When I was younger I connected two 9v batteries to each other and they got really warm and started to expand. I disconnected them before anything else happened but I’ve heard they can explode if left like that. Why does that happen?
Also, how powerful is the explosion?
Thanks.
When you hook them up backwards like that, each battery is going to try and maintain 9 volts across its terminals, while the other battery is trying to force it to -9 volts. The battery tries to supply as much current as it can to bring the voltage back up to 9 volts. In this case the only thing limiting the current is the physical construction of the battery itself. If it could supply an infinite amount of current, it would, but it can’t. However, it does supply enough current that the chemicals inside start to heat up, which can cause the battery to expand and possibly explode.
I’ve never actually witnessed one explode, so I can’t say how big the boom is.
Well, I’m sorry, that doesn’t make a lot of sense to me. I don’t think the batteries would be hooked up “backwards.” I think they easily hook up positive to negative.
It would seem more obvious to me that you are shorting out the batteries, and providing very little resistance to current flow. Allowing current to flow so freely heats things up and could cause fires.
Chemically speaking, the oxidation, reduction reactions within the batteries are going like gang busters and uncontrolled. The net reaction is exothermic; hot things expand, and if there’s any liquid in there it could conceivably turn to gas (water to steam) building even more pressure. Eventually, I could see the casing woul fail due to the increased pressure. I don’t know how big a boom you would get.
Depends on different interpretationS of “backwards” which is not explicit.
Two 9 v. cells in parallel still have 9 v. output but double the current of one cell.
The energy is dissipated into the air around the connecting wire.
When shorted either one or two cells will put max current into the short and die rather quickly, even explode.
OTOH two 9 v. cells connected in series, i.e. connectors snapped together have max. current circulating through each other and the temperature tends to rise quickly to explode one or both cells.
DON’T TRY THIS AT HOME OR ANYWHERE ELSE!
You don’t even need two batteries. Just drop one in your pocket with your keys and see what happens.
What if you used one fresh battery and one dead battery?
I should point out that “explode” isn’t really the right word for it. What will happen is that the plastic end caps of the batteries will pop out, possibly landing a few feet away in ideal circumstances - not exceptionally spectacular.
However! If there is a source of ignition nearby - for example, if whatever is causing the short has managed to get hot enough - the electrolyte released from the battery may catch fire, and do some damage. It’s also likely to be fairly corrosive, even if it doesn’t ignite. Standard “don’t try this at home” disclaimers apply, of course.
To some extent, it will depend on the batteries’ internal or equivalent resistance. Most 9V batteries have a very low internal resistance, so you’ve effectively created a circuit with 18 volts applied across a tenth of an ohm or so. Resulting current is 180 amps (I=E/R) and the power is 3240 watts (P=I*E)
Little 9 volt batteries aren’t designed to handle that much power, so they’ll sooner or later get too hot and suffer an unfavorable outcome.
Conversely, you do this with a string of 12 carbon-zinc 1.5 volt D cells and nothing will happen except the batteries all discharge slowly and go dead as they have comparatively high internal resistance.
My son brought me two rechargeable 9 volts, one of which he had just managed to explode. I wasn’t sure if they were defective and came across this post so thought I’d share. He said he had them end-to-end and they just made a loud noise and exploded, at which point he hit the deck, since he was pretending to be shooting them at each other at the time. Suffice to say he was a little shocked that his play became reality. Doesn’t appear to be any acid involvement, but the top is at a jaunty 45 degree angle. Was going to post a picture but doesn’t appear to be an option.
Here’s a link to my amazon review with short video showing the battery.
Nice phrase. I may not remember it long enough to steal it, but I’m going to enjoy it for awhile.
To clarify some wrong (or maybe just confusingly conveyed) information above:
When you do this, you are connecting the two batteries in series (18V) and also shorting the the pair. They will supply current limited only by their own internal resistance, and all the power will go into heating the batteries.
Since each battery nominally has the same voltage and internal resistance, the same thing will happen when you just short out one battery…say with a copper penny. 9V batteries are not something you want to carry in your pocket with your loose change.
If you connected them (with jumper cables, say) positive to positive and negative to negative (I feel as if I ought to be asking this of Doc Cathode) wouldn’t you merely have created a big 9v battery with the ability to put out twice the amperage? It would simply sit there on your table, just like any other battery, until you connected it to some kind of circuit.
Correct.
I should point out that 9v batteries have very little energy in them. Best case seems to be around 450mAH, or 4.05 WH. Since gasoline has about 9.7 KWH/l of energy, this means that a 9v battery is equal to about .000404KWH/9.7 KWH/l = .417ml of gasoline. So, the biggest possible “bang” you could get would be equivalent to burning about 1ml of gasoline (two batteries). That might make a nice “pop,” but you aren’t going to take down a building that way…
180 amps is way off; 100 milliohms is more like the ESR of a single AA battery; a 9 volt battery contains 6 cells in series, roughly similar to AAAA batteries. This Duracell datasheet says the ESR is 1.7 ohms*, so only 5.3 amps will flow, regardless of how many batteries are in series (18 volts / 3.4 ohms for two batteries/12 cells).
*Note that this is measured at 1 kHz; the DC resistance is likely higher because the reaction rate also limits the current a battery can produce.
I think the explosion blew your link apart, too.
I actually found AAAA batteries at a store once. I bought them..because, well, AAAA batteries!
This is a good video of what you can do with 244 9V batteries in series. I know we’ve all wanted to try this before.
Yes, with the caveat that this depends on the batteries being identical. If they’re a little bit different (one discharged more than the other, or different chemistries), then you’ll get a current flow, the weaker being charged (inefficiently, if they’re not made to be charged) by the stronger, and both of them heating up. The heating won’t be nearly as much as with them connected positive-to-negative, of course, but it’s still not generally advised.