempt at shortish answer
Power and charging before USB-C
- PC ports :
- USB1 or USB2 root hubs (≈the port inside a PC/laptop)
- are more or less guaranteed to give 100mA without ever running into trouble
- negotiation may allow (5*100=)500mA (0.5A at 5V is 2.5W)
- USB3 root are similar but slightly higher:
- guarantees 150mA
- negotiation may allow (6*150=)900mA after negotiation (0.9A*5V=4.5W)
- external hubs on PC
- if they have their own power supply, they may act work out like a root hub
- if not, they may not allow more than 100mA
- (seems to be part of why they often only have 3 or 4 ports(verify) -- one layer of hub could ask for 500mA and provide into four 100mA devices)
- generic USB-A wallplug chargers
- when a device detects that the 'host' side doesn't present as a host at all, it is up to devices to behave reasonably
- before PD and BC stuff, we relied on conventions, which were messy and not very predictable
- but also, most chargers can easily provide 0.5A so most devices would often assume that. Devices tended to draw 500mA, though 100mA would be safer.
- USB Battery Charging (BC) added a way to signal 1.5A
- there were some variants that fake BC (it's fairly simple), leading some devices to draw up to 1.5A (often fine even if not to standards/spec)
- ...but generally limited to (5V*1.5A=)15W
- device-specific specific USB-A chargers
- ...could add something proprietary on top of the generic behaviour, that just their own chargers+chargees would understand
- some variants negotiating for a higher voltage as well.
- they may draw up to 1A or 2A, at 5V or sometimes higher.
- The classic range was often 5W to at most 20W, largely because wires used in generic USB2 cables cannot safely support a lot more.
- there were various non-standardized ways of doing this (many involved putting some voltage dividers on the data lines) (not Battery Charging or Power Delivery, they came later)
- Names seem to include
- QuickCharge, QC (from Quallcomm)
- Adaptive Fast Charging, AFC (from Samsung) (apparently based on Qualcomm Quick Charge?)
- SuperCharge, SCP (from Huawei), apparently a successor to FastCharge Procotol, FCP
- Pump Express, PE (from Mediatek)
- TurboPower (from Motorola)
- VOOC (from OPPO), a.k.a. Dash Charge
- PowerIQ (from Anker)
- AiPower (from Aukey)
- VoltIQ (from TronSmart)
- iSmart (from RavPower)
- ...and more variations, as well as more rebrandings.
- Most are tend to be designed for at most 10W, 15W or 20W (because of the mentioned wire thing) but are more likely to actually do more than standard 2.5W.
- introduced later (since 2007?), at least after many not-so-standards mentioned above were introduced
- up to 1.5A (which is 7.5W), but devices may still play it safe and don't try more than 0.5A (2.5W).
- BC seems to have no specific logo, so it either has a longer label, or you have to know(verify)
- also less flexible, e.g. doesn't touch on allowed voltage drop, whereas PD seems to require charged devices to monitor VBus(verify)
USB Power Delivery (since 2012, though the 2014 version (2.0) for USB-C seems like the first interesting one)
- introduced more fault tolerance (but also a lot more footnotes, not least of which the eMarker stuff)
- ...that more safely allowed things to do up to (20V*5A=)100W or, more practically, (20V*3A=)60W because PD and USB-C specs(verify) mandate checking cable capability above 3A (eMarkers are a common method)(verify).
Power and charging with USB-C
- without PD, the specific case may limit you to USB2 and USB3 limits (500mA; 900mA or 1,500mA for dual-lane), or not limit you to that (allowing 3A) (verify)
- ...at 5V, that may mean any of 2.5W, 4.5W, 7.5W, or 15W
- USB-C with Power Delivery
- and standard cable, it probably won't try for more than 3A at 20V (for 60W)
- and a heavier cable, it can try up to 5A at 20V (for 100W)
- ..but there may be practical reasons any one case may not go that high - like that most devices don't need that
- Some of those are now enough to meet the basic power draw of a laptop, meaning that laptops can now choose to use one of their USB-C plugs as a power plug
- at the same time not all chargers or cables will be equal here (and it's possibly there are preferred ports to do this on(verify))
- various smarter things may choose to charge slower
- (e.g. when it notices the adapter or cable doesn't seem to manage)
- ...so it can be hard to estimate how much faster something will charge, particularly for smarter devices
PPS
Using USB to charge batteries
In general, don't count on classical USB supplying more than 2.5W (standard)
...or 5W or 10W if slightly special (not so standard).
Since phones may have batteries on the order of 10Wh, that means charge will rarely take less than 4 hours - and easily double that when some part of the system is being careful
Since laptops have batteries on the order of ~100Wh, that means a charge would take dozens of hours -- which is why we, before USB-C, we didn't try to use USB to charge them
PC port limits
It's often mentioned that USB ports have a polyfuse - a physical component specced so that drawing more than e.g. 500mA will lead to that power being disconnected.
(There are other solutions. A few implementations actually monitor current, and use transistors to disconnect(verify), and in some rare cases the protection is missing)
Implementations play it a little wider than that, to avoid disconnecting devices due to transients and slight deviation -- because polyfuses may, by nature, not reset until power is removed, which is why you sometimes need to completely power off a computer (and sometimes even take the laptop's battery out) to get that port working again.
So USB2 polyfuses seem to have a trip current around 1.5A, and also a higher-than-500mA hold current (I've seen 750mA mentioned)
Roughly twice that for regular USB3 ports (ports can supply 0.9A as per specs)
More so for charging ports (how much?(verify)).
DIYers will know from trial and error that you can get away with just ignoring the intended negotiation and drawing 0.5A from a USB2 PC port without requesting it (0.9A from USB3 as per specs)
...because most hosts/hubs just assume USB devices act according to specs, they assume you would always ask not just draw it. And if you ever made your project into a product, it would need to actually request it to pass USB certification.
Note also that since you are defeating the thing that lets USB guarantee power to its devices, you may be able to cause all USB devices that sharing that power to brown out, something normally prevented by in-spec USB (at least in theory).