# For How Long Can You Leave Your Headlights On Before Your Battery Dies? **URL:** \ **Category:** Factual Questions\ **Created:** [December 5, 2003, 4:12pm UTC](https://boards.straightdope.com/t/for-how-long-can-you-leave-your-headlights-on-before-your-battery-dies/217163 "2003-12-05T16:12:02Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![Surreal](https://avatars.discourse-cdn.com/v4/letter/s/eada6e/32.png) [@Surreal](https://boards.straightdope.com/u/Surreal)\ **Post date:** [December 5, 2003, 4:12pm UTC](https://boards.straightdope.com/t/for-how-long-can-you-leave-your-headlights-on-before-your-battery-dies/217163/1 "2003-12-05T16:12:02Z")
On average, what is the length of time it takes to fully drain a car battery simply by leaving the headlights on? I normally don’t bother to turn off my headlights when I pull into a gas station to fill up, as I figure it only takes me a few minutes and this isn’t nearly enough time to drain the battery. Also, if I did shut them off, there is the slight possibility that I might forget to turn them back on, which could be dangerous. However, whenever I leave my lights on at the pump someone smugly ‘reminds’ me of this fact, as if there is some sort of tremendous urgency surrounding the matter of my turning them off. So how long can you leave your headlights on before your battery dies? Thanks. ---
**Author:** ![Mojo\_the\_Monkey](https://avatars.discourse-cdn.com/v4/letter/m/eada6e/32.png) [@Mojo\_the\_Monkey](https://boards.straightdope.com/u/Mojo_the_Monkey)\ **Post date:** [December 5, 2003, 4:30pm UTC](https://boards.straightdope.com/t/for-how-long-can-you-leave-your-headlights-on-before-your-battery-dies/217163/2 "2003-12-05T16:30:51Z")
The headlights on my current car, a 1995 Honda Civic, will drain the battery in less than an hour in moderately low temperatures (30-40 F). On the other hand, my previous car, a 1900 Volkswagen Fox, could go almost 8 hours before the battery would drain past the point of restarting the car. Damn, I miss my Fox. ☹ I think this is a question without a good answer. It just depends on the car. ---
**Author:** ![SlickRoenick](https://avatars.discourse-cdn.com/v4/letter/s/e68b1a/32.png) [@SlickRoenick](https://boards.straightdope.com/u/SlickRoenick)\ **Post date:** [December 5, 2003, 4:34pm UTC](https://boards.straightdope.com/t/for-how-long-can-you-leave-your-headlights-on-before-your-battery-dies/217163/3 "2003-12-05T16:34:50Z")
> [@](#): > > \*Originally posted by Mojo the Monkey \* > \*\*I think this is a question without a good answer. It just depends on the car. \*\* . . .and the battery. ---
**Author:** ![KnK](https://avatars.discourse-cdn.com/v4/letter/k/edb3f5/32.png) [@KnK](https://boards.straightdope.com/u/KnK)\ **Post date:** [December 5, 2003, 4:37pm UTC](https://boards.straightdope.com/t/for-how-long-can-you-leave-your-headlights-on-before-your-battery-dies/217163/4 "2003-12-05T16:37:34Z")
It varies, on the amperage draw of the headlights and the capacity of the battery. Mathematically it can be figured out if you know those two. KnK ---
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [December 5, 2003, 4:42pm UTC](https://boards.straightdope.com/t/for-how-long-can-you-leave-your-headlights-on-before-your-battery-dies/217163/5 "2003-12-05T16:42:55Z")
Most will crap out after 30 minutes to an hour or so of drain. Some super heavy duty batteries night be able to go for 2+ hours with low draw lights. Pulling a car battery all the way down to failure or near failure is not recommended or good for the battery. ---
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [December 5, 2003, 4:48pm UTC](https://boards.straightdope.com/t/for-how-long-can-you-leave-your-headlights-on-before-your-battery-dies/217163/6 "2003-12-05T16:48:30Z")
