Big Mess o’ Wires:
Is a constant 14.2 mA draw enough to worry about discharging the car’s battery? Probably not. From a few quick searches, I learned that a typical car battery has a capacity of around 40 ampere hours. At 14.2 mA, it would take 2817 hours or 117 days to completely discharge the car’s battery. Assuming I drive the car every day, then, it’s not a concern. Even parking the car for a week or two should be fine. But if I ever need to leave the car in storage for an extended period of time, that 14.2 mA could add up. Of course the car itself has its own standby current draw for the anti-theft system and keyless entry, so the USB charger may not even be the largest concern. For typical driving, at least, it appears the USB charger’s standby current draw won’t be a problem.
I thought it was unlikely that leaving a car charger plugged in would drain your battery, but I never had the courage to just leave it that way. The last thing I need is a car that won’t start because I was too lazy to unplug the thing. But now that I know it’s safe to do so, I won’t even worry about it.