Does Usb Drain Car Battery. I dont believe there is any power output when the car is turned off. But that little LED is using only a fraction of a watt and with nothing else plugged in there should be no additional drain.
Is it still possible its causing some sort of short in the system and draining the battery or is it definitely a coincidence. Of course modern vehicles do tend to already have plenty of other electronics and systems in them that consume varying levels of power in the background while the car or truck is parked alarm systems keyless entry receivers automatic software updates etc and overall battery performance does decline with the age of the battery a 3 year old battery wont be as robust as one. And by the way heres how to tell which apps are draining your phones battery.
If the lights lit then there is a drain on the battery.
Also the OP asked about USB devices not just phone chargers. Hence you could at most draw 25W instantaneously running at max. Or your battery might be old and the energy draw could be enough to drain it. The tech stating the USB cable being plugged in will drain battery is full of you know what since without being plugged into anything there is no completion of a circuit.
