# What happens if you hook 2 9-volt batteries together?
**URL:** https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321
**Category:** Factual Questions
**Created:** [February 28, 2005, 6:03pm UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321 "2005-02-28T18:03:29Z")
**Posts on this page:** 20
**Page:** 2
### Author:  [@UncleFred](https://boards.straightdope.com/u/UncleFred)
#### Post date: [July 21, 2011, 12:04pm UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321/21 "2011-07-21T12:04:53Z")
> [@spingears](#):
>
> OTOH If you connect the two + terminals together and the two - terminals together you will get 9 volts but twice the apmpeage capacity and hence twice the life at the same load.
Not necessarily. Batteries typically don’t do well directly in parallel. The voltages usually aren’t exactly the same, so one ends up getting partially discharged into the other, limited by the “2 R” resistance described earlier.
Depending on the load, if you use a diode (undirectional flow) from each of two batteries to the load, you avoid that problem. Whichever battery presently has the higher voltage will be driving the load. The downside is the 0.6 (or so) voltage drop through the diode.
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### Author:  [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)
#### Post date: [July 21, 2011, 9:27pm UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321/22 "2011-07-21T21:27:04Z")
> [@johnpost](#):
>
> any rechargeable battery if shorted out can get hot enough to start a fire or cause burns.
Yeah, but the point was that even having terminals on opposite ends is not necessarily enough to prevent shorts with a big knot of keys in a pocket.
Rechargables aren’t the only cells with this problem, anyway–although alkalines have a lot of internal resistance, lithium primaries do not.
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### Author:  [@beowulff](https://boards.straightdope.com/u/beowulff)
#### Post date: [July 21, 2011, 9:53pm UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321/23 "2011-07-21T21:53:19Z")
> [@Chronos](#):
>
> Which would mean 180 watts. Which is a lot of power, for something that small, and not designed to dissipate heat.
I seriously doubt that even a brand new alkaline 9v could supply 10A for more than a few seconds - best case, 9v alkalines have less than 300mAH capacity (at a 1A discharge rate), which means they could supply 10A for all of 108 seconds. A 10A discharge rate probably brings that down to 20 seconds or so.
> **[Discharge tests and capacity measurement of 9 volt transistor...](https://www.powerstream.com/9V-Alkaline-tests.htm)**
>
> 9V battery amp hours, 9v mah, Resources for designing equipment using 9V batteries from PowerStream
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### Author:  [@asterion](https://boards.straightdope.com/u/asterion)
#### Post date: [July 21, 2011, 11:30pm UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321/24 "2011-07-21T23:30:38Z")
> [@engineer\_comp\_geek](#):
>
> Well, to be fair, while the batteries can explode, it’s not a grenade type of explosion. The battery case pops open and hot battery goo spurts out. Your 4 year old might have taken a trip to the doctor for minor cuts and burns, but he wouldn’t have lost a hand or anything like that.
>
> Because the terminals on a 9 volt battery are both physically on the same side of the battery and are close to each other, putting one in your pocket can easily cause it to short out on anything conductive, like a coin or a key. It can give a new meaning to the word “hot pocket”. 😉
For a while, the safety department at work was making us tape over the terminals of 9V batteries and put them in a separate bin for recycling. The story I was told was that there had been a report of a building catching fire that had been traced to shorting 9Vs heating up enough to ignite surrounding material. I can’t find anything along those lines on the Internet and we no longer have to separate out the 9Vs. Have there been any fires traced to 9V batteries?
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### Author:  [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)
#### Post date: [July 22, 2011, 12:39am UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321/25 "2011-07-22T00:39:58Z")
> [@engineer\_comp\_geek](#):
>
> …Because the terminals on a 9 volt battery are both physically on the same side of the battery and are close to each other, putting one in your pocket can easily cause it to short out on anything conductive, like a coin or a key. It can give a new meaning to the word “hot pocket”. 😉
So when I used to put the battery terminals on my tongue, what was the cause of the unpleasant sensation that made me pull the battery away in a split second? This is not a rhetorical question…
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### Author:  [@johnpost](https://boards.straightdope.com/u/johnpost)
#### Post date: [July 22, 2011, 12:57am UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321/26 "2011-07-22T00:57:19Z")
> [@Leo\_Bloom](#):
>
> So when I used to put the battery terminals on my tongue, what was the cause of the unpleasant sensation that made me pull the battery away in a split second? This is not a rhetorical question…
the 9V battery terminals are close enough together and the moist living tissue on your tongue conducts electricity so that you get a mild electrical shock.
