12V USB Charger Will Probably Never Drain Your Car Battery

The 12V socket in a car (was once a cigarette lighter) is a pretty common place for many of us these days when we put in a USB charger. In many car models, this 12V connector does not stop receiving power, which means that it technically drains the battery even when the car is turned off. How many? Big Mess O ‘Wire decided to figure it out.

Using crocodile clips and a multimeter, Big Mess o ‘Wires figured out what the USB charger consumes in standby mode, and then figured out what effect it would have on the car’s battery, if any:

Is the constant current draw of 14.2mA enough to worry about draining the car’s battery? Probably no. After doing some quick searches, I found out that the capacity of a typical car battery is around 40 Ah. At 14.2 mA, it will take 2817 hours or 117 days to fully discharge the vehicle’s battery. Assuming I drive every day, that’s not a problem. Even parking your 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 can add up. Of course, the car itself has its own standby current for the anti-theft system and keyless entry, so a USB charger might not even be the biggest issue. At least for typical driving, the standby USB charger’s current draw will not be an issue.

So not that much. This means that you can freely leave this USB charger plugged into the 12V car socket if you like, provided that you do not plan on storing the car for about 117 days, that is.

Standby current USB car charger | Big Mess O ‘Wires via Adafruit


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