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[HS93 (RIP)]

This Happend to me some Years ago so when I saw this picture I thought it may be worth a post , They  get very very hot and gas ,this is normally caused by an internal short , you don't have to be doing any thing wrong just normal charging , although I have seen it happen after an impact.Just disconnecting does not stop it , just get away from them until they have finished then bin them as the surrounding cells are normally damaged by the heat.

Peter

[Ghost in the shell]

does that only really apply to nickel type batteries?

[Umi_Ryuzuki]

does that only really apply to nickel type batteries?

No, I had a friend that used to run a Blueback RC submarine model.
I missed the two different times that this occured, but this was the sequence...

He had a sealed gell cell battery in the watertight compartment of the submarine.
He could charge it by pluging into the box via receptors he had built in.
After several years of running, he placed a new battery into the boat, and sealed it back up and
then charged for the next day. When he got out to the pond, he switched on his radio then
the boat, and sailed off. When he activated the motor to flood the tanks, apparently there was
a large thud, and a huge volume of water was thrown into the air, and his sub sank.

He recovered and and spent a couple weeks to repair the submarine. The water tight chamber had
blown open, So once he repaired that, he put everything back in its proper place and went back out to
the pond... Switch on transmitter, boat, sail off, switch to flood the boat, and Boom,,, another catastrophic
explosion and sinking. ....

Turns out that charging a brand new battery in a sealed water tight compartment fills the compartment with
hydrogen gas.  ::)  The submarines motor that ran the lead screws for the ballast  plungers had brushes that
arc'd when run. Hydrogen gas,... sparky motor... Boom!!

Best to charge the batterys where they can air a bit...

[malcolmfrary]

Any rechargeable battery has water as part of the electrolyte.  During charging, some of this will be split into hydrogen and oxygen.  In most, there is a bit of chemical magic inside the cell to recombine them, but it is always possible to produce the gasses faster than they can be recombined.  In a confined space, you will wind up with not only hydrogen, but oxygen in the exact proportions to recombine, which will happen very rapidly with the production of a lot of heat.  The heat will cause a sudden expansion of the contents of the chamber, and will cause a rupture at the weakest point.  Or, in normal terms, a "bl00dy big bang". 
All it needs is a vent at the highest point, as hydrogen is very light and will dissipate due upwards very rapidly.
If you get a faulty cell, its going to give trouble whether in or out of the boat, but normally, unless there is external damage, an internal short will grow gradually, and you should get warning when the battery seems to lose charge rapidly.

[HS93 (RIP)]

[quote
If you get a faulty cell, its going to give trouble whether in or out of the boat, but normally, unless there is external damage, an internal short will grow gradually, and you should get warning when the battery seems to lose charge rapidly.
[/quote]

Thats not the case they can Be new (just one or two charges old) with no sign of loss of capacity . in racing this was a common occurrence when it happend to me it was the forth or fifth charge and individual cells here being monitored during the charge. the Reason why said remove them was that if id did happen the heat would not damage the boat.
You only have to look at the recent recalls of new  DELL and SONY lap tops for evidence
peter

Peter

[malcolmfrary]

Fair enough, but I did say "normally". 
I believe the laptop recalls were for Lithium batteries, which I am glad to have little experience with, apart from this laptop and a vid camera, and they are both sort of proprietary "plug in and follow the instructions" sort of things.

[portside II]

i did see a li-polly destroy a model aeroplane on Blackpool beach a few years ago ,it was still burning even when the sea washed over the model .
The only time i had a prob with a nicad pack which started shorting out ,i dumped it in the shallows for safety :angel: and at the end of the day took it home stripped it down and repaired the connection and put it back on charge .Yes it was ok and 12 months down the road its still working  :)
daz