What killed my battery if all my "accesories" were turned off?

Dear Car Talk

Dear Car Talk | Mar 01, 2002

Dear Tom and Ray:

I accidentally left my key in the "accessory" position over the weekend. On Monday morning, I had a dead battery. I was surprised, since the car is new. Nothing in the car was turned on. Where did all my little electrons go? -- JR

RAY: Good question, JR. Turning the key to the "accessory" position doesn't actually draw any electricity from the battery. It simply ALLOWS you to draw a limited amount of electricity to run certain accessories, like the radio, the power windows and the interior lights.

TOM: So SOMETHING had to be on, despite your protestations, JR. And since you've already admitted to being spacey enough to leave the keys in the ignition all weekend, we have to assume you're spacey enough to leave other stuff on -- like the dome light, for instance.

RAY: If you're absolutely certain that nothing was switched on over the weekend, then the answer might lie in the ventilation system. In most newer cars, the blower fan cannot actually be turned off. Even when you move the switch to the "off" position, the fan continues to turn at a very low speed whenever the key is in the "accessory" or "run" position. And that would certainly be enough to kill the battery in 48 hours.

TOM: So that's probably where your electrons went, JR. They were keeping the air moving inside your car all weekend. What do they know? They figured you were spending a nice, quiet, relaxing weekend in there.

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