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### Author:  [@njaohnt](https://boards.straightdope.com/u/njaohnt)
#### Post date: [April 7, 2012, 4:38pm UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321/27 "2012-04-07T16:38:39Z")
What if you don’t connect the positive to the negative on one side(so that there is just a positive goes to a negative)?
Wouldn’t the current go through the positive to the negative causing one battery to get hot? I’m asking this because I am wondering if it is okay to put a whole bunch of batteries in a box, when they may connect a current, and use up their electricity/explode.
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### Author:  [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)
#### Post date: [April 7, 2012, 5:00pm UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321/28 "2012-04-07T17:00:26Z")
> [@njaohnt](#):
>
> What if you don’t connect the positive to the negative on one side(so that there is just a positive goes to a negative)?
> Wouldn’t the current go through the positive to the negative causing one battery to get hot? I’m asking this because I am wondering if it is okay to put a whole bunch of batteries in a box, when they may connect a current, and use up their electricity/explode.
If you simply connect the positive terminal of one battery to the negative terminal of the other battery, you have created an 18 V battery. There will be no current if you do not connect something between the other two terminals.
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### Author:  [@johnpost](https://boards.straightdope.com/u/johnpost)
#### Post date: [April 7, 2012, 5:07pm UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321/29 "2012-04-07T17:07:00Z")
> [@njaohnt](#):
>
> What if you don’t connect the positive to the negative on one side(so that there is just a positive goes to a negative)?
> Wouldn’t the current go through the positive to the negative causing one battery to get hot? I’m asking this because I am wondering if it is okay to put a whole bunch of batteries in a box, when they may connect a current, and use up their electricity/explode.
if you only connect for example a positive of one battery to the negative of another battery no current will flow, it is called an open circuit. if you connected the negative of battery one to the positive of battery two with a wire.
it is not OK to connect batteries this way in a drawer, accidents are too likely. it is better to tape a strip of paper over the terminals.
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### Author:  [@KneadToKnow](https://boards.straightdope.com/u/KneadToKnow)
#### Post date: [April 7, 2012, 5:37pm UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321/30 "2012-04-07T17:37:22Z")
I’m thinking of writing one of those classic/monster mashups based on Thomas Carlyle and calling it _[Zombie Rezombus](http://en.wikipedia.org/wiki/Sartor_Resartus)_.
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### Author:  [@njaohnt](https://boards.straightdope.com/u/njaohnt)
#### Post date: [April 9, 2012, 9:51pm UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321/31 "2012-04-09T21:51:02Z")
Thanks, putting a strip of paper is a good idea.
Why wouldn’t it create a current? What is the difference if a the current flows to a negative terminal on the same, or a different battery?
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### Author:  [@johnpost](https://boards.straightdope.com/u/johnpost)
#### Post date: [April 9, 2012, 10:10pm UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321/32 "2012-04-09T22:10:15Z")
> [@njaohnt](#):
>
> Thanks, putting a strip of paper is a good idea.
> Why wouldn’t it create a current? What is the difference if a the current flows to a negative terminal on the same, or a different battery?
there needs to be a wire (or other conducting material) between the the farthest positive and negative of a battery or bunch of batteries. there isn’t current flow if that doesn’t happen.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [April 10, 2012, 10:02pm UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321/33 "2012-04-10T22:02:14Z")
Strictly speaking, there would be a slight current flow immediately after connecting the two together, but it’d be limited by the capacitance of the arrangement, which would be very low. The flowing charge would result in a charge buildup, which would in turn inhibit the flow of any further charge.
