Charging the battery produces hydrogen gas, a spark from the electrical system (when you started the car) ignited the hydrogen gas.

Always open the hood when charging a battery. Go get a few boxes of baking soda and pour them on everything dry, then wet it down again 3 or 4 times. A relay sparking may be the cause in a confined space full of hydrogen gas!

I've never seen a battery explode but I had a coil explode 1 foot from my face. Had blisters for days. Not fun.

Glad you're OK, Arm & Hammer Baking Soda from the grocery store is the same thing.

I suspect the battery wasn't topped off with water, hydrogen gas built up in and around the battery and there was a spark, probably at the negative terminal.

What type and what brand battery was it?

In that case, if you leave the hood opened, the hydrogen gas spread in the house and kill you when you sleep if it's concentrate enough?

See post #6 . . .

I had a capacitor explode while pluging the power source into the circuit board in a lab. The cap was around the width of a hotdog. It almost knock some of us down the stool :D. I imagin the size of the car's battery might blow some of us away?

To get the hydrogen to light off inside the battery requires the spark to be right at the battery and almost certainly inside of it. It probably was arcing between some of the plates which means the battery was partially failed.
The battery resolved the question of if it should be replaced in a most definite way...

Besides the obvious of cleaning out the underhood area of any hint of residue with a water/baking soda mix, take special care in neutralizing the carpets. Any hint will bleach out the color. I would get them soaking in a wash tub and replace the water a number of times. Also look at every crevice and seam where moisture can hide. Acid will cause amazing rust so over do the cleanup.

Never start a car with a battery charger attached. Unless specifically designed for that use it will most likely damage the charger.
Never leave a battery on a charger for days unless it is a trickle charger expressly designed for the purpose. Over-charging a battery is a sure way to produce excess hydrogen gas.
Always ensure there is a full fluid level (but not over-full) before starting to charge. Afterwards, check to see that it is still full. It is not a good idea to charge a battery is a small compartment that is not well vented to ensure the hydrogen gas will not build up to dangerous levels. While charging it is always a good idea to check the charging rate, and the battery temperature. There are not many, if any, battery chargers that can tell if a battery has one dead cell or is just well discharged. A battery should start to charge at a high rate and slowly drop until charging is completed. If it refuses to charge at a high rate in the beginning or refuses to drop from a high rate after 10 hours then you have a real problem and should replace the battery. If the battery gets really hot during charging, then stop and let it cool down. Wait until it is cool before starting.
Using a battery charger properly requires attention, and knowledge; they are not idiot-proof. The bigger problem is determining what the true state of the battery is. The older the battery the more difficult it is; five years old, and they can be as crafty as a career criminal.

SCARY!!

damn..any pics? i cant imagine how much baking soda and water you are going to need.

I had a '68 Coupe de Ville that blew up it's battery. I think it is from battery gas leaking up around the terminals, then if you have loose battery clamp the spark will jump from battery post (with combustible gas leaking up) to the battery clamp.....and boom up she goes. It's pretty unforgettable when it happens.

Uh, NO.
I work in 129vdc lead acid battery rooms in my trade.
The volume of gas produced from fully discharged to fully charged, and then to an overcharged state would be easily dispersed/diluted by the volume of air in a single car garage for a 12v car battery.
Most battery rooms on 24 hr charge only need a 1/2 hour of exhaust fan running per day to remove the gas accumulation from 60 battery cells that are about 4-5 times the size of a car battery each. Yeah, you'll smell it in a garage if it's really gassing, just open the garage door...

This battery likely failed internally or due to a spark in the immediate vicinity with a lot of trapped gas in the area. Flame went back into battery and kaboom! One of the reasons we install flame arrestors in our battery banks.

I blew up the battery in my '87 325 when trying to give it a jump start. My dad (engineer) was convinced I reversed the polarity. I heard the bang when I turned the key on the BMW (the dead car) which did start and run. I wonder if it wasn't for the reasons you guys mention rather than crossed wires. I had jumped it a hundred times and was always hard to believe I had hooked it up backwards.

Hydrogen is highly volatile, and the LFL is 4%. I know it because we just went through the safety reviews and designed a ventilated enclosure for testing lead acid batteries at my work. Most likely hydrogen accumulated inside the battery, because it was defective. Could have been shorted plates, or low electrolyte level.

I have had a few batteries explode on me. Some for no apparent reason. I'm seeing a trend here though with 930s and exploding batteries.

Since the battery in the 911's isn't exactly in area that gets airflow, I took some clear plastic tubing and attached it to the vent on my battery, drilled a hole through the plastic duct work for the blower in front and sneaked it out through the bottom of the car. This way any excessive nitrogen will vent outside of the car. For winter storage I use a trickle charger/battery maintainer that shuts off when the battery is fully charge.

The charger is a Battery Tender with wires permanently installed on the battery which is always on the car when it's in the garage.

I'm pretty sure the issue was with the acid/water level since I hadn't checked it in a while. I've also had issues with negative battery cable connection and should have replaced it long ago. There's plenty of blame on me so I'm admitting my fault to help prevent this from happening to somebody else.

My plan is to install a sealed battery like an Optima. Anybody see an issue with leaving a Battery Tender on them?

http://forums.pelicanparts.com/uploa...1335619460.jpg

I have an Odyssey battery on my 911, which I use a trickle charger on over the winter. I charge it once every month or so, and have had no issues. I do not feel 100% comfortable keeping a battery tender hooked up all of the time to any battery.