A few volts here, a few amps there, and before you know it there might be nothing left!
May 20, 2010 1:07 PM   Subscribe

Does leaving my phone's car charger plugged in to the car at all times significantly drain my car's battery?

I can't believe I'm spending my weekly question on this, but I've just got to know!

I have a car dock for my Nexus One. Up until now, I've been plugging it (the car dock) in to the outlet in my car as often as I use it, and unplugging it when I'm done. However, I'd prefer to just leave it plugged in all the time and just pop my phone in and out the the dock as needed. This is a very very minor annoyance, but still.

My car (a 2008 Ford Focus SES) is the type that has always-on power outlets. Even when I take the keys out of the car, open the door, and get out, anything plugged into the outlet continues to receive power until unplugged. Of course, when I leave my car, I take my phone with me so my phone would not drawing any power from the car. However, there's an orange light on the car adapter that lights up when it's plugged in, and that light is always on when it's plugged into the car, even when there's no phone attached to the other end. So I have to believe the car adapter is drawing SOME power at all times, even if it's a really small amount.

My question is: what is the danger of leaving the car adapter with its small power draw plugged in at all times? How long would it take the little orange light and whatever power leakage might occur to drain my car's battery? If I didn't drive my car for a week, would it be a problem? Does this shorten the life of my car battery at all?

I imagine you guys are going to tell me "dude, it's such a small power drain, it'd take forever before it'd be a problem". But I'd just like to be sure.

Thanks MeFites!
posted by Vorteks to Technology (22 answers total) 1 user marked this as a favorite
Best answer: Well, I accidentally left my iTrip plugged in one time, to my always-powered cigarette lighter (in a '02 VW GTI). I don't drive my car a lot, didn't notice, and 3 days later came back to a dead battery. I'm pretty sure I've left it plugged in overnight without issue, however. But I'm no longer taking any chances.
posted by cgg at 1:10 PM on May 20, 2010

Best answer: We accidentally left an iPhone charger plugged into my husband's 2005 VW Beetle when we went out of town for a week. When we got back to the airport (an hour from home late on a snowy winter night in Philadephia, of course), the battery was dead as a doornail. These days, the charger is only plugged in when the iPhone is plugged into it.
posted by immlass at 1:15 PM on May 20, 2010

Best answer: I leave my phone plugged into my car charger 16 hours a day (work phone, leave it in the car when I'm not using it for work)... never a problem..

there are constant drains on a battery, your radio, gps systems, alarm systems, etc..

don't worry about it unless it causes it a problem, which I doubt will happen with that new a car.
posted by HuronBob at 1:17 PM on May 20, 2010

hmmm..looks like you're good if it's not a V-dub... :)
posted by HuronBob at 1:18 PM on May 20, 2010

Best answer: How long would it take the little orange light and whatever power leakage might occur to drain my car's battery?

Geez. Like AGES. Possibly two weeks. Probably longer, just from that little LED and maybe a voltage step down. The iTrip mentioned may have more draw in terms of power, running an FM transmitter and all, but it's unlikely that even that was what flattened a healthy battery in 3 days.

If I didn't drive my car for a week, would it be a problem?

Hell, no. There are probably as many small leaks from things staying powered in your car anyway.

Does this shorten the life of my car battery at all?

Not at all.

Leave it plugged in.
posted by Brockles at 1:18 PM on May 20, 2010

Best answer: We accidentally left an iPhone charger plugged into my husband's 2005 VW Beetle when we went out of town for a week. When we got back to the airport (an hour from home late on a snowy winter night in Philadephia, of course), the battery was dead as a doornail.

My Jag occasionally does that in very cold situations. It can flatten the battery inside a week, or it can be fine for two. It's weird, but is very definitely temperature related - I still haven't found out why.

Cold weather is awful for batteries = especially. They give radically different performance in cold weather than hot. It's possible the battery drain was absolutely nothing at all to do with the charger.
posted by Brockles at 1:22 PM on May 20, 2010

Best answer: I've also had a dead battery after 2-3 days with a charger plugged in (and nothing even plugged into the charger.)
posted by callmejay at 1:50 PM on May 20, 2010

Battery docks and rechargers, power strips and surge protectors do drain power even when they have nothing plugged into them, sometimes over five times as much energy as they need to just do the recharge, and a lot more than just the amount it takes to keep the little light lit. (Here's someone on salary at Slate saying this.)