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### Author:  [@JSexton](https://boards.straightdope.com/u/JSexton)
#### Post date: [April 10, 2012, 10:10pm UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321/34 "2012-04-10T22:10:58Z")
It’s possible to start a fire with clean fine steel wool anda fresh 9-volt battery. I’ve done it myself, though it’s tricky and requires a lot of oxygen. It would be highly unlikely to happen accidentally in a drawer somewhere, but not impossible.
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### Author:  [@Cliffy](https://boards.straightdope.com/u/Cliffy)
#### Post date: [April 10, 2012, 10:44pm UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321/35 "2012-04-10T22:44:25Z")
This may be a zombie thread, but that’s ok – I have OFTEN wondered if it’s OK to click two 9-volts together. It just seems so obvious. I’m sure at some point my curiosity would have gotten the better of my caution and I would have ended up burning my hands. (I once put two AA batteries in series in my mouth to see if they’d taste any different than one – they don’t, but you do go blind for a millisecond when you close the circuit.)
Actually, I’ll probably still do it someday, but now I know to take precautions.
–Cliffy
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### Author:  [@Chessic\_Sense](https://boards.straightdope.com/u/Chessic_Sense)
#### Post date: [April 11, 2012, 3:46pm UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321/36 "2012-04-11T15:46:17Z")
> [@njaohnt](#):
>
> What if you don’t connect the positive to the negative on one side(so that there is just a positive goes to a negative)?
> Wouldn’t the current go through the positive to the negative causing one battery to get hot? I’m asking this because I am wondering if it is okay to put a whole bunch of batteries in a box, when they may connect a current, and use up their electricity/explode.
Chances are you’ve already done this at some point in your life. This is the arrangement of the batteries in any multi-battery device, such as a flashlight, while the switch is in the “off” position. The switch only breaks the connection from the front battery to the back battery. The other terminals are always touching.
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### Author:  [@Satchmo](https://boards.straightdope.com/u/Satchmo)
#### Post date: [April 11, 2012, 9:05pm UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321/37 "2012-04-11T21:05:56Z")
> [@JSexton](#):
>
> It’s possible to start a fire with clean fine steel wool anda fresh 9-volt battery. I’ve done it myself, though it’s tricky and requires a lot of oxygen. It would be highly unlikely to happen accidentally in a drawer somewhere, but not impossible.
I’ve seen that done with two ‘C’ or ‘D’ cell batteries, too. Could help get a fire started when camping with damp wood.
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### Author:  [@Irishman](https://boards.straightdope.com/u/Irishman)
#### Post date: [April 11, 2012, 9:53pm UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321/38 "2012-04-11T21:53:05Z")
> [@JSexton](#):
>
> It’s possible to start a fire with clean fine steel wool anda fresh 9-volt battery. I’ve done it myself, though it’s tricky and requires a lot of oxygen. It would be highly unlikely to happen accidentally in a drawer somewhere, but not impossible.
It’s not that tricky. I’ve done it, too. Just takes getting the 9v and steel wool in contact, such that you get a short across the terminals. Low likilihood of happening in a drawer primarily because of having loose fine steel wool and batteries floating around in the same place.
I’ve recently noticed new 9V coming with plastic caps on the positive terminal to prevent accidental shorts.
I haven’t been putting tape on old 9V prior to bagging for disposal, but may start that now.
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### Author:  [@njaohnt](https://boards.straightdope.com/u/njaohnt)
#### Post date: [April 12, 2012, 10:32pm UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321/39 "2012-04-12T22:32:08Z")
> [@johnpost](#):
>
> there needs to be a wire (or other conducting material) between the the farthest positive and negative of a battery or bunch of batteries. there isn’t current flow if that doesn’t happen.
There is, the metal of the terminals.
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### Author:  [@tempaccountforasinglepost](https://boards.straightdope.com/u/tempaccountforasinglepost)
#### Post date: [April 2, 2014, 5:11am UTC](https://boards.straightdope.com/t/what-happens-if-you-hook-2-9-volt-batteries-together/292321/40 "2014-04-02T05:11:47Z")
Hi im a college student and we just did this as an experiment. After several minutes there was a pop and a puff of smoke as one side blew out. A few minutes after (long enough to find this thread on google and start reading people saying an explosion would not happen) the other side popped with smoke as well.
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