Okay folks, let's not blow this out of proportion. Here is what's
happening in your friendly lead-acid battery during discharge:
at the anode:
Pb + HSO4- --> PbSO4 + H+ + 2 electrons
at the cathode:
PbO2 + HSO4- + 3H+ + 2 electrons --> PbSO4 + 2H2O
overall:
Pb + PbO2 + 2HSO4 --> PbSO4 + 2H2O
It is the spontaneous transfer of electrons from the anode to the
cathode that creates the current in a good battery. When the battery is
dead, it has used up enough of the reactants (and created enough of the
"waste" products) that the potentials at the two electrodes are the
same; there is no potential difference and current does not flow. When
you charge the battery you are reversing these reactions and forcing
electrons in a thermodynamically unfavourable direction. The above
reactions are reversed, but as far as exploding batteries are concerned
it is the water that was generated during discharge that is the problem.
The water can be electrolysed to diatomic hydrogen and oxygen:
2H2O + 2 electrons --> 2H2 + O2
It is the mixture of these two gases is explosive. A few things to keep
in mind are that yes, this is an explosive mixture, but H2 needs a high
vapour density to combust (this is why it is actually safer in this
respect than gasoline). So when you are boosting your battery, if the
circuit is grounded to some metal component of the car (preferably not
right next to the battery) and this is disconnected first it is unlikely
that you will have enough hydrogen in that immediate area to ignite if a
spark is generated. If you are boosting in a well ventilated place,
especially outdoors, it is even less likely that you will have enough H2
around for that spark to ignite. Newer, sealed batteries are also much
less likely to generate gaseous hydrogen and oxygen. This is because
the electrodes are made of a calcium lead allow that helps to prevent
water electrolysis. This is why you don't have to add water to these
batteries. So, the take home message is that it is important to be safe
around batteries, but they are nothing to fear. On another note, the
combustion of hydrogen gas around a battery, while not a good thing, is
not remotely related to a hydrogen bomb. A hydrogen bomb is a fusion
type reaction, which if that occured in your battery would make a very
big bang indeed.
James
roger crabtree wrote:
>I keep seeing this "blowing up battery" thing on the
>club page. I case you don't know a wet or acid bath
>battery is a miniature hydrogen bomb just waiting to
>go off. The battery by nature will discharge hydrogen
>gas in the state of charge or discharge. This means it
>can and will explode at anytime. There are three
>major causes of this explosion. Smoking, open flame,
>sparks from jumping with jumper cables or spark from
>loose or dirty cable connection. I used to work on 24
>volt systems in the winter. This was for oil field in
>vary cold weather. If cable ends were dirty or loose
>you will see batteries blow up when you hit the
>starter button. It works the same with your auto
>battery. I don't know if the gell battery does the
>same thing or not. RC
>
>
> ---
>
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