If it's going to be enough to drain your car battery (which is recharged for free from the extra power your car produces), depends on how good your battery and generator are, and things like how cold it is. With all things being in good shape, and not going a really long time with out running the engine, it shouldn't be a problem.
posted by Some1 at 1:50 PM on May 20, 2010

Best answer: Datapoint: my husband always leaves his phone charger plugged in (Ford Ranger) and has done so since at least 2006. He's never had a problem with the battery. He doesn't drive the truck everyday either; sometimes he can go a couple weeks without using it.
posted by rabbitrabbit at 1:55 PM on May 20, 2010

Best answer: I'm curious about the answer to this as well, but anecdatally ... I drive a 2000 Ford Focus and leave my charger plugged in all the time. Sometimes I go 3 or 4 days without driving, no problem.
posted by Eyebrows McGee at 2:12 PM on May 20, 2010

Best answer: If you want to settle this definitively, get a multimeter and measure the current draw of the thing while it's plugged in but not charging anything. I suspect that there's enough variance in the design of chargers that there is not going to be any one definitive answer here. If you can find an amp-hour rating of your battery (not cold cranking amps) then you can just divide and get an approximate time period that it would take to discharge based on that load, but keep in mind that the charger is not the only load on the battery when the car is sitting idle -- things like the car's clock or security system will have a small drain as well.
posted by Rhomboid at 2:13 PM on May 20, 2010

Some vehicles have outlets that are only powered when the ignition is on or in the ACC position. I imagine that was designed to address this issue.
posted by M Edward at 4:02 PM on May 20, 2010

Best answer: I have a 12V extension cord with a switch and an LED power-on light on the plug end. I keep my phone charger plugged into the extension cord and keep the switch turned off most of the time. It takes up a little more room but works. I got it because the car is old and the 12V socket was threatening to come apart, so now I'm not plugging and unplugging stuff all the time.

Or maybe you could get someone to install a switch and power LED in the dash next to the socket. Or get someone to make up a one-piece plug/socket combo with switch and power LED, if you don't want to deal with the cord. Maybe they even sell those, who knows?
posted by Crabby Appleton at 4:24 PM on May 20, 2010

Just to clarify, I bought the 12V extension cord somewhere, I can't remember where, but it was probably either someplace like Walgreens or Target or one of the auto parts chains like Pep Boys or AutoZone. Wish I could remember.
posted by Crabby Appleton at 10:44 PM on May 20, 2010

Response by poster: Thanks everyone! All the answers were good. I like having all these different datapoints, even if it is just anecdotal evidence. I guess the answer to my "can it drain the battery" question is "sometimes, in certain circumstances, but you're probably OK".

@Eyebrows McGee - Thanks for your comment especially. Since you're driving a comparable car (albeit older), I'd assume that what works for you would work for me.

@Some1 - I think you misread the Slate article. It said people may have up to 5 chargers (you know, 1 phone charger, 1 laptop charger, 2 iPod chargers, and a camera charger, or something like that). It didn't say the charger may take 5 times as much power when not charging a device as it does when charging something. I would think that would be quite scandalous if true. Given how hot my laptop charger can get when my laptop is plugged into it, I imagine it would burst into flame if it somehow managed to pull 5 times as much juice.
posted by Vorteks at 6:26 AM on May 21, 2010

Best answer: On many late model cars, it's common to have up to 500mA drawing from the battery when the car is at rest (500mA would be on a high tech car with many accessories).
This amperage allows the various computers and systems to maintain certain memory that'd be erased if they were totally disconnected from their power source. So, unless it takes a whole amp to keep that tiny little LED lit up, you're probably fine.
posted by Jon-o at 1:26 PM on May 21, 2010

Y'know what? I'll test it this week and get back to you with some results. I'll hook up an accessory charger with an LED power indicator and take an amperage draw measurement and report back. Metafilter can settle this definitively.
posted by Jon-o at 1:58 PM on May 21, 2010

Response by poster: @Jon-0 - Thanks! That'd be awesome.
posted by Vorteks at 2:40 PM on May 21, 2010

Have a Griffin PowerJolt Dual USB plug sitting in my always-on 12V plug with a USB-iPhone cable plugged in my Mazdaspeed3. It's been there within a few weeks since buying the car and the longest stint without turning the car on was 10 days.

Started up just fine.

Curious what you find out Jon-o!
posted by liquoredonlife at 2:58 PM on May 21, 2010

Best answer: So, here's the data:

The little LED in the charger for my navigation draws .04 amps. That's going to take a very long time to drain your battery. And that's even if your cigarette lighter is hot at all times, meaning its circuit still gets power with the key in the OFF position. In my case, the circuit is only hot with the key on, so that charger can't possibly drain the battery. Your mileage may vary, of course. A quick check: If the LED turns off when you turn the key off and remove it from the ignition, then your charger will never drain your battery since the cigarette lighter powers down with the key off.
posted by Jon-o at 1:11 PM on May 26, 2010

Just in case anyone wants to double check my data, I used an inductive amp probe with a conversion of 100mv/A (or 1mV=.01A) and my reading was 4.0mv with the charger hooked up and 0mv with nothing in the cig lighter.
posted by Jon-o at 1:16 PM on May 26, 2010

Response by poster: Thanks Jon-o. That's good to know. It's possible my dock uses a bit more than that (the Nexus One dock has a bluetooth radio in it and stuff), but hopefully it's smart enough to keep that fully powered down when the phone's not in it. I'm convinced from the answers in this thread that I don't need to worry unless I'm going to be out of town for more than a week or two.
posted by Vorteks at 9:54 PM on May 31, 2010

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