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Old 03-23-2014, 08:32 PM
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Is it possible to drain a car battery by charging a phone?

Yesterday I needed a jump start because somehow my battery got drained. I have a 2013 Honda CRV with less than 10K miles on it. I drove about two hours to a soccer tournament and between games sat in the car with the key turned to "accessory" mode. The sound system was on, playing music from my daughter's phone, and my phone was charging, evidenced by the little lightning icon. (That's an iPhone and an Android plugged in simultaneously, if it matters.)

At one point I looked down and saw that the battery indicator on the phone hadn't increased at all, so I went to turn the car on so it would charge more effectively. Lo and behold, the battery was completely dead. We couldn't have been there more than 30 minutes.

I called roadside assistance and the guy who came to give me a jump said yep, it was the fact that the car wasn't on and we had things plugged in. But I find it hard to believe that two phones could actually drain a car battery. And in any event, if that's what happened, wouldn't the phone have actually charged?
Old 03-23-2014, 09:00 PM
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Two smartphones can't drain a healthy car battery in 30 minutes.
Old 03-23-2014, 09:25 PM
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The car speakers were playing, yes? That and any running lights, cabin lights, or other accessories will consume a lot more juice than your phone charger.

Playing a car stereo for hours can easily drain a car battery. The few drive-in movies that exist these days tend to use FM radio transmitters for sound. At the end of the movie it's not uncommon for a few cars to need a jump start.
Old 03-23-2014, 09:28 PM
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The speakers were on, yes, but it certainly wasn't hours. But it is possible that my daytime running lights were on; I don't know if they go on when the accessory setting is used. I'm going to check that next time I have a chance. Otherwise I can't think of anything that could have been running it down.
Old 03-23-2014, 10:04 PM
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A lot of newer cars like mine won't let you charge your phone unless the car is running or in the accessory position. The charger simply doesn't work when the ignition is keyed off.
Old 03-23-2014, 10:19 PM
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Quote:
Originally Posted by FoieGrasIsEvil View Post
A lot of newer cars like mine won't let you charge your phone unless the car is running or in the accessory position. The charger simply doesn't work when the ignition is keyed off.

Yep, the key was in the accessory position. Nothing happens when the car is off completely. But in accessory mode the battery is just draining.
Old 03-23-2014, 10:38 PM
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Car batteries will drain VERY rapidly if the car is not running and the key is in the accessory mode even if no phone or music player etc. is drawing from the lighter charge ports. The phones and the radio were a miniscule drain compared to activating all the car electronics without the generator running. Even with nothing pulling on the battery if I leave my key in the accessory position it will kill my big Denali SUV battery in about an hour.
Old 03-23-2014, 11:09 PM
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I'm not an expert in car electronics, but I know quite a bit about charging cell phones. Charging two phones will take (about) one to two amps. A quick good search indicates that the stereo speakers are drawing more like 5 amps. So playing the stereo probably had more of an impact on your battery than charging the phones did.
Old 03-23-2014, 11:14 PM
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If the cars network is up and running, the draw from that dwarfs that of the cell phone.
I'm not familiar with a Honda but a Volvo draws about 15 Amps.
I haven't measured my Hyundai but after about 10-15 minutes I get a start the engine message on the Nav screen.
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Old 03-24-2014, 09:08 AM
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My 2006 Nissan has a charging port that works no mater what. You don't even need the key in the ignition. You might want to check to see if you have similar ports.
Old 03-24-2014, 10:04 AM
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I asked my brainiac brother if he had a guess, and I think he may have solved the mystery. He thinks I likely turned the key to "run" instead of accessory by mistake, which would indeed have turned on the DRL and lots of other systems. Normally I would have noticed that, because the dash lights and so forth would have come on, but I had my camera bag sitting between the steering wheel and the dash so I couldn't see it. Since I hadn't actually started the car, it didn't feel any different to us inside.

Next oil change I'll have the charging system and battery checked, but I'm about 99% convinced this is what happened. Nice to know that in the battle between a cell phone and a car battery, the car battery would win.
Old 03-24-2014, 01:27 PM
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At the last company where I worked as a truck driver, the boss would go round on Friday evening making sure that no phones had been left on, and all the night heaters were off.

Truck batteries are BIG and 24 volts, but a phone left on over a holiday weekend would make them struggle on a frosty morning. Modern night heaters sense battery voltage and cut out if it drops below a certain point, but the older ones would happily flatten a battery in 12 or 14 hours. They also have timers so we could come in to a warm, defrosted truck.
Old 03-24-2014, 02:10 PM
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I'm disappointed in the dearth of actual numbers provided in this thread. Is anyone enough of an expert on electricity to give at least a rough estimate of how much power is used up by, say, an hour's worth of charging a phone, operating stereo speakers, DRLs, etc. and how that compares with the amount of power a car battery can put out before it loses its ability to start the car?
Old 03-24-2014, 02:16 PM
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Quote:
Originally Posted by ASanders View Post
Yesterday I needed a jump start because somehow my battery got drained. I have a 2013 Honda CRV with less than 10K miles on it.
My 2012 CRV had a battery charging issue where once every few months it wouldn't charge and I'd end up with a dead battery. The last time it happened, about a year ago, the service department at the dealer told me it was a known problem and a software fix had just come out for it. They updated my car's firmware and it hasn't happened since.

I don't know if the 2013's had the same problem, but I'd suggest you ask the dealer about it.

Last edited by Pixel_Dent; 03-24-2014 at 02:17 PM.
Old 03-24-2014, 03:17 PM
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Quote:
Originally Posted by Thudlow Boink View Post
I'm disappointed in the dearth of actual numbers provided in this thread. Is anyone enough of an expert on electricity to give at least a rough estimate of how much power is used up by, say, an hour's worth of charging a phone, operating stereo speakers, DRLs, etc. and how that compares with the amount of power a car battery can put out before it loses its ability to start the car?
Got something against post #9?
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Old 03-24-2014, 03:23 PM
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For the benefit of any future Dopers who check in here from a search, I'll say that I once drained a battery by leaving my cell plugged in over the course of a three-day weekend. The car was turned completely off, and that was the only thing running (no lights were left on or anything.)

So, 30 minutes, probably not. But over a three-day weekend? I'm here to testify.
Old 03-24-2014, 11:18 PM
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Just another anecdote, but I've drained batteries in multiple cars by using them as a power source for my laptop. Granted it's a 17in gaming laptop, but still, if it can drain one in a few hours...
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