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Vent in Battery

The vent is to allow gas/fumes to vented to the outside viva that little
hose which is probably laying in the battery box.  When you start your 
car you draw down the battery. So your alternator then tries to charge 
the battery back up. The alternators put out say 25 to 30 amps right after 
starting but this only lasts for maybe 4 or 5 minutes at the most. When 
the battery is being charged at a high rate if produces hydrogen gas if 
the gas becomes concentrated and is exposed to a flame or spark a 
exposion could result. I have seen this happen in a 911 which was being 
charged at a high rate for a hour or so. Someone then removed the 
charger without turning the charger off. A spark resulted and BOOM 
battery acid everywhere.

Could your Audi blow up? Probably not, your alternator only charge your 
at high rate when your start you car. The faster the car starts the short 
the high charge rate. In order to produce much hydrogen gas the battery 
acid need to be boiling, not likely under normal conditions. Then you 
need a spark or flame at the battery. If you have the little cover 
over the positive terminal and you don't smoke under the back seat, this 
too is not likely. 

So all in all the risk is quite low that you would ever have a problem 
by not having a external vent on your battery. I have used batteries 
without the external vent for years in Audi and never had a problem.  You 
can get external vented battery from the dealer and I believe that 
Interstate Battery makes either an external vented battery or a kit which 
relpaces the standard vented battery caps with external vented caps.

- Tom Feller