Does a cell phone battery drain back to the car overnight?

I keep my cell phone plugged into my car charger when I’m driving. At night, I just turn off the car, but not the phone. In the morning, sometimes the cell phone is dead. Not undertanding electricity OR car battery systems, I don’t know why the electricity would seem to flow BACK into the car overnight. Does it? This doesn’t happen when I just leave my phone on at night while it’s sitting on my desk. Can someone explain to me what’s happening and why? Thanks, Dopers. xo, C.

That only seems likely if your phone is REALLY old and has primitive charging circuitry, which won’t be the case with anything that uses a Li-ion battery and even then a good charger would have a diode in series (as in the example here) with the charger to prevent this (the charger is built-in with most Li-ion designs; the external charger is just a power adapter and therefore could be anything that supplies the right voltage and sufficient current).

In my car, the cigarette lighter turns off when the car is off, hence no longer charging. My phone, an HTC Sensation can completely discharge over the course of an evening.

Where do you park your car? If it’s in a place with poor reception the phone will try harder to maintain connection with the cell towers, using more power to do so.

GPS also uses a lot of power, so if that is on it can easily drain your battery in 8-10 hours.

Many phones stay in a high power mode while charging–the screen stays on, the cell radio doesn’t go to sleep as quickly, etc. There’s a chance that the phone is staying in this high-power state while plugged in, even if no power is being supplied (if, as has been said, the cigarette socket turns off when the engine is off).

Also, batteries drain more quickly in temperature extremes. Is it very cold at night where you are?

Seems like you are greatly increasing the risk of a car break-in.
Seeing a valuable (and easily pawnable) cellphone sitting there in the car may be all the incentive some thief needs!

Every phone I’ve worked on has a charge FET to disable the path when there isn’t a valid charger detected. And the detection mechanism, at least in part, involves sensing 5V on one of the pins for the charger. So when you turn your car off, the charge voltage goes away, and we open the charge FET. So there would be no return path, and therefore no discharge.

I’ve never seen radio code behave like that. The display, yes…including the backlight. And that’s noticeable current draw. It would be a serious bug to leave it in this mode when the charger goes away, but I suppose a device that is looking for something BESIDES the 5Vs on v_USB could have this issue.
-D/a

Thanks. It’s an older phone - probably at least a couple years. The display goes dark after a couple of seconds, certainly when the car is turned off. Weather isn’t cold. Yet. (And no one is going to steal it - it’s in a garage.) I just don’t know why it discharges so fast in the car, but not when it’s at home.

The comment about signal strength is a good one. Do you have bad coverage in the specific spot you leave it? Phones will search for a signal, using more electricity, when they don’t see a good cell. It’s a classic problem.
-D/a

If your cigarette lighter sockets go off when the car is turned off, that may be easy to fix. On my truck, it only required unplugging a relay and jumpering the terminals. This is a very popular modification among people who like to customize their vehicles.

In our integral garage phones get poor or no signal. If I leave my phone in the garage it’ll die overnight. Left upstairs it dos fine overnight.

Hmm. It sounds like I should try to get a signal while I’m inside the garage, and if there is only one bar, for example, it may be that the signal is so weak that the phone is using up juice searching for a better signal? I’ll give it a shot. Thanks.

Just put it in airplane mode overnight.

And if it is enabled for wifi, it may also be using up juice searching for a wifi connection.

Why don’t you:
1 unplug it and leave it in the car
2 unplug the charger, leave it connected to the phone and keep it in your house
And you might see what’s the cause?

[quote=“Alley_Dweller, post:14, topic:634325”]

Just put it in airplane mode overnight.QUOTE]You may not realize the advances/changes in phone technology in the past 2-3 years. My cell phone, which I only use to make and receive calls (!) has no “airplane mode,” nor do I get wi-fi or internet, nor does it have GPS. It’s a cell phone - not a tracker - so I’ll try some of the other suggestions to diagnose the problem. Heh…airplane mode. Thanks.

The answer to the original question is very simple.

If your car battery was still connected to the phone, the phone would stay charged all night, but would (slightly) drain the car battery. The car battery normally has way more amp-hours than the phone needs.

Note that if your car battery was near end-of-life, this could drain the car battery enough so the car might not start. Also, if something is wrong with the phone, it could drain even a healthy battery. Also, if left long enough, a normal phone will eventually drain a normal car battery. But this isn’t your case.

Because your phone is dead in the morning (and your car isn’t), cutting off the ignition must cut off the power to the socket. As mentioned above, turn your phone off. Or you can take your car to any mechanic and ask for the socket to remain powered when the car is off: this is usually an easy modification.

BTW, lack of “airplane mode” on your phone is not due to your phone being more advanced. Evidently it’s not a smart phone, so during flights, you just turn it off, because it has no other useful functions.

In any case, it’s very unlikely that your cell phone is draining into the car at night, unless there’s something wrong with the car wiring. The power socket (e.g., cigarette lighter or USB socket) should be an “open circuit” when the car is off, given that it won’t charge your phone when your car is off. An “open circuit” basically means it won’t drain any power even if your phone charger could do that (which I doubt, and which Digital commented on above).

For the record, internet access first came to mobile phones in 1999 and went mass-market in 2002. The first phone I’m aware of with WiFi is the Nokia N93 from 2006, and they added GPS in 2007 with the N95.

So it’s not at all unreasonable to assume you might have had all of that on a phone made 2-3 years ago. I certainly did – I bought an N95 when it was released and I’d been using mobile Internet long before that on almost every phone I’ve had.

The first phone I worked on started at the very end of 2002, and was released in 2005…and had WiFi. I don’t believe we were the first in my company.

I’d have to dig one up to test it, but I’m pretty sure we supported airplane mode, too.
-D/a