# Reversing Dry Cells in a Flashlight... **URL:** \ **Category:** Factual Questions\ **Created:** [February 16, 2010, 4:09am UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164 "2010-02-16T04:09:27Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [February 16, 2010, 4:09am UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164/1 "2010-02-16T04:09:27Z")
Why won’t a flashlight work if the batteries are inserted upside down? I imagine the terminals do not make proper contect. But, why would they bother to design it like this? I can only WAG they didn’t want someone orienting the batteries such that like terminals would touch? Thus, by making the flashlight work only one way, so to speak, one would be strongly encouraged to get the polarities oriented correctly??? Other than that, it really shouldn’t really matter how the batteries complete the DC circuit as long as opposite terminals touch, true? ---
**Author:** ![Gbro](https://avatars.discourse-cdn.com/v4/letter/g/c6cbf5/32.png) [@Gbro](https://boards.straightdope.com/u/Gbro)\ **Post date:** [February 16, 2010, 4:23am UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164/2 "2010-02-16T04:23:39Z")
I just reversed the battery’s in my mini-mag light and it turns on. I believe it is the bulb holder on some lights that prevent the negative end from making contact. Also remember that in a flashlight with 2 or more dry cells the battery’s can be rearranged when they appear dead and get some limited use rather than none at all. ---
**Author:** ![UncleRojelio](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/unclerojelio/32/3160_2.png) [@UncleRojelio](https://boards.straightdope.com/u/UncleRojelio)\ **Post date:** [February 16, 2010, 4:37am UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164/3 "2010-02-16T04:37:35Z")
So that you can put them in backwards when packing your flashlight and the batteries won’t get drained if the switch is accidentally bumped into the ‘on’ position. ---
**Author:** ![Gbro](https://avatars.discourse-cdn.com/v4/letter/g/c6cbf5/32.png) [@Gbro](https://boards.straightdope.com/u/Gbro)\ **Post date:** [February 16, 2010, 4:42am UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164/4 "2010-02-16T04:42:18Z")
> [@UncleRojelio](#): > > So that you can put them in backwards when packing your flashlight and the batteries won’t get drained if the switch is accidentally bumped into the ‘on’ position. Very reasonable, I just cannot do that with my mini-mag. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [February 16, 2010, 5:11am UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164/5 "2010-02-16T05:11:12Z")
Cylindrical batteries have different shapes for the positive and negative terminals, and most devices that use them have different design terminals to most effectively make contact. For a standard flashlight, it’s not that they care about polarity, it’s that they care about a good connection. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [February 16, 2010, 8:42am UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164/6 "2010-02-16T08:42:29Z")
> [@Gbro](#): > > Also remember that in a flashlight with 2 or more dry cells the battery’s can be rearranged when they appear dead and get some limited use rather than none at all. Can you elaborate on this? If they’re connected in series, how can it make any difference? ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [February 16, 2010, 12:10pm UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164/7 "2010-02-16T12:10:24Z")
Never mind… ---
**Author:** ![Harmonious\_Discord](https://avatars.discourse-cdn.com/v4/letter/h/74df32/32.png) [@Harmonious\_Discord](https://boards.straightdope.com/u/Harmonious_Discord)\ **Post date:** [February 16, 2010, 1:10pm UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164/8 "2010-02-16T13:10:45Z")
The spring on the bottom often didn’t touch the positive terminal if they were reversed. Think of the hole in the middle of a spring. The bulb contact problem was on ones that the positive terminal directly contacted the bulb and not the ones that had a metal spring tab between them. Some flash lights did have contacts that happened to work either way. Some modals used to have more complicated switches that couldn’t accidentally slide to on. They required you to press down on a small button on the switch to release it to turn on. They cost a little more also. ---
**Author:** ![Harmonious\_Discord](https://avatars.discourse-cdn.com/v4/letter/h/74df32/32.png) [@Harmonious\_Discord](https://boards.straightdope.com/u/Harmonious_Discord)\ **Post date:** [February 16, 2010, 1:12pm UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164/9 "2010-02-16T13:12:03Z")
> [@Mangetout](#): > > Can you elaborate on this? If they’re connected in series, how can it make any difference? It did work as did a short rest. I don’t remember it working on 2 cell flashlights, but more on 3, 4, or 5 cell lights. The more cells the better switching order helped. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [February 16, 2010, 1:28pm UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164/10 "2010-02-16T13:28:38Z")
If a flashlight uses LED’s instead of an incandescent bulb, it will only work with the batteries in the correct orientation; such is the nature of LED’s. That limitation can be overcome if the flashlight is designed with a rectifier in the circuit - but it’s cheaper to leave the rectifier out and just tell the user to put the batteries in the right way. ---
**Author:** ![Harmonious\_Discord](https://avatars.discourse-cdn.com/v4/letter/h/74df32/32.png) [@Harmonious\_Discord](https://boards.straightdope.com/u/Harmonious_Discord)\ **Post date:** [February 16, 2010, 1:42pm UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164/11 "2010-02-16T13:42:57Z")
> [@Harmonious\_Discord](#): > > It did work as did a short rest. I don’t remember it working on 2 cell flashlights, but more on 3, 4, or 5 cell lights. The more cells the better switching order helped. I’m referring to the old carbon batteries that were the common battery of the time. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [February 16, 2010, 2:06pm UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164/12 "2010-02-16T14:06:02Z")
