This was discussed somewhat a long time ago. Basically with the USB units there is a circuit inside which converts the 12volts to 5 volts. This circuit is active at all times and so it draws a few milliamperes even when nothing is plugged in. Then when you plug an item in the current draw goes up as it starts to charge the device. The cheaper the USB unit the more it uses at rest. I did find out the approx current draw at rest but can't remember now. So under most applications it will not run the battery down BUT on a small bike with a low capacity battery there is a chance if the bike is not used for a week or two the battery may go flat. HTH And most converters these day's come with a LED fitted to show its on .. I went away to work and forgot to hook up the bike to the Optimate charger, came home to find the Busa battery completely flat and shot, l found out that the permanent usb supply l had wired in for my phone was the culprit, it's now on the switch side of the circuits. I thought like some, that without a load attached there would be no current draw, i was proven wrong the hard way once again.