Ethernet adapter disconnects when AC adapter unplugged

Hi,

I have an xps 13 bought in September 2014, and an apple ethernet adapter. It is well supported by linux, but:

- When the AC adapter is plugged, the "xps 13 whine" stops when the keyboard backlight is on. However, it is turned off when the keyboard is idle for a moment (for instance when reading news with the trackpad), which resumes the whine. This is distracting.

- A potential solution is then to unplug the AC adapter: indeed, when running on battyr, the whine starts when the keyboard backlight is on, and stops when turned off.

But this disconnects the USB Ethernet adapter (apart from consuming battery for nothing)!

This computer seems to have serious power issues, and this bug is not only annoying (like the coil whine), it also makes the computer barely usable. How was this laptop tested?