USB car charger with LED
How much drain on the battery would the LED and empty dock cause? How long would it take to drain the battery enough to cause starting problems?


Assuming the cigar lighter is powered when the ignition is off (which is not the case in any car I have owned)

A LED might draw 20 mA

A typical car battery might have a capacity of 60 Amp hours

A very rough calculation would say that the battery can sustain this current for 60 / 0.02 hours or 125 days.

In practice, there are probably lots of other loads on your car's battery. For example it probably has a remote control for unlocking the doors, the receiver for this has to be powered.