Data port always powered? Why you might think that, what your USB device has to do with it, and how to prevent 12V battery drain. Potentially...

TL;DR:
Hypothesis: Some USB devices plugged into the USB data port prevent deep sleep by leaving data lines in an ambiguous state. For example, my Carlinkit Mini (and dumb cables) disconnect properly, allowing deep sleep. Others don't, and trigger a different, power-consuming sleep state, which is why people report 12V drain with certain devices. Experimental evidence presented.

It's being claimed quite often that the center console USB data port is constantly powered and thus a potential source of 12V battery drain if certain devices are kept plugged into the port and the car is off.

I already knew that the port isn't constantly powered in my car, despite people's persistent claims, but I wanted to collect some solid data on it, so I monitored that port overnight using a logging multimeter powered by its own power source, i.e., not with power from the port. This is what I found:

OWON B41T+ voltage data of USB data port after turning off the car

I turned the car off and measured USB data port voltage overnight. The pattern I saw is identical to the 12V battery voltage pattern after turning off the car, as seen here for example for the time period 6:40-9:10 am:

BM2 voltage trace of my 12V battery (Duralast Platinum Elite, 4 months old)

That's is, after turning off the car, the car runs its usual 5 maintenance jobs in 30 minute-intervals. During those jobs, the 12V system is alive, and the USB data port is powered. In between, the port is off.

The kicker is that the above 12V battery trace is what would normally happen. It is not the trace that went along with my data port measurements. That is this one:

BM2 voltage trace of my 12V battery (Duralast Platinum Elite, 4 months old) while monitoring the USB data port (0-7:00 am)

The relevant timespan here is until 7:10 am when I drove my kids to school. The trace clearly shows that the 12V battery was being drained and recharged by the LDC twice in a very short amount of time (~6 hours)! Normally, it may take a whole day or even longer for the voltage to drop low enough for the LDC to kick in.

So, somehow, my monitoring the port caused increased 12V power usage, despite the fact that the port was off most of the time.

To monitor the port, I used a cable that came with my USB tester. It plugs into a USB port on one end, and it has alligator clips on the other that can be connected to a multimeter. What I ignored was that it also contained some electronics in a small box in the middle of the cable. That is used by the USB meter to analyze a USB port via the D+ and D- wires (the data lines) from the USB.

So, I repeated the experiment with a dumb cable, i.e., one that connects the V+ and GND wires to the multimeter but does not use the data lines at all. Lo and behold, the 12V power drain was gone, and everything was back to normal! I conclude that the data-line sensing electronics were the culprit that caused increased power consumption, not the presence of a device at the power lines.

So, why was there power drain despite the fact that the port itself was not powered?

Here is my theory, and here is where electrical engineers can chime in, because I am not one: we know that the sleep state of the car is affected by many things, for example, doors unlocked vs locked, Bluelink access or not, just to name two situations. I believe that the fact that a device with active data lines was plugged into the port when the car was turned off does not allow the car to go to proper sleep. It entered a sleep state that is way more active and that consumes a lot of 12V power. My Carlinkit Mini does not do that. Many people have reported 12V drain with their devices, though. The connection between the car and those devices isn't cleanly severed when they shut down, and that sets a flag in the car that prevents it from going to deep sleep.

Some devices might do this intentionally, maybe to speed up connection times when there is a request, so I don't really know whether this is sloppy on the device side, sloppy on the car's Infotainment side. Maybe, the car's USB subsystem needs a more robust timeout logic, perhaps at the expense of slower start-up or device-connection times.

There are lots of follow-up experiments one could do here. Anyway, it might be helpful to characterize devices, such as wireless adapters, with respect to this sleep-state behavior, so that people can avoid them, or petition Hyundai to update their software.