Why did my 9 volt battery explode -- twice? The McGuyver in me wants to know more

Factual Questions
1

I’m not even sure why it exploded once.

As I was sitting at my desk last week, there was a small pop (maybe not quite so lous as popping open a beer can), and a couple of papers auddenly rustled. A second or two later, there was a loud BANG (somewhere between a firecracker and M80 but closer to the latter), and a soda can tipped over and a pen shot out from under a couple of papers and onto the floor.

After changing my underwear I investigated. A 9-volt battery had exploded; a Duracell Procell Alkaline to be specific. I believe it was lying on its side and both electrodes came into contact with the side of the soda can, and a currecnt resulted.

Some other known or unlnown items:
I dont know how long it took to run up to this.
The bottom of the battery away from the electrodes was blown out. I saw no sign of flame or smoke, didn’t even smell any odd chemical smell before or after. No liquid or powder came out. I’m sure I touched the body of the battery wthin 30 seconds of the blast, but it wasn’t hot.

Tell me, physicists and chemists, how and why this happened. Especially the second explosion.

Anyway, if you’re trapped by your enemies with a battery and soda can and need a distration, here you go.

2

If you open up a PP3 9 volt battery, you’ll find that it is a ‘battery’ in the true sense of the word (most other types are ‘cells’). Anyway, a 9v battery contains six AAAA size 1.5v cells wired in series (BTW AAAA is a real cell size you can buy as single cells if you look around hard enough). This would at least explain the potential for mutliple explosions.

3

The current that a battery supplies depends on the voltage of the battery and the resistance of the material that the current flows through. The resistance of metal is very very low, which is why we make wires and stuff out of metal. The lower the resistance, the higher the current. If you have a really low resistance (like a chunk of a metal can) then the battery is going to try and supply a really large amount of current. Of course, since it’s only a tiny 9 volt battery, it’s only capable of supplying a relatively small amount of current. But, small or not, it’s enough current to make the battery heat up. Sometimes they’ll just swell and leak, sometimes they’ll actually pop. Depends on exactly how the battery is designed and what’s inside of it. Your typical alkaline battery is one of the more likely ones to go poof, compared to other typical battery types.

It usually only takes a few seconds for the battery to heat up and explode.

Something like a car battery, which can supply a lot more current, will probably melt the can instead of damaging the battery. However, if you short a car battery with a thick enough piece of metal you can make a car battery explode too (kids, don’t try this at home…).

4

[QUOTE=Mangetout]
If you open up a PP3 9 volt battery, you’ll find that it is a ‘battery’ in the true sense of the word (most other types are ‘cells’). Anyway, a 9v battery contains six AAAA size 1.5v cells wired in series (BTW AAAA is a real cell size you can buy as single cells if you look around hard enough). This would at least explain the potential for mutliple explosions.
[/QUOTE]

By jillickers you are right. I had imagined the interior to be some single battery unit. But yes, not that I’ve peeled the side walls back a bit with a pliers, I can see that it’s a bundle of six little cylinders, And indeed, two of them are missing end caps.

So, why do they explode at all?

5

But, engineer_comp_geek, is it something in the nature of the battery design, or of electricity itself, that causes these explosions?

Could you design a battery that had some kind of “discharge governor”? What kind of compromises would you have to make to do such a thing? are there some kinds of equipment that couldn’t be powered by “safe” batteries?

6

[QUOTE=Boyo Jim]
So, why do they explode at all?
[/QUOTE]
The cells in most 9 volt batteries use water in their electrolyte. If you pull too much current, the water boils, pressure builds up, and the cell casing fails with a hiss, pop or bang.

7

[QUOTE=Boyo Jim]
Could you design a battery that had some kind of “discharge governor”?
[/QUOTE]
I suppose the manufacturer could incorporate a pressure relief valve into the battery, similar to what some large electrolytic capacitors have.

8

[QUOTE=Boyo Jim]

Could you design a battery that had some kind of “discharge governor”?
[/QUOTE]

For transportation on passenger aircraft, batteries above a certain size are required to have such a device. Usually it is a PTC (positive temperature coefficient themistor). This acts like a self-resetting fuse. Thermal fuses may also be required. Too many years since I had to worry about this to provide a cite.