> [@](#): > > \*Originally posted by Mojo the Monkey \* > \*\*The headlights on my current car, a 1995 Honda Civic, will drain the battery in less than an hour in moderately low temperatures (30-40 F). > > On the other hand, my previous car, a 1900 Volkswagen Fox, could go almost 8 hours before the battery would drain past the point of restarting the car. > > Damn, I miss my Fox. ☹ > > I think this is a question without a good answer. It just depends on the car. \*\* Your VW Fox car battery could power the head lights for _8 hours_? I have to admit I find this difficult to believe unless the battery output fell off to some “super dim stage” stage after an hour or two and just rested there. . ---
**Author:** ![aerodave](https://avatars.discourse-cdn.com/v4/letter/a/f08c70/32.png) [@aerodave](https://boards.straightdope.com/u/aerodave)\ **Post date:** [December 5, 2003, 6:56pm UTC](https://boards.straightdope.com/t/for-how-long-can-you-leave-your-headlights-on-before-your-battery-dies/217163/7 "2003-12-05T18:56:24Z")
You ever see the headlights on a 80s/early-90s Audi/VW? They were at “super dim stage” from the factory to begin with. A 9V Duracell could power them for a couple hours at least, I’m sure. ---
**Author:** ![FatBaldGuy](https://avatars.discourse-cdn.com/v4/letter/f/ecd19e/32.png) [@FatBaldGuy](https://boards.straightdope.com/u/FatBaldGuy)\ **Post date:** [December 5, 2003, 9:59pm UTC](https://boards.straightdope.com/t/for-how-long-can-you-leave-your-headlights-on-before-your-battery-dies/217163/8 "2003-12-05T21:59:52Z")
> [@](#): > > \*Originally posted by Mojo the Monkey \* > **On the other hand, my previous car, a 1900 Volkswagen Fox, could go almost 8 hours before the battery would drain past the point of restarting the car.** I didn’t realize VW was making cars in 1900.😃 ---
**Author:** ![Mr.Blue\_Sky](https://avatars.discourse-cdn.com/v4/letter/m/d26b3c/32.png) [@Mr.Blue\_Sky](https://boards.straightdope.com/u/Mr.Blue_Sky)\ **Post date:** [December 5, 2003, 10:29pm UTC](https://boards.straightdope.com/t/for-how-long-can-you-leave-your-headlights-on-before-your-battery-dies/217163/9 "2003-12-05T22:29:32Z")
Wouldn’t the type of headlight be a factor? Sealed beam vs halogen vs high-intensity discharge? ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [December 5, 2003, 10:53pm UTC](https://boards.straightdope.com/t/for-how-long-can-you-leave-your-headlights-on-before-your-battery-dies/217163/10 "2003-12-05T22:53:47Z")
The OP can’t be precisely answered because many factors are involved, such as: - Internal resistance of battery - Battery’s rated energy capacity (amp hours) - ``` Battery's discharge curve ``` - Charge on battery - Temperature - Resistance vs. voltage curve for headlights (an incandescent bulb is very nonlinear) It also depends on some “known” factors such as open circuit battery voltage (12.6 V) and chemistry (lead acid). And because by definition “dead” means you can’t start the car, you would also have to know the minimum charge on the battery that is capable of starting the car. At any rate, you should _not_ run your headlights any more than is absolutely necessary when the engine is not running. ---
**Author:** ![Mojo\_the\_Monkey](https://avatars.discourse-cdn.com/v4/letter/m/eada6e/32.png) [@Mojo\_the\_Monkey](https://boards.straightdope.com/u/Mojo_the_Monkey)\ **Post date:** [December 5, 2003, 11:21pm UTC](https://boards.straightdope.com/t/for-how-long-can-you-leave-your-headlights-on-before-your-battery-dies/217163/11 "2003-12-05T23:21:55Z")
> [@](#): > > \*Originally posted by Fat Bald Guy \* > \*\*I didn’t realize VW was making cars in 1900.😃 \*\* I told you it was a very special car! Make that 19 **9** 0.