Hopefully this explanation will be sufficiently technical to help, without being either overly-simplified or EE level . . .

There's a few modes that USB can be used for power (including charging). Your wall charger and car charger are probably both case 3 here, whereas connecting to a computer should start as case 1 and negotiate case 2.


1. low current. Initially, a device (like a phone) plugged into a host (a computer, etc.) will get a small amount of max current, 50mA IIRC so it can start negotiating with the host for more. Some devices may be able to charge slowly from this without negotiation, some may not.

2. negotiated current. After establishing communication, a device can request a higher maximum current by talking with the host, which the host can grant or refuse.

3. high current "dumb" charger. Newer wall warts, car chargers, etc. may use this. It's called BC1.1 DCP (Battery charging protocol version 1.1, dedicated charging port). This requires no data transfer between the charger and the device, the D pins are shorted through a small resistor. Some devices will follow this protocol precisely, and only pull high current from the wall wart if they detect the low-impedence between the D pins. If this isn't working on the device, it'll charge very slowly since the device will think it's not plugged into a DCP charger. This spec allows manufacture of low-cost chargers that don't need USB circuits and uC's inside.


(Oh, there's also a newer charging specification (BCP 1.2, I think) introduced circa 2012 by the USB industry association, but that's beyond our scope here.)

This ins't meant to be a textbook on USB charging or a replacement for reading the USB specs, which can be accessed at usb.org if one wants to build their own. I've simplified and left out quite a bit that I (personally) didn't think would help with the question.