9

[QUOTE=Boyo Jim]
But, engineer_comp_geek, is it something in the nature of the battery design, or of electricity itself, that causes these explosions?
[/QUOTE]

It’s the nature of electricity itself. Every material has a certain amount of “resistance” to electricity flowing through it (ignoring superconductors). Metal has a fairly low resistance. Something like wood or glass has a very high resistance. Even if you have a very low resistance, like a piece of metal, you still have some resistance, and as electricity flows through the material you end up with energy that gets “lost” due to the resistance. It actually isn’t lost, but is converted into heat. The more electricity flows through a battery, the hotter it gets. The same with just a piece of wire, too. The 15 amp rating on house wiring comes from how much current you can push through the wire without causing it to heat up and cause a fire.

For some batteries, the chemical reaction inside the battery also produces heat. I don’t think this is the case with alkalines, but my memory for chemistry is a little fuzzy.

[QUOTE=Boyo Jim]
Could you design a battery that had some kind of “discharge governor”? What kind of compromises would you have to make to do such a thing? are there some kinds of equipment that couldn’t be powered by “safe” batteries?
[/QUOTE]

Sure. The most common type of “discharge governor” is a series current limiting resistor. This will protect you from dead shorts, but it has a disadvantage in that the resistor is always wasting power by converting it into heat, and also you’ve got a problem where the more current you try and draw from the battery, the more the voltage at the terminals drops because of the resistor in the circuit.

You could also put in a fuse which would blow when excessive current was drawn from the battery. The drawback to this is that any momentary minor short would likely blow the fuse and ruin the entire battery.

You could also get fancy and put in a self resetting thermal fuse, or even a semiconductor circuit that would go to a high impedence state (effectively disconnectnig the battery) for some period of time if a short is detected.

The main compromise you’d have to make is cost.

Be glad it was a tin can. I’ve heard of people putting batteries in their pocket and have them short out on coins. I’d much rather have a battery explode on my desk than in my pants near my sensitive bits.

10

[QUOTE=Mangetout]
If you open up a PP3 9 volt battery, you’ll find that it is a ‘battery’ in the true sense of the word (most other types are ‘cells’). Anyway, a 9v battery contains six AAAA size 1.5v cells wired in series (BTW AAAA is a real cell size you can buy as single cells if you look around hard enough). This would at least explain the potential for mutliple explosions.
[/QUOTE]
I have disected a few 9 v. dry cells and all of them consisted of a stack of 6 fairly thin rectangular cells wraped in insulating plastic and foil combo.
Will have to do a few more looking for the AAAA cells!
3 new 9v. 2 National 1 House brand no indication of type of construction.

Snap two 9 volters together.
TWICE THE CURRENT — TWICE THE FUN
== DON’T TRY THIS AT HOME. ==
Reqires safety glasses and blast shield protection.

11

[QUOTE=engineer_comp_geek]

Be glad it was a tin can. I’ve heard of people putting batteries in their pocket and have them short out on coins. I’d much rather have a battery explode on my desk than in my pants near my sensitive bits.
[/QUOTE]
Now you can say that rather than hearsay, you have actual anecdotal evidence. I barely got the damn thing out of my pocket before it caused 2nd degree burns. :smack:

12

To expand on what ECG said…

All batteries (and cells) have “internal resistance,” a.k.a. “source resistance.” It’s not a discrete resistor inside the battery. It is simply the equivalent series resistance when the battery is modeled as an ideal voltage source in series with a resistor.

The internal resistance of a battery dissipates heat when current is flowing through it. The amount of power (watts) dissipated in the form of heat is proportional to the value of the internal resistance (ohms) and the square of the current (amps[sup]2[/sup]). The internal temperature of the battery is a function of the amount of power dissipated by the battery’s internal resistance, the battery’s size & shape, and the thermal conductivity of the materials used in the battery. The pressure of any gas inside the battery is a function of temperature.

So…

short circuit → maximum current through battery’s internal resistance → maximum power dissipated by battery’s internal resistance → maximum core temperature of battery → maximum gas pressure inside battery

13

[QUOTE=spingears]
I have disected a few 9 v. dry cells and all of them consisted of a stack of 6 fairly thin rectangular cells wraped in insulating plastic and foil combo.
Will have to do a few more looking for the AAAA cells!
3 new 9v. 2 National 1 House brand no indication of type of construction.
[/QUOTE]
Ah, I suppose some manufacturers have switched to rectangular-section cells to pack more capacity into the same sized battery. PP3s used to contain six AAAA cells, anyway.