Dear Car Talk:
I propose to leave my iPhone 4 permanently plugged into my car's cigarette-lighter socket, engine running or not. When the car is not running, the ignition key would be turned to the "accessory" position. My questions: Would this keep my phone always charged? Could this cause my car's battery to discharge completely and be unable to start the car, or cause any other problem? -- Mark
Yeah. The "other problem" is that your car could get stolen. If the key is in the ignition, and it's set to accessory, you'll light up a few dashboard lights, which will be a neon sign that says "Steal me!" to any passing car thief or delinquent teen.
And they'll take your iPhone, too. Although an iPhone 4 is borderline -- you might get lucky, and they might unplug it and toss it out the window as they pull away.
In terms of the battery, I'm not so worried about your iPhone, on its own, draining your car's battery, Mark; the car battery is huge compared to your iPhone's battery. But without knowing what kind of car you have, I don't know what else gets powered when you put the key in the accessory position.
In the old days, accessory mode basically powered up the radio and the cigarette lighter -- which is what counted as sufficient entertainment back in the day. But now, when you put the ignition in the accessory position, depending on the car, the blower motor may kick in, the instrument panel may light up and the infotainment screen may come on. So I think you'd be better off circumventing the accessory position.
Ask your mechanic to safely wire up one of your power ports so that it stays hot even when the car is shut off. And make sure it's still fused. You don't want a car fire, either.
Some cars have power ports wired that way, but lots -- including, we assume, yours -- don't. But you can change that. Then all you'll be drawing is the about .08 amps your phone will use, plus whatever the car usually draws when it's turned off (alarm, emissions computer, keyless entry sensors, etc). And, crucially, you'll be able to take your key with you.
And if the phone is all you've got plugged in, and your battery and charging system are in good working order, you shouldn't have any problem starting the car the next day. And your Sleepy LaBeef playlist will always be charged up and ready to go, Mark. Rock on.