This issue comes up often so I always make an attempt in vehicle forums to explain it. USB charging is actually a lot more complicated than most people realize. The latest USB ports are designed to supply a specific electrical current based upon a standard specification depending upon how a device is connected and interacts. In order to understand this better, you have to look at it from a design engineer's viewpoint. I'll try to explain this as non-technically as possible because not everyone interested in this here is an electrical engineer. There is a lot more to this but it's beyond the interest of most people here who just want to know what's going on.
Standard USB cables usually have four wires: Power: VCC (+5VDC) and Ground, and Data: D+ and D-. Using a combination of these wires there are three basic modes of operation:
When the data lines are in normal use (i.e. transferring information), up to 500ma (.5 amp) maximum power is normally supplied by the USB specification. This is the Standard Downstream Port (SDP) connection configuration and is the most common connection when using a smartphone as a connected device.
However, if the data lines are properly manipulated, a handshake protocol occurs and the connected device can call for more current (up to 1.5A for USB 3.0). This is the Charging Downstream Port (CDP) connection configuration. This is the most sophisticated connection. Your phone may or may not employ this feature based upon its software and hardware configuration.
If the two data lines are shorted by the connected device or the cable, with no data communication at all, then the current will be set to deliver up to 1.5A, depending on what the available power source can handle. This is primarily used for chargers. This is called the Dedicated Charging Port (DCP) connection configuration.
This is primarily why you may see different charging times and capabilities when connected to the Ascent infotainment system. If you wish to know more, keep reading, if not you can stop here.
In a USB connection there is usually a "host", most often a computer, but in this case the Ascent's infotainment system (which is a computer), and the connecting device. The connecting device can be a smartphone, a USB thumb drive, a USB hard drive, etc.
It's not the primary function of the host to charge a connecting device. The primary function is to exchange data. However, it's also recognized that the host is responsible for powering a connecting device that does not have its own source of power, such as a thumb drive. Therefore the USB spec calls for 2.5ma-500ma (900ma w/USB 3.0) of current to be nominally available to power a connecting device. This is the Standard Downstream Port (SDP) connection configuration. If a device needs more than 500ma, then it's expected to supply its own power. This is why some USB disk drives have their own external power source and some don't, depending on whether it can operate with only 500ma. A smartphone supplies its own power, but its battery runtime is limited, so it may wish to be charged using this available host current. You can charge a smartphone's battery with only 500ma, but it'll probably be significantly slower than desired.
Thanks in part to Apple (who didn't like supplying chargers for their connecting devices such as the iPod), it was later commonly recognized that the host should also supply power for charging. However, the host may be a laptop on battery power or with limited internal power capability, or in the case of the Ascent, it may be running only on the car's battery at times. So the host is not going to simply give up more of its potentially limited power to a connecting device by default without being asked. Also, simply supplying more current by default may potentially cause issues with or even damage some sensitive connecting devices. Therefore, the connecting device needs to somehow ask the host for more power. Since the USB connectors prior to USB 3.0 only had two power wires and two data wires (plus an extra ground), the easiest way to do this was simply to effectively short out the data wires. Then the host knew the connecting device was in "charge mode" rather than "data mode" and would (at its own discretion) supply up to 1.5A for charging purposes. This is the Dedicated Charging Port (DCP) connection configuration. In this mode, data cannot usually be exchanged. Also, please note that some so-called "charging cables" attempt to force this mode by shorting out the data connectors internally. This may or may not work depending on how the host is designed to react. This also often causes a lot of confusion about USB cables. People think "charging cables" have thicker wires and thus can draw more current. The ampacity of USB cables has only a minor effect on current draw in short lengths. Usually most decently made USB cables can draw enough current to supply 1.5 amps in lengths under a few feet. Only the cheapest cannot. So it's rarely just a cable issue when you feel USB charging is too slow.
Finally, it was later recognized that a USB connecting device may want power for both data use and charging at the same time, such as in the case of smartphones, or that a connecting device simply needed more power than the default 500ma. So a more sophisticated way was designed for the data lines to request more current from the host. In this way, data could still be exchanged while the device was being charged and the required amount of current could be supplied (up to 1.5A). However, this mode requires that both the host and the connecting device are designed and programmed to establish and understand the handshake protocol and that the host is willing and able to supply more power. This is the Charging Downstream Port (CDP) connection configuration.
So the Ascent may have all three modes, but unless the connecting device (your smartphone) is designed to use CDP mode, it won't perform the necessary handshake and it'll be stuck in the lower current SDP mode. You can use a charging cable to attempt to force it into the higher current DCP mode, but then you'll lose the simultaneous ability to data transfer while charging.
So again, it's more complex than you'd think, but I hope this clears up USB charging for everyone.