> [@Harmonious\_Discord](#): > > It did work as did a short rest. I don’t remember it working on 2 cell flashlights, but more on 3, 4, or 5 cell lights. The more cells the better switching order helped. But how could this work? (changing the order, I mean, not the short rest, which works for many types of battery). Aside from the fact that rearranging the cells _implies_ a short rest, because it takes time to perform, how can it be explained in electrical circuit terms? ---
**Author:** ![Harmonious\_Discord](https://avatars.discourse-cdn.com/v4/letter/h/74df32/32.png) [@Harmonious\_Discord](https://boards.straightdope.com/u/Harmonious_Discord)\ **Post date:** [February 16, 2010, 2:12pm UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164/13 "2010-02-16T14:12:46Z")
> [@Mangetout](#): > > But how could this work? (changing the order, I mean, not the short rest, which works for many types of battery). Aside from the fact that rearranging the cells _implies_ a short rest, because it takes time to perform, how can it be explained in electrical circuit terms? I’m no longer able to answer stuff like why. You should have tried me a few decades ago. this is one of those things where some one else will have to answer. I think it had something to do with internal resistance. I don’t know any longer. I just know when we were out too long that changing the inner batteries to the end got us a little more light. ---
**Author:** ![Colophon](https://avatars.discourse-cdn.com/v4/letter/c/f05b48/32.png) [@Colophon](https://boards.straightdope.com/u/Colophon)\ **Post date:** [February 16, 2010, 3:05pm UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164/14 "2010-02-16T15:05:26Z")
> [@Harmonious\_Discord](#): > > I’m no longer able to answer stuff like why. You should have tried me a few decades ago. this is one of those things where some one else will have to answer. I think it had something to do with internal resistance. I don’t know any longer. I just know when we were out too long that changing the inner batteries to the end got us a little more light. It’s because all the electricity sinks to the bottom of the batteries. It’s like getting the last bit out of the ketchup bottle by standing it upside down overnight. 🙂 ---
**Author:** ![UncleRojelio](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/unclerojelio/32/3160_2.png) [@UncleRojelio](https://boards.straightdope.com/u/UncleRojelio)\ **Post date:** [February 16, 2010, 7:35pm UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164/15 "2010-02-16T19:35:30Z")
> [@Gbro](#): > > Very reasonable, I just cannot do that with my mini-mag. You can however reverse ONE of the batteries. That will prevent accidental activation. ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [February 16, 2010, 7:50pm UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164/16 "2010-02-16T19:50:54Z")
> [@UncleRojelio](#): > > You can however reverse ONE of the batteries. That will prevent accidental activation. Sorta. You have to reverse HALF of the batteries. With half “trying” to push electricity one way and half pushing the other way, nothing gets pushed, therefore no work gets done and no energy is expended. Works great for devices that use an even number of batteries. Not nearly as much for odd battery jobs (but still better than doing nothing). I’ve been doing this for decades. No use having an emergency/backup light that you find doesnt work because it got turned on accidently. I also use disposable lithuims (usually) in these back up lights because they have several times the capacity of Alkalines and have a shelf life of many years. Oh, and when you go to use the light you gotta remember you reversed half rather than thinking “oh crap, this one is dead too”. ---
**Author:** ![wolfman](https://avatars.discourse-cdn.com/v4/letter/w/a8b319/32.png) [@wolfman](https://boards.straightdope.com/u/wolfman)\ **Post date:** [February 16, 2010, 7:52pm UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164/17 "2010-02-16T19:52:48Z")
Isn’t it easier to just throw a piece of paper in there eather than turning the batteries around? ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [February 16, 2010, 7:59pm UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164/18 "2010-02-16T19:59:30Z")
> [@wolfman](#): > > Isn’t it easier to just throw a piece of paper in there eather than turning the batteries around? And if the paper gets wet or slips out of place or the batteries wear through that paper due to being jostled around? And, you STILL have to open the device to get the paper out. I know plenty of people that do the reverse the battery trick. I don’t know anybody thats decided the paper trick was better enough to choose that method. ---
**Author:** ![Enright3](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/enright3/32/2897_2.png) [@Enright3](https://boards.straightdope.com/u/Enright3)\ **Post date:** [February 18, 2010, 3:51pm UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164/19 "2010-02-18T15:51:48Z")
> [@Colophon](#): > > It’s because all the electricity sinks to the bottom of the batteries. It’s like getting the last bit out of the ketchup bottle by standing it upside down overnight. 🙂 That’s why you whack a flashlight when it’s not working. It shakes up the electrons. I always thought that batteries were as strong as their average power. So reversing the order of the batteries wouldn’t help any. I also remember seeing the warning on items that take multiple batteries to change all the batteries at the same time. I’m reminded of the joke: Q: What’s the definition of a flashlight? A: Tubular metal container for storing dead batteries. ---
**Author:** ![daveg](https://avatars.discourse-cdn.com/v4/letter/d/b5ac83/32.png) [@daveg](https://boards.straightdope.com/u/daveg)\ **Post date:** [February 18, 2010, 4:46pm UTC](https://boards.straightdope.com/t/reversing-dry-cells-in-a-flashlight/529164/20 "2010-02-18T16:46:51Z")
If proper contacts are made in a non-LED flashlight, it should work regardless of the polarity of the batteries. Someone beat me to the definition of flashlight. 😉