A 12V USB Charger Probably Won't Ever Kill Your Car's Battery

A 12V USB Charger Probably Won't Ever Kill Your Car's Battery

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.

Standby Current of a USB Car Charger [Big Mess O' Wires via Adafruit]


    A 12V USB Charger is much more likely to damage your phone.

    That's not research.

    If they've worked it out on paper that one charger will be powered by a battery for 117 days, it's time for them to wire 10 of them up to a fully charged battery and see if it lasts 11.7 days.

    Also, you can't start your car if the battery is 99% empty, so they first need to determine the minimum level of charge that is still enough to start an average car, then test how long it it takes for a USB charger to bring a fully-charged battery down to that level.

    Send `em back to the lab and tell them to go and do the job properly.

      They got told.

      Actually, that experiment would not be accurate ;)

      The very important factor it and the original calculations don't account for is that the 40Ah rating has another component - the rate of current draw.

      40Ah is probably a 20 hour rate, which in this case is a 2A current. So it has a 40Ah capacity if discharged over 20 hours. At a 10 hour rate it will have a lower capacity. A lower discharge current and thus a higher hour rate can give more than a 40Ah capacity.

      At a very low current like 14.2 mA the capacity could be very high, depending on the battery. Using some battery specs of the internet and Peukert's law, some very simplified calculations suggest a capacity over 100Ah at 14.2 Ma.

      This isn't useful in the real world either of course, and factors (aside from other power consumption by the car) such as the battery age, condition, self discharge rate and temperature can drastically change the result.

      So testing with 10 devices and a higher current draw will reduce the usable capacity, so is not a useful experiment in itself.

      The 117 days isn't accurate even as a ballpark. But the conclusion is the same - leaving the USB charger in will have little impact in normal use.

        Wow. That's obscure, interesting and remarkably practical. Thanks!

        I'm going to go and hook a Raspberry Pi up to a Car battery now. Sounds like it's likely to be running for well over 40 hours!

          Yeah, I thought so too!

          For various such experiments, I have found cheap second hand deep cycle lead acid batteries on Gumtree before. They are great because if they are accidentally run dead flat, they charge back up ok. Whereas a car battery does not like going under about half charge.

          The batteries I got where 100AH, ex race yacht starter batteries. About 85% of the original capacity left, and $50 each.

          Also good is a cheap solar regulator and panel off eBay. It's super easy to make a system that will run something like a Raspberry Pi off grid indefinitely.

    i can't think of a single car i have come across that the 'jack' aka cigarette lighter socket that stays live after the car is off. That is for AU, EU, Jap, and American cars. The ONLY exception i can think of is a secondary plug in the boot area.

      I had a VW Passat and its jack in the cabin of the car stayed live all of the time.

    Battery chemistry is another factor. Lead Acid batteries generally get damaged (sulfated) after being depleted 20% or more of their full capacity. Deep cycle batteries, not so much. Battery type and chemistry are a very important factor. Amp Hour ratings are a guide to current draw. Not overall lifetime. Batteries and life can get quite complicated.

    More info here: http://batteryuniversity.com/learn/article/whats_the_best_battery
    Advantages and Limitations of Battery Types

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