The 12V jack in a car (once the cigarette lighter) is a pretty common place for many of us to put a USB charger these days. In a lot of car models, that 12V jack doesn't stop getting power, which means it's technically draining your battery even when the car's off. How much? Big Mess O' Wire decided to figure that out. Photo by Alan Levine.
Using some alligator clamps and a multimeter, Big Mess o' Wires figured out much the USB charger draws in standby mode, then figured out what effect, if any, it'd have on a car's battery:
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.
So, not much, really. Which means you can freely leave that USB charger plugged into the car's 12V jack if you want, provided you don't plan on putting the car in storage for around 117 days, that is.