Will using my car's speakers at the drive-in movie theater drain my battery and leave me stranded?

Dear Car Talk

Dear Car Talk | Oct 01, 1998

Dear Tom and Ray:

My co-worker and I got into a discussion of drive-in theaters. We were getting
nostalgic, when I said I don't like to go anymore because they don't have the
speakers, and you have to use your own car radio to get the sound. My Ford
Ranger is 6 years old and has the original battery. And I don't want to be
stranded at the drive-in. He laughed at me and said he heard you could turn the
key to the accessory position for 24 hours, and it would only drain the battery
as much as starting the car one time. Is this true? --Frank

TOM: Frank. The whole IDEA is to get stranded at the drive-in!

RAY: Your friend is right. Assuming you're just using the car radio, and it's
not one of those mega-boombox-highway-vibrator models, you could expect to draw
about half an amp of current per hour.

RAY: So if the movie lasted 24 hours (or if you sat through three and a half
consecutive showings of "Lawrence of Arabia"), you'd draw 12 amp hours.

TOM: That's fairly insignificant. According to our battery guru at Exide, the 12
amp hours you drained would represent about 20 percent of the battery's starting

RAY: And since the average battery will give you five or six starts at a sitting
before petering out, you'd still have enough juice to give you four or five
tries at starting the car -- that's after 24 hours of radio listening in the
accessory position. Plenty for all but the poorest-running, most out-of-tune

TOM: Let's not talk about my Dodge Dart today, please.

RAY: And then, when you run the car, you charge the battery back up and,
assuming your charging system is working correctly, your battery goes back up to
full charge.

TOM: Now, if you have a really marginal battery (say, a cheap one that's 6 years
old!), you may just luck out and kill it during your 24 hours at the drive-in.
So be sure to choose your date carefully, Frank, just in case.

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